REC-ERC-82-8

COLORADO COOPERATIVE

FISHERY RESEARCH UNIT

STUDIES OF TWIN LAKES,

COLORADO: 1980 REPORT

OF FINDINGS

December 1981

Engineering and Research Center

u. S. Department of the InteriorBureau of Reclamation

Colorado Cooperative Fishery Research Unit Studies atTwin Lakes, Colorado: 1980 Report of Findings

7-2090 (4-81)Bureau of Reclamation

1. REPORT NO.

REC-ERC-82-84. TITLE AND SUBTITLE

7. AUTHOR(S)

Eric P. Bergersen andMelo Maiolie9. PERFORMING ORGANIZATION NAME AND ADDRESS

Colorado Cooperative Fishery Research UnitColorado State UniversityFort Collins, Colorado

12. SPONSORING AGENCY NAME AND ADDRESS

Engineering and Research CenterBureau of ReclamationDenver, Colorado 80225

1S. SUPPLEMENTARY NOTES

TECHNICAL REPORT STANDARD TITLE PAGE3. RECIPIENT'S CATALOG NO.

s. REPORT DATE

December 19816. PERFORMING ORGANIZATION CODE

8. PERFORMING ORGANIZATIONREPORT NO.

REC-ERC-82-8

10. WORK UNIT NO.

11. CONTRACT OR GRANT NO.

7-07-83-V070013. TYPE OF REPORT AND PERIOD

COVERED

1980 Report of Findings

14. SPONSORING AGENCY CODE

DIBR

Microfiche and/or hard copy available at the Engineering and Research Center, Denver, Colo.Editor: EJH

16. ABSTRACT

The objectives of these investigations were to gather baseline data on the Twin Lakes system during the preoperationalphase of the Mt. Elbert Pumped-Storage Powerplant. and to develop aquatic environmental monitoring techniques suitablefor use during postconstruction operating periods. The five areas investigated were chosen specifically to fill gaps in presentknowledge of the two-lake system, test methodologies, and point out monitoring problems which might be encounteredduring the operational phase of the project.

Study 1, "Lake Currents." characterized and documented natural. seasonal patterns in surface and bottom currents inthe two-lake system. Data gathered during this study will allow detection of changes in natural current patterns produced bythe powerplant.

Study 2, "Lake Trout Utilization of Shallows for Spawning," showed that it is unlikely that lake trout in Twin Lakes spawnin water less than 1.5 m deep, and thus the expected daily water level fluctuation of 0.7 to 1.1 m should not result in the ex­posure of lake trout eggs.

Study 3. "Mysis Population Estimates- A Photographic Technique," developed a photographic method of estimatingMysis shrimp densities that is not only more reliable than the previous bottom sled trawl method, but also eliminates deple­tion of the population. It was concluded that shrimp are at least 2.4 times as abundant in Twin Lakes as previously thought,with densities in the vicinity of the powerplant as high as 232 shrimp/m2 •

Study 4. "Sedimentation Rates," used both sediment traps and inflow/outflow suspended sediment sampling toestimate sedimentation rates in Twin Lakes. Estimates from these two methods differed greatly. although backgroundsedimentation rates and probable sources were established for comparative purposes. It was concluded that the best timeto monitor powerplant impacts on sediment movement and deposition would be during the winter months.

Study 5, "Organic Contribution of Vegetation Inundated by New Lake Levels." examined the possible consequences forthe lakes' energy budget of inundating vegetated land areas on the west end of the upper lake when water levels are raisedby the new Twin Lakes Dam. Potential input from this terrestrial vegetation was estimated to be over four times the presentaverage annual primary production of the system. Suggestions are advanced to possibly minimize the harmful effects of thisinput.

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCR IPTORS-- / *Iake currents/ *Iake trout/ spawning/ *opossum shrimp/ fish foodorganisms/ *sedimentation rates/ * vegetation/ energy budget/ ecology/ eutrophication/Iimnology/ pumped storage/ powerplants/ environmental effects

Colo.! Mt. Elbert Pumped-Storage Powerplant, Colo.! Frying-b. IDENTIF IERS-- / Twin Lakes,pan-Arkansas Project, Colo.

c. COSATI Field/Group 06F COWRR:0606 SRIM:

18. DISTRIBUTION STATEMENT

Available from the National Technical Information Service, OperationsDivision, S285 Port Royal Rood, Springfield, Virginia 2216/.

19. SECURITY CLASS(THIS REPORT)

UNCLASSIFIED20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO.OFPAGE

5922. PRICE

REC-ERC-82-8

COLORADO COOPERATIVE FISHERY RESEARCH UNIT STUDIES AT

TWIN LAKES, COLORADO: 1980 REPORT OF FINDINGS

Prepared and submitted to the

Bureau of Reclamation

Denver, Colorado

by

Eric P. Bergersen

Melo Maiolie

Colorado Cooperative Fishery Research Unit

Colorado State University, Fort Collins

December 1981

Applied Sciences BranchDivision of ResearchEngineering and Research CenterDenver, Colorado

II81 METRIC

UNITED STATES DEPARTMENT OF THE INTERIOR * BUREAU OF RECLAMATION

ACKNOWLEDGMENTS

This report presents the results of studies conducted by per­sonnel of the Colorado Cooperative Fishery Research Unit, Col­orado State University, Fort Collins, Colo., for the Bureau ofReclamation, Engineering and Research Center, Denver, Colo.,under USBR Contract No. 7-07-83-V0700. The AppliedSciences Branch of the Bureau's Division of Research is pub­lishing this report as part of its research on the ecological ef­fects of operating the Mt. Elbert Pumped-Storage Powerplantat Twin Lakes, Colo. This research is being funded by theEngineering and Research Center's Division of Research andthe Lower Missouri Region's Fryingpan-Arkansas Project.

As the Nation's principal conservation agency, the Department of the Interiorhas responsibility for most of our nationally owned public lands and naturalresources. This includes fostering the wisest use of our land and waterresources, protecting our fish and wildlife, preserving the environmental andcultural values of our national parks and historical places, and providing for theenjoyment of life through outdoor recreation. The Department assesses ourenergy and mineral resources and works to assure that their development is inthe best interests of all our people. The Department also has a major respon­sibility for American Indian reservation communities and for people who live inIsland Territories under U.S. administration.

The information contained in this report regarding commercialproducts may not be used for advertising or promotional pur­poses and is not to be construed as an endorsement of anyproduct by the Bureau of Reclamation.

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LAKE CREEKINFLOW

Frontispiece-Map of Twin Lakes, Colorado.

FOREWORD

Twin Lakes are a pair of connected montane drainage lakes of glacial origin located onLake Creek at the eastern foot of the Sawatch Range in the Upper Arkansas RiverValley of central Colorado. The lakes lie at an elevation of 2802 m above mean sealevel at 39°05' N. latitude and 106°20' W. longitude. Maximum surface areas areabout 263.4 ha for the upper lake and 736.5 ha for the lower, with correspondingdepths of approximately 20 and 27 m, respectively. A controlled outlet works wasbuilt on the lower lake around the beginning of this century. The channel connectingthe two lakes was dredged at about the same time, so that today both lakes fluctuateessentially as one. Twin Lakes are dimictic, with maximum surface temperaturesreaching up to 18 ° C during late July to mid-August, and ice cover lasting usually fromDecember to May. Chemically, Twin Lakes are soft, dilute calcium bicarbonate lakes,with pH being usually neutral to slightly basic. Phytoplankton flora consists mainly ofdiatoms and other golden-brown algal species, while the zooplankton fauna is a mysisshrimp-copepod-rotifer association. At present, Twin Lakes support a self-reproducinglake trout fishery and a put-and-take rainbow trout fishery. Both lake trout and mysisshrimp were introduced into Twin Lakes.

The northwest corner of the lower lake is the site of the Mt. Elbert Pumped-StoragePowerplant, Fryingpan-Arkansas Project. The powerplant houses two 100-MW pump­generators, the first of which began operating in September 1981, while the second isscheduled to begin operation in 1983. A forebay has been constructed on the ridgenorth of the lake, and a new dam has been built below the lower lake. When this newdam is closed, the maximum water surface elevation of Twin Lakes will be raised toabout 2805 m, inundating the isthmus between the two lakes and creating one largereservoir.

Baseline studies of the limnology and fishery of Twin Lakes have been conducted bythe Bureau of Reclamation and its cooperators since 1971, with the object ofdocumenting and quantifying the effects of the Mt. Elbert Pumped-Storage develop­ment on the aquatic environment. The present report is one of a series presenting theresults of these ongoing investigations. Information on pumped-storage effects ac­quired at Twin Lakes will be used to improve the Bureau's project planning capabilitiesso that the environmental impacts of future pumped-storage developments can bemore accurately evaluated.

James J. Sartoris, Research Civil EngineerDivision of ResearchEngineering and Research CenterBureau of ReclamationDenver, Colorado

iv

CONTENTS

Page

Chapter 1, Lake currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Introduction 1Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Open water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Wind measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Surface currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Bottom currents ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Under ice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Open water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Under ice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Testing and temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Upper lake ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Lower lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Discussion 8Surface currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Upper lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Lower lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Bottom currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Upper lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Lower lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Under ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Testing 12Upper lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Lower lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Lake currents and the powerplant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Open water 14Under ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Conclusion 15

Chapter 2, Lake Trout Utilization of Shallows for Spawning. . . . . . . . . . . . . . . 17Introduction 17Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Chapter 3, Mysis population estimates - A photographic technique. . . . . . . . . . 19Introduction 19Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Speed and distance measuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Off-bottom trawling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Net trawl efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Shrimp density estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Distribution 23

Discussion 24Problems with past trawling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Shrimp density estimates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Distribution 24

Conclusion 24

v

CONTENTS - ContinuedPage

Chapter 4, Sedimentation rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Introduction 25Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Sediment trapping 25Suspended sediment sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Conveffi~nfactoffi........................................ 25

Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Conversion factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Sediment trapping 26Suspended sediment sampling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Discussion 30Sediment trapping 30Suspended sediment sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Chapter 5, Organic contribution of vegetation inundated by new lake levels. . . . 33Introduction 33Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Discussion 34

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Appendix - Surface, Bottom, and Under-ice Current Maps for Twin Lakes,Colo. 37

TABLES

Table

4-1 Suspended sediments passing through Twin Lakes andrespective organic and inorganic fractions . . . . . . . . . . . . . . . . . . . . 28

5-1 Cover dominance, weight, and caloric content of the majorvegetation types found in the study area above the upper lake . . . . . . 33

FIGURES

Figure

Frontispiece-Map of Twin Lakes, Colorado. . . . . . . . . . . . . . . . . . . . . . . . . . iii1-1 Drogue used to measure lake surface currents during daylight

hours at Twin Lakes, Colo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21-2 Strobe-lighted drogue for measuring nighttime lake surface

currents at Twin Lakes, Colo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21-3 Calibration curve for leaning-tube current indicator . . . . . . . . . . . . . . . . 21-4 Probe used to measure under-ice currents by releasing ink . . . . . . . . . . . 41-5 Current velocities at various depths . . . . . . . . . . . . . . . . . . . . . . . . . . 41-6 Temperature profiles of the upper lake. . . . . . . . . . . . . . . . . . . . . . . . 41-7 Temperature profiles of the lower lake . . . . . . . . . . . . . . . . . . . . . . . . 41-8 Sediment temperature profiles, made at various water depths,

in the upper lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51-9 Sediment temperature profiles, made at various water depths, in

the lower lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

vi

Figure

1-101-111-121-131-141-151-161-171-181-19

1-20

1-211-221-232-1

3-1

3-23-3

3-4

3-5

3-63-74-14-24-34-44-54-64-7

5-1

CONTENTS- Continued

Under-ice currents of the upper lake on February 2, 1981 .Under-ice currents of the upper lake on March 10, 1980 . . . . . . . . . . . .Under-ice currents in the lower lake on January 21-22, 1981 .Under-ice currents in the lower lake on March 17-19, 1981 .Surface currents measured on the upper lake, July 16, 1980 .Wind roses for the daylight hours of July 16, 1980 .Surface currents measured on the upper lake, November 21, 1980 .Wind roses for the daylight hours of November 21, 1980 . . . . . . . . . . .Surface currents measured on the upper lake, October 22, 1980 .Surface currents measured on the lower lake during the night of

November 5-6, 1980 .Wind roses for the night of November 5-6, 1980 during the time

lake currents were measured . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bottom currents of the upper lake on October 28, 1981 .Bottom currents of the lower lake on November 15, 1980 .Bottom currents of the lower lake on August 21, 1980 .Shore areas in which search for shallow spawn lake

trout eggs was conducted .Bottom sled trawl as used in the Twin Lakes study with both camera

equipment and removable net-scoop assembly attached .Results of General Oceanic's Digital Flowmeter testing .Results of testing the float passage method of measuring boat

speed against flowmeter results . . . . . . . . . . . . . . . . . . . . . . . . . . .Trawl speed and its relation to the amount of time trawl was

on bottom .Photographic shrimp density estimates and simultaneously made

net trawl density estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Trawl speed and estimated shrimp density relationship .Frequency histogram of 195 photographs taken in 1981 .Schematic of sediment trap used in Twin Lakes .Sonic-activated float release device used to retrieve sediment traps . . . .Location of nine sediment traps placed in the upper lake . . . . . . . . . . . .Location of twelve sediment traps placed in the lower lake . . . . . . . . ...Sedimentation rates in the upper lake based on sediment trapping . . . . .Total suspended sediment budget in Twin Lakes, Colorado .Comparison between sedimentation rates in the upper lake as

estimated by sediment trapping and suspended sedimentsampling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Study area west of the upper lake which will be inundated bynew lake levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

Page

66778899

10

10

11111313

17

1921

21

22

222323262627282929

31

33

CHAPfER 1. LAKE CURRENTS

INTRODUCTION

Although lentic waters are often characterizedas being still or sluggish, lake waters in actualityare in constant motion. This motion plays an im­portant role in determining the physical andchemical nature of lake environments and in thedistribution of planktonic organisms andorganisms which feed upon them. Becausenatural currents in Twin Lakes will be modifiedby the Mt. Elbert Powerplant and the increasedsize of the lake, it was important to documentthe nature of existing currents prior to these ma­jor environmental changes. In doing so, a betterunderstanding of lake dynamics was gained anda baseline against which powerplant-inducedchanges can be compared.

It is suspected that the combined impact ofoperation of the powerplant and the increasedlake size will affect currents, and consequently,lake biota in several ways. The increased fetchmay result in higher velocity currents whichcould affect the mysis shrimp which are knownto be sensitive to turbulence (Gregg and Berger­sen, 1980 (1)1). Larval fish, which are highlydependent on protected shoreline areas, couldbecome dislodged by increased currents and bemore susceptible to predation in less protectedareas. Bottom sediments may also be resus­pended by stronger lake currents. Powerplant­induced currents may also affect mysis and lar­val fish. Furthermore, these currents will add anew dimension to the situation in that they willcontinue throughout the winter. This could havetwo effects: (1) Changes may occur in the icesheet in the form of erosions, pitting, or unequalfreezing, making some areas of the lakes unsafefor recreational uses such as ice fishing, jeepand snowmobile racing, etc.; or (2) under-icecurrents could be sufficiently strong to keepsediments suspended during the time when theywould be expected to settle. This would result inincreased turbidity and a subsequent change inthe total energy budget of the lake. The conse­quences of this event are far reaching and couldresult in a total change in the Twin Lakes en­vironment as we now know it.

1Numbers in brackets indicate references listed in the3ibliography.

The purpose of this report is to characterize anddocument seasonal patterns in surface and bot­tom currents in the two-lake system.

METHODS

Open Water

Wind measurements. - Wind data were gath­ered with a recording wind vane made by placinga Minolta time-lapse movie camera over a con­ventional wind vane. A Trade-Wind anemometermodel 11 0 was mounted above the moviecamera with its windspeed meter on the windvane. The intervalometer on the camera was setto take one frame every 12 to 15 minutes. Inthis manner, one roll of movie film recorded awhole month's windspeed and direction. Thevane and camera were placed on a building100 m from the lake. A street light, 5 m away,provided light for nighttime exposures.

Wind roses were used to describe the winddirection from the 16 compass points. Each daywas divided into fourths and one wind rosedrawn for that quarter. Each arm segment of thewind rose radiating from the center representsone reading of the wind vane. The orientation ofthe 'wind rose arms indicate the direction fromwhich the wind was blowing. Complete calmwas seldom recorded at the lake. In the center ofeach wind rose is the corresponding averagewindspeed in meters per second.

Surface currents. - Surface currents were meas­ured with vertical floating drogues. Duringdaylight hours small flags were attached to thetop of each drogue. Lighted flashing strobedrogues were used at night (figs. 1-1 and 1-2).Drogues were designed to move passively withthe surface currents and be minimally affectedby wind.

Each month, approximately 15 drogues were seton selected standard transect lines and followedfrom 8:00 a.m. to 4:00 p.m. Night surface cur­rents, measured in November, were monitoredbetween 5:00 p.m. and 2:00 a.m. Periodically,the position of each drogue was recorded by tri­angulation on known shore positions with a sex­tant. Velocities of surface currents were based

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Figure 1-1 - Drogue used to measure lake surfacecurrents during daylight hours at Twin Lakes, Colo.

Figure 1-2 - Strobe-lighted drogue for measuringnighttime lake surface currents at Twin Lakes, Colo.

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2

on the straight line distance between recordedpositions and therefore represent a minimumestimate. Anemometers at the center of eachlake were used when precise windspeeds wereneeded (used in expressing currents as percentwindspeed) .

Bottom currents. - Direction and velocity of bot­tom currents were measured with modified"Pisa" current indicators (Carruthers, 1958[2] ). Indicators were calibrated at the Bureau ofReclamation's Denver Hydraulics Laboratory(fig. 1-3). Weighted Masonite boards (30 by 60by 0.5 em) were used as indicator bases to pre­vent sinking into bottom muds.

Bottom currents were measured monthly alongthree transects in the upper lake and fourtransects in the lower lake. Six to nine indicatorswere set along each transect. All sets on eachlake were made the same day. Position of the setwas triangulated by sextant and depth and timerecorded. At least 30 minutes after setting, indi­cators were retrieved and degree tilt of gelatinemeasured. The direction of the current was de­termined by comparing the direction of tilt of thebottle to the north-south orientation of the barmagnet which was held in the solidified gelatine.Velocities could only be measured down to3 cm/s due to limitations of the indicator.

Under Ice

An ink tracing technique was developed tomeasure the velocity and direction of water cur­rents under the ice sheet. On the bottom end ofa 2-m (1.9-cm by 2-m PVC pipe (fig. 1-4))probe, a 10-cm-long measuring bar was at­tached at right angles. A Quanterron 10-mL oralsyringe was fitted onto the probe and a cap witha 0.8-mm hole in the tip was fitted over the endof the syringe.

When the probe was lowered through a 15-cmhole cut in the ice, a trail of ink would streamfrom the syringe tip. This line of ink was thendisrupted by moving the probe several centi­meters to one side. The break in the ink trail wastimed as it moved the length of the measuringbar. Three velocity measurements were aver­aged at each location. Current direction wasmeasured with a sextant in terms of degreesfrom a known point on shore. The sextant wasalso used to triangulate the location of each cur­rent measurement station. Current measure-

3

ments were made in the months of April 1980and January, February, March, and April 1981.

To note changes in current due to the frictionalinfluence of the ice cover, four measurements ofwater velocity and direction were made at fivedepths from just under the ice to 2.6 m.Measurements at each depth at three locationswere averaged.

Temperature profiles of the water and bottomsediments were recorded in each lake with aHydrolab Model 8002 or T-4 Marine Ther­mometer. Temperature profiles within the bot­tom sediment were made by mounting theHydrolab Model T-4 temperature probe in theend of a sectionalized 4-cm-dia. aluminum rodwhich was sufficiently long to reach the bottomsediments. This rod was forced into bottomsediments and readings taken at various depths.

RESULTS

Open Water

Current maps and corresponding wind data areshown in the appendix.

The maximum surface current velocity meas­ured on the upper lake was 15.2 cm/s. Maxi­mum observed velocity of surface current in thelower lake was 14.5 cm/s. Maximum observedbottom currents for the upper and lower lakeswere 9.8 cm/s and 8.9 cm/s, respectively.

The percent of windspeed for each vector on thecurrent maps was calculated. These proved tobe quite variable, ranging from 0.4 to 17 per­cent windspeed within the same day. All vectorpercentages were combined to obtain anaverage percent windspeed for the lake on thatparticular day. The lake average ranged from alow of 0.9 percent windspeed in November (up­per lake) to a high of 4 percent windspeed inSeptember (upper lake).

Under Ice

Testing and temperatures. - No change in cur­rent direction occurred from just under the ice toa depth of 2.6 m. Current velocity increasedwith depth; however, at depths greater than 2 mlittle additional increase occurred (fig. 1-5).

Water temperatures were below that for themaximum density of water (3.94 °C) in April1980 and January and February of 1981. ByMarch 1981 water temperatures were at orabove maximum density temperature (figs. 1-6and 1-7).

ICE

Bottom sediment temperature profiles measuredin the shallow regions of the upper lake werewarmer than temperature profiles measured indeeper regions (fig. 1-8). Under the shallowwater (1.8 m) of the upper lake, temperaturechanged 2 °C/m into the sediments. Tempera­ture changed 0.6 °C/m at deeper regions of thelake (13.6 m water depth). A similar, though notas pronounced, relation existed on the lowerlake (fig. 1-9). Such profiles indicate that rela­tively more heat is being added to shallow por­tions of the lake than at the lake center.

Upper lake. -Anticyclonic (clockwise) circula­tion predominated in the upper lake in April1980, and January, February, and April 1981 .

-PROBE

oMax density

oMax density

3/12/81../

oMax density

/MEASURING

t-­II---SYRINGE

s

BAR

5 5 54/1/81 ------

10 10 10E

!I~

2' ;!, 3/3/81Il 15 15 15w :j0 4/14/80-

II2/4/81---:

20 20 20

Figure 1-4- Probe used to measure under-ice currentsby releasing ink.

0.....-----------------ICE

12345 12345 12345

T E M PER A T U R E ('C)

Figure 1-6-Temperature profiles of the upper lake.

3--L----r----r----,----,

E

If-a..wo 2

.----"'" "'-

"- '\.\

\.\

DEPTH OF \CURREN T ---------------------+-----MEASUREMENT I'

\I

Max. density Max. density Max density

0 0 0-,5 5 5

10 10 10~ 2/4/81 3/20/81 4/1/81E

I15f-15 15

Ilw0

20 20 20

Figure 1-7- Temperature profiles of the lower lake.

10 20 30 40VELOCITY (em I min)

Figure 1-5- Current velocities at various depths. Eachpoint represents the average of four readings at threedifferent locations.

4

12345 12345

TEMPERATURE ('CI

1 2 3 4 5

The average measured current velocity for these4 months was 24, 33, 38, and 16 em/min, re­spectively. A maximum velocity of 67 em/minwas observed in January 1981 . Currentsrecorded in February 1981 were typical of thoseobserved under the ice (fig. 1-10). Some eddy­ing occurred near the lake's perimeter with the

center of the main gyre located toward the northshore.

In March 1981, current in the main gyre re­versed direction (fig. 1-11). It became cyclonic(counterclockwise) with the perimeter of thelake anticyclonic. Current velocities dropped toan average of 13 em/min with the gyre centerlocated toward the south shore.

Figure 1-8- Sediment temperature profiles, made at variouswater depths, in the upper lake.

0

E

+-'cQ)

E-0Q)

vi

0+-' 2c-~

+-'Q.Q)

03

3.5

0.... .....\. Lake bottom'o~k~~+ t8m water depth

"" Jil ...........~, ................

\'''-.. ". 37m water depth.',""." '."" '-.. 9.7m water depth

' ...... '0"",-......., ......., ..

" '-" ."', .........

......... 0 .................. ' ...........

..... , .~ ....... o....136m water depth ~'" '0

4.0 4.5 5.0 5.5Temperature ("CJ

Lower lake. - Under-ice currents in the lowerlake in April 1980 were often characterized byopposing pulses of water movements, some­times occurring as a complete 1800 change indirection in a matter of minutes. As a result, dis­tinct circulation patterns were difficuh: toidentify.

In January and February 1981 a general circula­tion pattern began to emerge (fig. 1-12). Watermovement was much more complicated on thelower lake than the upper lake with numeroussmall gyres occurring around the lake. Apparentareas of upwelling and downwelling were alsonoted. The circulation pattern discerned was un­stable, changing monthly. Average measuredcurrent velocity for January and February was32 em/min. Maximum observed velocity was100 em/min, which occurred in January.

3-+----,----....----,------..----r-----.------r

E

+'c:4i

E1J4i

(f)

o+'c:

.c+'0.4io

o

2

4.0

LOWER LAKE

5.0Temperature (0C)

6.0

9.1 m wat er depth

7.0

Figure 1-9-Sediment temperature profiles, made at various water depths, in the lower lake.

5

Figure 1-10-Under-ice currents of the upper lake on February 2, 1981. Current velocities aregiven in em/min. Flow direction lines, fitted by inspection, were added for clarity.

19/

10---~

I ~10..--

500 M

SCALE

Figure 1-11 - Under-ice currents of the upper lake on March 10, 1980. Current velocities are givenin em/min. Flow direction lines, fitted by inspection, were added for clarity.

6

"\\ N

~. ~ f/.. I,. ~ 35\\~ /~O

( ~ '\ \"~ /" ~.t: ~ ~7 ~.

\35 '" 2. !, t40

500 M ~ /35 ./SCALE ~ ~7

Figure 1-12-Under-ice currents in the lower lake on January 21-22, 1981. Current velocities aregiven in em/min. Flow direction lines, fitted by inspection, were added for clarity.

500 MSCALE

'0

4 ........ 4

1"-0-440-44

'0

Figure 1-13- Under-ice currents in the lower lake on March 17-19, 1981. Current velocities aregiven in em/min. Flow direction lines, fitted by inspection, were added for clarity.

7

500 M

SCALE

____0

......---. 3.8__e 3.7

4.~ •

---:,2.0. 3.8.~~..o ~ ° :::-:--.Z82.6 6.6 2.3.~.;)-.;-..

O.~.'.1 4.' O~

r~O,3.6 o~o1.3\ .........

'\. O~o 6.'2.5'-.. ~~.

~ 7.5.2.8

J

Figure 1-14-Surface currents measured on the upper lake, July 16, 1980. Velocities betweendrogue sitings are given in cm/s.

The current pattern changed to a central anti­cyclonic gyre in March. Current velocities at thistime were greatly reduced with an averagemeasured velocity of 13 cm/min. The easternend of the lake appeared calm at this time(fig. 1-13).

DISCUSSIONEARLY AM LATE AM

Surface Currents

Upper lake. - A very strong dependence wasnoted between upper lake surface currents andwind velocity and direction. A good example ofthis was noted on the upper lake on July 16,1980 (figs. 1-14 and 1-15). When windsshifted from a southeasterly to a northwesterlydirection, corresponding changes were noted incurrent direction. As windspeeds increasedthroughout the day, lake current velocities, par­ticularly in the middle of the lake, also increased. EARLY PM LATE PM

On November 21, 1980 (figs. 1-16 and 1-1 7)windspeeds never exceeded 1.6 m/s. Underthese calm wind conditions, current velocities

Figure 1-1 5- Wind roses for the daylight hours of July16, 1980. Numbers within the circles indicatewindspeed in m/s.

8

500 M

SCALE

0.'O.6~•.'fO·3

118

.%- •21' i'2.y 10.4. /..

1.3[ •

. f'}.2

.--07'_0.8

• 0.30.1

J0.7 0.3

0,9.........

10." '-.----~

Figure 1-16- Surface currents measured on the upper lake, November 21, 1980. Velocities be­tween drogue sitings are given in cm/s.

were the lowest ever recorded. Current veloci­ties at this time, expressed as a percent of wind­speed, were also the lowest ever recorded (0.9percent) suggesting that low-velocity winds arepoor current inducers.

Currents in November also showed a slight ten­dency toward clockwise rotation, particularlynearshore. It is suspected that on calm days,these nearshore clockwise movements of waterare induced by convection currents. Nonlinearcurrents may also be in part responsible forthese clockwise lake currents; however, thesewere also noted under the ice indicating aminimal wind influence.

In the upper lake, currents generally moveddownwind at 1 to 4 percent of windspeed.When the downwind shore was reached, cur­rents moved alongshore with decreased velocity(fig. 1-18). Undoubtedly, much of the surface­blown water sank to greater depths (dependingon density gradient) when reaching the down­wind shore.

EARLY AM

EARLY / PM

LATE AM

LATE PM

lower lake. - Under moderate to strong wind:onditions, greater than 2 to 4 mis, currents of

Figure 1-1 7- Wind roses for the daylight hours ofNovember 21, 1980. Numbers within the circlesindicate windspeed in m/s.

9

N

~~:~~/>...~I 9

3

4.~~",• ':,: 5.6.~

" /91 • .~ 1 •

\fi.~:/91. ~'03~ ~. 30 fu. 8.5 • 2.5 • ......-- ---.•

•~4 • 101 2(~••

'b· .~ /. 39 41 \5.4/ ••~ 7,0· J4.6// ~9'0 /;V29 9'~~.~.~. 149

2 1. ~~•• / • .....---.//. 8.1 26

.-:;~6• 10.6 .3.6/- e.7 1/ •.a.t. • 2·~~O

3.;" 500 M.~. SCALE

25J

Figure 1-18-Surface currents measured on the upper lake, October 22, 1980. Velocities be­tween drogue sitings are given in em/so

N

f

500 MSCALE

''''~, '\'\:13 ~ . .

~. '~'z':"~ ,"\..',,' ~\-~ \.. \

.19.~\6:~0.~~·~~5 \~2·\9~ 19 __ 33

36 e___._. ~....l. 9 49

20

.\- 15· JI 7

~'2 • 0.4.~

3.1 2.0.

"""-----------

Figure 1-19-Surface currents measured on the lower lake during the night of November 5-6,1980. Velocities between drogue sitings are given in cm/s.

10

N

i

~-~EARLY PM MID PM

the lower lake proceed downwind at an averageof 1 to 4 percent of windspeed. The flow to thedownwind side of the lake is complicated by anintrinsic circulation pattern which appears as aclockwise oval circulation about the long axis ofthe lake. The center of this gyre may move fur­ther north or south but it is always clockwise.

The intrinsic pattern is evident in the first vectorof lower lake surface current maps on calm days(app. figs. A-9, A-11, and A-13), when an initialclockwise circulation pattern emerges. As windincreases, (often very early in the day), theclockwise circulation pattern is overridden andsurface currents proceed to the downwindshore. It is a combination of thermal- and wind­induced forces which are likely responsible forproducing this intrinsic movement pattern.

Figure 1-20-Wind roses for the night of November 5-6.1980. during the time lake currents were measured.Numbers within the circles indicate windspeeds in m/s.

LATE PM EARLY AM

In November 1979 and 1980, currents weremeasured at night (figs. 1-19 and 1-20). Aswould be expected under similar atmosphericconditions, surface currents were very similar.Currents initially proceeded downwind. Aswindspeeds dropped, current velocities slowed,and drogues were found to cluster in the south­eastern corner of the lake. A change was then

J500 M

SCALE

\30'/;

8.'--

.,

I·,

\8'

\"

• (3.0

.< 3.0

5. ,---

-( 3.0

.,--8.9

---

98---

Figure 1-21-Bottom currents of the upper lake on October 28. 1981. Current velocities areshown in cm/s. Current direction is indicated by arrows.

11

noted in the paths of several drogues. Appar­ently, with the decrease in windspeed thetypical clockwise circulation pattern againbecame established.

Bottom Currents

Upper lake. -During the summer of 1979 and1980, bottom currents along the southeasternshore of the upper lake were relatively strong inresponse to the predominantly northwesterlywinds (fig. 1-21). Currents here were suffic­iently strong to keep rocky bottom areas free ofsilt and other sediment commonly found else­where in the lake.

Apparent irregularities in the bottom caused cur­rents to be quite variable in direction. Althoughno consistent pattern was evident, overall veloc­ities recorded will be useful in assessing anyfuture current changes in the lake.

Lower lake. -A relationship between lake troutspawning areas and areas of strong bottom cur­rents was evident in the lower lake. HartmanPoint, Mackinaw Point, Rocky Point, and thesouth shore all were found to have currentsexceeding 6 cmls and all are suspected laketrout spawning areas (Walch, 1979 [3] ). Con­striction of the water mass by these projectingland features appears to have caused these in­creased velocities. For example, on November15, 1980, a current of 7.0 cmls was recordedon Hartman Point, the strongest reading on theeastern half of the lake that day (fig. 1-22). InAugust 1980, Mackinaw Point had the strong­est current recorded that day (7.2 cm/s) (fig.1-23). Similarly, strong currents were found onthe south shore and Rocky Point (7.9 cmls and6.4 cmls, respectively) (fig. A-27).

Construction of a road along the south shore andits subsequent removal resulted in a largeamount of silt and sand being deposited on thelake trout spawning habitat in this area. (Seech. 4.) It is suspected that without further dis­turbance, the strong currents in this area willflush the rocks clean.

Although bottom currents in the lower lake areusually variable, a circulation pattern did be­come established on November 15, 1980,when strong easterly winds occurred (fig.1-22). The lake was unstratified at this time andas expected, bottom currents moved west to

12

east. Currents along the easternmost transect,however, moved in the opposite direction (i.e.,east to west) and were generally of lower veloci­ty. The west-to-east currents apparently liftedoff the bottom before encountering the lasttransect and a back eddy occurred there. Similarbottom currents occurred on September 9,1980, only with currents from east to west.Again, currents along the downwindmosttransect changed direction and were of lowervelocity. Such patterns, however, becameestablished only after strong winds persistedfrom a constant direction.

Under Ice

Testing. - The smooth underside of the ice sheetapparently does not affect current direction. Asemilog plot of current speed vs. depth approx­imates a straight line, indicating current velocityincreases logarithmically with depth within thefirst 2.6 m. Beyond 2 m, little change in currentspeed occurred; thus, the readings made at2.1 m are below most of the ice's influence.

Upper lake. - The energy necessary to move thegreat mass of water in the lake and cause the re­sulting circulation was believed to come fromthe heating and cooling of the lake's water. Thesediments in the shallows of the upper lake werewarmer than sediments at the center of the lake.Thus, relatively more heat was supplied to thewater at the perimeter of the lake. Heated waterthen rose or sank, depending on the water'sdensity gradient. Conversely, the upper layers ofwater at the center of the lake were cooled.Again, this water either rose or sank, dependingon ambient water density.

The upper lake's bowl-like shape provided anearly perfect location to study these currents.Regular contours caused a minimum of currentdisturbance. Waters were below 4 °C through­out the winter of 1980 and in January and Feb­ruary of 1981. Under these conditions, waterheated in the shallows sank. Upper layers ofwater that became cooled rose. The result wasascending water at the center of the lake and de­scending water near the perimeter. Fultz (1 951 )[4] found that a dishpan of water (above 4 ° C)heated at the bottom near the rim had a narrowclockwise current at the perimeter, with thecenter region moving counterclockwise. If thewater temperatures were below 4 ° C, currents

""II

500 MSCALE

...-;....­e.1

• (3.0

/ .../7.'

...........

-­...• <3.0

N

f·<~.O

--.:.9

.....:...-"i.e

Figure 1-22- Bottom currents of the lower lake on November 15. 1980. Current velocities are shown incm/s. Current direction is indicated by arrows.

;.7

N

f-< 3.0

.( 3.0

-< 3.0

"•..

• <3.0

SCALE500 M

"IIII----------

Figure 1-23- Bottom currents of the lower lake on August 21. 1980. Current velocities are shown incm/s. Current direction is indicated by arrows.

13

moving in the opposite direction would be ex­pected. Fultz (1960) [5] found that water-filledrotating cylinders, heated at the bottom, devel­oped convection cells with ascending motion intheir cores and anticyclonic rotation about theiraxes. Those cells driven by cooling at the tophave descending motion in their cores andcyclonic rotation about each cell's axis. Thesemodeling studies mimic closely the circulationpattern found on the upper lake in April 1980,and January and February 1981 .

In March, water temperatures were greater than4°C. Water heated around the rim of the lakewould now rise and cooler water at the center ofthe lake would sink. A similar situation was re­ported in Tub Lake, Wis. by Likens & Ragotzkie(1966) [6]: a cyclonic rotary movement at thecenter and anticyclonic movement near theperimeter. A very similar pattern existed in theupper lake during March. The decreased currentvelocities during this month were likely due tocurrents switching direction only a shQrt timebefore measurement.

In April 1981, the upper lake was unexpectedlyfound to have anticyclonic rotation over most ofthe lake. Sufficient data were not available to ex­plain why the March pattern was overridden;however, it is likely due to rapid heating at thistime of year.

It is interesting to note that throughout thisstudy the upper lake had only one large gyre;thus, it appears the lake functions as one verylarge convection cell. Such was not the usualcase on the lower lake.

Lower lake. - Under-ice current patterns in thelower lake were more variable as a result of theirregular contours and suspected irregularheating and cooling. Currents were believed tobe thermally driven, with many convection cellsforming the irregular pattern. Strong currentswere found to converge and diverge, suggestingareas of downwelling and upwelling.

Velocities on the lower lake were similar to theupper lake. Again, as the water reached 4 ° C,current velocities dropped by more than half.This would tend to lend credence to theconvection-induced current theory. At 4°C, acurrent reversal was expected and partiallyobserved. During this reversal, currents wouldneed to stop, then start in the opposite direction;hence, the low velocities noted in March.

14

Circulation during March was in the form of alarge anticyclonic gyre in the center of the lake.The reason for anticyclonic rotation and the ap­pearance of only one gyre is speculative;however, rapid heating at the center of the lakecould cause water to ascend, hence the rotation.Possibly, the heat influx was strong enough tooverride the many smaller gyres and create onelarge gyre.

Pulsed waterflows in varying directions werecommonly found in the lower lake, particularly inApril 1980 (app. figs. A-43 and A-44). Thesepulsed flows were much more common and of amore variable direction on the lower lake, indi­cating that lake size is a factor in their occur­rence. Verber (1964) [7] concluded that differ­ential pressure on the ice cover of frozen LakeMichigan by wind could have been responsiblefor some water movement in midwinter. Con­ceivably, changes in differential pressure couldpulse under-ice currents of Twin Lakes, makingthe net flow and direction difficult to interpret.Because of its smaller size the upper lake couldbe expected to have less differential pressure,hence a less frequent occurrence of pulsedflows.

Lake Currents and the Powerplant

Open water. - The powerplant tailrace is locatedin the northwest corner of the lower lake. Thisarea of the lake, because of its sheltered loca­tion, is not conducive to producing strong lakecurrents. Rarely did bottom currents in this areaexceed 3 cm/s. The hillside to the north andwest of this corner shelter it from the wind;thus, strong surface currents also did not occur(fig. 1-19). With calm water predominating inthis area, the sedimentation rate here may begreater than elsewhere in the lake. If this has oc­curred, a higher degree of turbidity can be ex­pected to eminate from this area than elsewherein the lake once the powerplant begins opera­tion, or at least until these sediments are flushedfree.

Under ice. - Currents in the vicinity of thepowerplant are generally well below the lake av­erage because of the restrictive nature of thiscorner of the lake. Currents in 1981 flowed to­ward the mouth of the powerplant. Such cur­rents could have several effects on the power-

plant-generated currents. They may slow watercoming from the powerplant, causing its energyto dissipate quicker, or lake currents could de­flect powerplant flows, changing their direction(possibly downward).

Even in winter, lake currents are sufficientlystrong to continually carry mysids into the vicini­ty of the powerplant. Assuming a straight linedrift at an average of 32 em/min, a passivelydrifting shrimp could travel from Interlaken tothe powerplant (1.7 km) in less than 4 days.Winter powerplant operation may therefore sub­ject major portions of the shrimp population toentrainment.

15

CONCLUSION

Surface currents in both the upper and lowerlakes are strongly influenced by surface winds.During low wind or ice-cover periods an intrinsicclockwise pattern emerges and predominateswhich may be somewhat altered by local fea­tures. Bottom currents in both lakes are variableand are generally lower in velocity than surfacecurrents, which may be as high as 15 cm/s.

With the data collected thus far during this partof the work at Twin Lakes, natural currents inthe two-lake system can be characterized and, inmany cases, predicted, with confidence. Anychanges in natural current patterns produced bythe powerplant should be readily apparent.

CHAPTER 2. LAKE TROUT UTILIZATION OF SHALLOWSFOR SPAWNING

INTRODUCTION

Because of the fluctuating lake levels which willoccur during the pump-generation cycle, it islikely that any fish eggs deposited in less than1.5 m of water would be exposed to the air andresulting extremes of environmental conditionswhich will not be favorable to their survival.Walch (1979) [3] reported catching ripe laketrout in as little as 1.5 m of water. It is notknown, however, whether these fish wouldhave spawned in water this shallow. The objec­tive of this investigation was to determinewhether lake trout do spawn in water less than1.5 m deep. The presence of live trout eggs inshallow water would be prima facie evidence ofspawning in this area.

METHODS

Fish eggs, if present, lake water, and loosegravel from interstitial areas of suspectedspawning habitat were pumped into a 1 by

0.45-m hardware cloth-covered frame through a5-cm-dia. hose with a Briggs and Strattongasoline-powered water pump. To test the effi­ciency of this method, 100 lake trout eggs werescattered over about 2 m2 of rocky bottom inwater 0.30 to 1.00 m deep; then, as many aspossible were recovered with the pump. Noeggs were visible during the search. Thirty­seven eggs were recovered, leading to the con­clusion that the method was capable of detec­ting lake trout eggs in shallow water.

During the 1979 spawning season (November20, 21, 27), approximately 250 m of suspectedshoreline spawning habitat were thoroughlyvacuumed to a depth of 1.5 m with the pumpdevice. Areas searched included the east side ofMackinaw Point and the west side of RockyPoint (fig. 2-1). During the 1980 spawningseason (November 6, 7, 10), both sides ofRocky Point, riprap near the powerplant, and asection of the south shore of the upper lake (fig.2-1) were searched. In total, 700 m of shorelinewere searched in 1980.

/PowerplantI, Tailrace

1979 = 000

1980 = ....",." ....

Mackinaw

Figure 2-1-Shore areas (dotted) in which search for shallow spawn lake trout eggs was conducted.

17

RESULTS AND DISCUSSION

Although small suckers, black-nose dace(Rhinichthys atratulusl. Mysis shrimp, scuds,and damselfly nymphs were collected during thesearch of over 950 m of shoreline, no trout eggs

18

of any kind were found. From this it was con­cluded that it is unlikely that lake trout in TwinLakes spawn in water less than 1.5 m deep andthat the expected daily water level fluctuationsof 0.7 to 1.1 m will not result in the exposure oflake trout eggs.

CHAPTER 3. MYSIS POPULATION ESTIMATES ­A PHOTOGRAPHIC TECHNIQUE

INTRODUCTION

When the Mt. Elbert Pumped-Storage Power­plant starts operation, Mysis shrimp (Mysisoculata var. relicta [Loven] ) will be entrained.Because these shrimp serve as a principal foodof lake trout (Salvelinus namaycush) in TwinLakes, it is important to monitor the Mysis popu­lation in relation to operation of the powerplant.

In the past, a bottom sled trawl was used tomonitor the abundance of the shrimp. Thisproved to be a rather difficult and inefficienttask. In an attempt to find a better technique, aphotographic method was developed. It wasbelieved greater accuracy could be achievedwith photographs since a known area of bottomwas sampled, the spatial distribution of theshrimp could be readily determined, and sampl­ing in this manner eliminated depletion of thepopulation and pointed out other problems in­herent in net-trawling such as lifting off bottomand net avoidance by the shrimp.

METHODS

A bottom sled trawl (1.5 m long, 1.3 m wide,and 0.7 m high) made of 19-mm-dia. aluminum

tubing was used in this study (fig. 3-1). Skids,15 cm wide by 1.5 m long, flanked each side ofthe trawl and prevented the trawl from sinkinginto the bottom. A removable net, frame, andscoop could be attached to the center of thephoto-trawl allowing shrimp samples to betaken. The net (45.4 by 28.8 cm) was 1.6 mlong, with l-mm bar mesh. Its cod end ter­minated in a removable bucket. When towed,the scoop preceded the net mouth and pushedthe shrimp off the bottom, directing them intothe net.

A bracket was mounted over the net mouth tohold a Farallon Oceanic Model 2001 Strobe. ACanon Fl camera, with a 35-mm lens and auto­winder, in a waterproof housing, was sus­pended in front of the trawl. With the 35-mmlens (0.083 m2 bottom exposure), shrimp largerthan 0.8 cm were clearly visible. A 24-mm lens(0.204 m2 bottom exposure), was tested and re­jected because of difficulty in shrimp image res­olution. Camera speed was set at 1/60 secondat f-ll with the film plane 56 cm off bottom. Aremote shutter cable 75 m long activated thecamera and enabled an entire role of film to beexposed without servicing the camera. Koda­chrome 64 ASA slide film and Kodacolor II, 100

Figure 3-1-Bottom sled trawl as used in the Twin Lakes study with both camera equipment and removable net-scoopassembly attached.

19

ASA print film were used. Infrared slide film(IE-135-20l was tested and rejected for failingto increase resolution.

A General Oceanics Inc. model 2030 digitalflowmeter, in conjunction with a stopwatch,measured boat speed and distance trawled. Ac­curacy of the flowmeter was tested by pulling italong a submerged line stretched between twoposts. The posts were 29.27 m apart and in 1 mof water. Twelve replicates of this test wereconducted. The distance as measured by theflowmeter was then compared to the actualknown distance.

Depths were recorded at the beginning and endof trawling using a Lowerance depthfinder. Alltrawls were conducted between 8 a.m. andnoon.

An outboard-powered boat was used to tow thephoto-trawl by a 75-m-long, 2.4-mm-dia. steelcable. Trawls lasted 2 minutes, with 9 to 11 pic­tures being taken at 12-second intervals. Whenshrimp samples were needed, the net-scoopassembly was attached to the trawl. In this man­ner the number of netted shrimp could be com­pared to photographic density estimates. Themajority of trawls were conducted in the north­west corner of the lower lake in the vicinity ofthe powerplant tailrace.

To evaluate past trawling methods, a standard­ized procedure of using a houseboat equippedwith power winch and 91 m of polyethylenetowrope (9.5 mm diameter) was used to tow thetrawl. Trawling speeds ranged from 0.5 to0.7 mls and were measured by timing threepaper floats as they passed the length of theboat and by using the flowmeter. Thus, the floatpassage method could be evaluated.

All photodensity estimates (195) taken in 1981were plotted on a frequency graph. Sterling'sapproximation was then used to calculate cor­responding probability values of a negativebinomial distribution. The two curves were com­pared for agreement using the Chi-square test. Ifthe two curves were found to agree, experi­mental data would be shown to approximate anegative binomial distribution, which is indica­tive of a clumped distribution pattern for theshrimp population.

20

RESULTS

Speed and Distance Measuring

The digital flowmeter was found to be very ac­curate. Its 95 percent confidence interval for thedistance measured was found to be ± 4 percentof the actual distance (fig. 3-2). Float passage,however, was considerably less accurate. Bycomparing float passage distance measure­ments to flowmeter results (fig. 3-3) its 95 per­cent confidence interval was found to be ± 14percent. Also, under windy conditions, floatpassage error increased to as much as 16 per­cent; whereas, flowmeter results remained un­changed.

Off-Bottom Trawling

Photographs showed that the sled trawl, using91 m of polyethelene towrope, frequently liftedoff the lake bottom. Of 10 standardized "tradi­tional" trawls, one remained on the bottom(0.48 mls trawling speed), and five were offbottom over 50 percent of the time (at speedsgreater than 0.6 m/s) (fig. 3-4). Water depthalso influenced the trawl's performance. Twotrawls conducted at the center of the upper lake,where the water depth is several meters deeperthan other areas trawled, remained off bottom100 percent of the time at 0.61 and 0.62 m/s.

Net Trawl Efficiency

By taking photographs and simultaneously net­ting shrimp on trawl runs, it was possible to cal­culate a conversion factor between the twomethods. Only trawls which stayed on the bot­tom more than 70 percent of the time were usedto calculate this factor. Trawl net samples werecorrected for the amount of time the trawl wasoff bottom. The net was found to capture only42 percent of the shrimp seen in the photo­graphs. Thus, at least 2.4 times more shrimpwere present than would be indicated· by thetrawl estimates (fig. 3-5). This is considered aminimum number since the photographic esti­mates increased at higher speeds (fig. 3-6). Ap­parently, the scoop which precedes the net wasscaring shrimp off the bottom (possibly by creat­ing a pressure wave) and thus out of the areaphotographed. At higher speeds, the shrimp hadless time to react and therefore, more appearedin the photographs.

40

35

30

E25

il>ucC1)+'<J1

0 20

• • Actual--------~----~-~----~-~------;----Distance. .

152 3 4 5 6 7 8 9 10 11 12

Figure 3-2-Results of General Oceanic's Digital Flowmeter testing.

w110U • •Z • •

~ •(/) • •0 • •

100DISTANCE

0:: • BYw • FLOWMETERf- • •W • •2 •~ •0 90 •-l •I..L

f- •ZWu 800::Wa..

1 2 3 4 5 6 7 8 9 1011 12 13 14 1516 17181920TRAWL NUMBER

Figure 3-3-Results of testing the float passage method of measuring boat speed against flowmeter results.

21

100+-0----- "1-

2: "- ",90 '\0 "- ...f- '\ \f- 80 \e ...,0 '\£I)

70 d-~z Explanation "-0 60 \

Depth trawled. Weight \0w Symbol meters Lake on trawl '\2: 50

~\of- + 22.9-24.7 lower 11.4 kg * \

40 X 14.6-20.1 lower 11.4 kg*LL0 30 0 18.3-20.4 lower 6.8 kgf- 24.7 lower 6.8 kgz 20w 20.1-22.9 lower 6.8 kgu

25.6 6.8 kga:: 10 upperw0- *Extra eight added to sled

0.1 02 03

TRAWL

0.4 05 0.6

SPEED (m / s)

07 08

Figure 3-4.Trawl speed and its relation to the amount of time trawl was on bottom. All trawls used9/m of polypropylene towrope (as used in the past) .

• •

AVERAGETRAWLESTIMATE

AVERAGE- PHOTOGRAPHIC

ESTIMATE

o

•

o

•

o

•

o

••

o

•

o

•

o

•

o

•

o

o

o

•

o-------------------------

200

C\Ja::wf-WL

a:: 100wQ..

Vl-Vl>-L

2 3 4 6TRAWL

8 9 10NUMBER

11 13 14 15

Figure 3-5-Photographic shrimp density estimates and simultaneously made net trawl density estimates.

22

250c/

//

Na::: 200wf-W2:

•........ 0 ./....... ..--....--

...- ,'--...-""- ...- __~ c_....... ...- __ ----0 .'0"'- e_- /

a:::w0-

(f)

(f)

>­2:

150

100

Legend

----- - 35 mm lens- - - - - 28 mm lens

o Weather station 3+ West lower lakeX Weather station 3o North Bay

Weather station 3~ Weather station 4

____ 6

L>.-

10/17/8010/29/8010/29/8011/17/8011/17/8011/17/80

01 02 03

TRAWL

0.4

SPEED05

(m /5)

06 0.7

Figure 3-6. Trawl speed and estimated shrimp density relationship.

Shrimp Density Estimates

Figure 3-7 - Frequency histogram of 195 photographs takenin 1981. Dots indicate expected frequency based onSterling's approximation to a negative binomial.

20

In 1980, shrimp density estimates in front of thepowerplant (15 to 20 m water depth) were 157shrimp/m2 on October 17, and 199 shrimp/m2

on November 17. On June 4, 1981, densitieswere estimated at 89 shrimp/m2. Densities in­creased to 112 shrimp/m2 on June 10, 1981,and then to 232 shrimp/m2 on June 20, 1981.By July 10, 1981, the density of shrimp de­creased to 142 shrimp/m2.

Distribution

Analysis of 195 photographs taken in 1981suggests that the shrimp exhibit a negative bi­nomial distribution (fig. 3-7). A Chi-square test(x 2 =5.36 < x20.95(4)=9.49) indicated that theobserved distribution was not significantly dif­ferent from that of a negative binomial, whichwould mean that the shrimp are distributed in aclumped pattern.

23

r-60

"/ \

/ \50 1,-

.\\

40

30 I

-4­I

20 II

10

10

\\\~\

r---\-

30Shr inr/photogra ph

40 50 60

DISCUSSION

Problems with Past Trawling Methods

With net trawling, an accurate measurement ofdistance is essential since any errors in thelength of trawl would be expressed as an error inMysis density. The float passage method, asused at Twin Lakes before 1980, with its14 percent error should be discontinued. Use ofa digital flowmeter not only greatly increased ac­curacy (4 percent error), but also saved con­siderably on manpower and sampling time.

The biggest problem with previous trawlingmethods was that the sled trawl frequently liftedoff the bottom. The bouyancy and large diam­eter of polyethylene towrope appeared to pullthe trawl off bottom at the start of trawling, giv­ing extremely variable density estimates. In deepwater, such as in the central region of the upperlake, lifting off bottom became more severe.Density estimates made in this region would beexpected to be more erratic and greatly under­estimate the actual population. Variability ofthese past estimates had been attributed to theclumped distribution of the shrimp. It appears,though, that this variability was more closely re­lated to poor sampling technique. Thus, pre­vious estimates of shrimp density using thesmall trawl and polyethylene towrope should beviewed cautiously.

The problem of the trawl lifting off bottom wassolved by using steel tow cable. When this wasdone, all trawls conducted under 0.9 m/s re­mained on the bottom 100 percent of the time.The use of steel cable is essential for deepwatertrawling.

Another serious problem discovered was thatthe net trawl was only 42 percent efficient. Pastestimates assumed the trawl would catch all theshrimp in its pathway. Many however were ac­tively avoiding the net, were run over by thetrawl, or were pushed away from the net mouthby a pressure wave of water. Netted shrimp esti­mates would need to be multiplied by at least2.4 times to more closely approximate the ac­tual population.

24

Shrimp Density Estimates

The shrimp population near the powerplant wasfound to fluctuate from 89 to 232 shrimp/m2 •

These fluctuations are likely due to lake currentsmoving shrimp in or out of the area. Growth ofyoung-of-the-year shrimp (missed in springtimephotographs) would not account for the260-percent increase in shrimp in 16 days.

Should whole lake population estimates ofshrimp be required once a year, the best monthfor sampling would be October. At this time,young-of-the-year shrimp would be largeenough to be readily identified in the photo­graphs, and bottom oxygen levels would notpreclude the presence of shrimp (Keefe, 1980[8] ).

Distribution

Based on an analysis of their frequency distribu­tion, Mysis shrimp in Twin Lakes are distributedin a contagious or clumped pattern on a generalbackground of low density. Successive photo­graphs taken during trawls (generally 10 to11 m apart) show a wide variation in shrimpdensity. By considering the time interval andspeed of travel between bottom photographs, itis estimated that the density clumps are lessthan 10m in diameter. This contention is inagreement with observations made during scubadives. Also, only rarely did a photo show lessthan 60 shrimp/m2

, indicating a backgroundscattering of shrimp between clumps.

CONCLUSION

The photographic bottom trawls have considera­bly changed the thinking regarding shrimp densi­ties. It is now believed that shrimp are at least2.4 times as abundant as previously thought,with densities in the vicinity of the powerplantas high as 232 shrimp/m2

•

The development of the photographic methodhas pointed out deficiencies in methods used inthe past. Information gained in this study willhelp to reduce errors encountered in estimatingshrimp densities and make future estimates con­siderably more reliable and useful when judgingthe impact of the powerplant.

CHAPTER 4. SEDIMENTATION RATES

INTRODUCTION

The sediments in Twin Lakes play an integralpart in the ecology of the lake system, both as anutrient sink and as habitat for lower food chainorganisms. Operation of the Mt. Elbert Pumped­Storage Powerplant, as well as the new dambuilt below the lower lake, could cause changesin sedimentation rates. Three possible methodsexist to measure sedimentation rates. The first isby aging bengthic core samples (Bergersen,1976 [9] ). The second method is to measureincoming and outgoing suspended sediments,and lastly, sediments can be collected in trapsplaced on the lake bottom. The latter twomethods were employed in this study.

METHODS

Sediment Trapping

Sediment trap frames were built of 1.9-cm PVCpipe and attached to sheet metal skids (1 5 by91 cm) to prevent sinking into the soft bottom.Each trap held two 1-L Imhoff cones in a verticalposition (fig. 4-1). A sonic-activated float re­lease device (fig. 4-2) was mounted to the top ofeach trap used during open water periods of1980. Traps deployed during winter were con­nected to a post mounted in the ice with a nylonrope. Three traps set in the spring of 1981 usedonly a float and line for recovery.

Nine sediment traps were placed in the upperlake between July 6, 1980 and July 11, 1981(fig. 4-3). Two traps were lowered through theice and seven were placed during open waterperiods. Twelve sediment traps were placed inthe lower lake between June 23, 1980 and July11, 1981, two traps through the ice, and tenduring open water (fig. 4-4). Periodically, thetraps were pulled, sediments filtered throughtared fiberglass filters, and dried at 104°C.

Processing filters involved drying at 104 ° C for1 hour, cooling in a desiccator, and weighing tothe nearest 0.1 mg. Filters were then incineratedat 550°C for 15 minutes and reweighed(American Public Health Association et aI.,

25

1975 [10] ). This analysis gave the total depos­ited sediment as well as an estimate of organicand inorganic fractions.

Suspended Sediment Sampling

At appr()ximately 3-week intervals (2-week in­tervals during runoff) between August 7, 1980and July 21, 1981, suspended sediments weresampled in Lake Creek above the upper lake, inthe channel between the lakes, and in LakeCreek below the lower lake. Six 400-mL waterbottles were filled at each station using a sus­pended sediment hand sampler (US DH model48). Two or three 800- to 1000-mL replicatesfrom each station were filtered through fiber­glass filters which had previously been dried andweighed. Filters were then redried and stored foranalysis at a later date by the same procedureused on sediment trap filters. To calculate thekilograms of sediment transported per day, theamount of sediment was multiplied by flow ratesobtained from the Colorado Division of WaterResources.

Conversion Factors

For both methods (sediment trapping and sus­pended sediment sampling), a conversion factorwas needed to calculate volume of wet sedimentfrom weight of dried filtered residue. To obtainthis factor, ten 17.6-mL samples of lake ben­thos were dried at 104 ° C to a constant weight(10 hours' drying time) and weighed to thenearest tenth of a milligram. Five samples werethen analyzed for organic and inorganic fractionsby incinerating them at 550°C until a constantweight was achieved (50 min).

RESULTS

Conversion Factors

By drying samples of lake benthos, it was foundthat 1 g of dry filtered residue equaled 3.86 mLof wet sediment. Composition of the lake ben­thos was found to be 92 percent inorganic and8 percent organic.

Float and line spool

PVC supports

Sonic release device

l-liter Imhoff flask

I

-C==:;;;:::;;;::;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;=~==;;;;;;;;;;;::=:=====::::L~ Sh eet me ta 1 base

Figure 4-1 - Schematic of sediment trap used in Twin Lakes.

C 106Y1 1N4001

105M

330 k

+ Float

Line spool

Flashbul b

Transducer

Circuit board

l8-kg monofilament line

Release device housing

Vent plug

< Device to be recovered

Figure 4-2 - Sonic-activated float release device used to retrieve sediment traps.

Sediment Trapping the time they were set, are shown in figures 4-3and 4-4.

Four traps were recovered from the lower lakeand one trap from the upper lake during openwater in 1980. One sediment trap set duringopen water in 1981 was recovered from the up­per lake and all four traps set under the ice wererecovered from the upper lake and all four trapsset under the ice were recovered. Positions ofthe traps, dates set, and sedimentation rates for

Three of the traps in the upper lake were pulledat different times of the year. Sedimentationrates were thus calculated for summer, winter,and during spring runoff. Between these timesof the year, the sedimentation rate was esti­mated by interpolating between known datapoints. The incoming suspended sediment load

26

J

4.

500 M

SCALE

Figure 4-3. Location (dots) of nine sediment traps placed in the upper lake. Sedimentation rate for dates given and inorganic andorganic fractions are listed for those traps recovered.

Explanation

Trap Number Result

1/28/81 - 4/20/81, 1.15 mm/a, 89% inorganic; 11 % organic

2 9/3/80 - 11/26/80, 1.68 mm/a, 83% inorganic; 17% organic3 5/23/81 - 7/11/81, 9.36 mm/a, 89% inorganic; 11% organic4 1/29/80 - 4/20/81, 0.183 mm/a, 63% inorganic; 37% organic

was used in the interpolation to derive the pro­portion of change over time (fig. 4-5). Byweighting for time, an annual sedimentation rateof 2.2 mm/a was estimated for the upper lake.

No sedimentation rate was calculated for thelower lake since sediment traps set during springrunoff were not recovered. Summer, fall, andwinter rates, as well as sediment outfall near thesouth shore road, however, were estimated (fig.4-4).

Suspended Sediment Sampling

Using the inorganic sediment figures (table 4-1),235 220 kg/a entered the upper lake and

27

177 544 kg/a left the lake. Thus, 57 676 kg ofinorganic material were deposited. This resultsin a sedimentation rate of 0.084 mm/a, basedon the entire lake surface area. Analysis of thefive 17.6-mL samples of bottom materialshowed bottom composition to be 92 percentinorganic and 8 percent organic. Thus, an addi­tional 8 percent should be added to the sedimen­tation rate for the organic fraction bringing thetotal sedimentation rate for the upper lake to0.091 mm/a.

The amount of sediment being transported tothe lakes was highest during the spring runoff of1981 (late May to early July). Lowest levels ofsediment transport occurred during the latewinter months of February and March (fig. 4-6).

•

N

f

..8

•

•

500 MSCALE

"IIII---------

Figure 4-4. Location (dots) of twelve sediment traps placed in the lower lake. Sedimentation rate for dates given and inorganicand organic fractions are listed for those traps recovered.

Explanation

Trap Number Result

5

6789

10

1/28/81 - 4/20/81, 2.96 mm/a, 80% inorganic; 20% organic

9/3/80 - 8/26/80, 4.01 mm/a, 87% inorganic; 13% organic1/29/81 - 4/20/80,0.28 mm/a, 81% inorganic; 19% organic7/1/80 - 10/9/80,3.7 mm/a, 88% inorganic; 12% organic7/6/80 - 8/26/80, 33.83 mm/a, 96% inorganic; 4% organic7/11/80 - 8/13/80, 3.06 mm/a, 88% inorganic; 12% organic

Table 4-1 Suspended sediments passing through Twin Lakes and respective organic and inorganicfractions

Lake Creekabove

upper lake

Channelbetween

lakes

Lake Creekbelow

lower lakekg/a

Organic fractionInorganic fractionTotal suspended sediment

90677235220325897

110565177 544288 109 726949

28

10

Weighted average sedimentation rate

Actua 1 sed imenta tion ra te by sediment trappi ng

8

i.. 6..,~

'"o..,..~ 4.~

1\In

2

Sedimentation rate

(~'

1.15

6.62

2101980

310

Time (Days)

441981

144

Figure 4-5-Sedimentation rates in the upper lake based on sediment trapping.

5000

4000

~ 3000E:;;:::"'""~ 2000~

"'"

1000

10782

7701 i9491

"' f:' 7143 II \ i~\ iI ';il\ i

• ''\·1 .I ".'l'i Ii I idi i\ lake Creek above upper lake ~1 : if 'i ii Channel between the lakes /'1., : i \ i ji ; 'f l\ i., lake Creek below lower lake i \ : 'i ; I. Ii ' i "; r,I''. iii !!\i {\ iil!j 1\11

"ii' ,I, .. ",i' :it IIii

\ \ i '. :/I iI,\ 'i i \ iii ; ~,iii :1i ! \

. i \ i i f't; i ..I

,. i I .'

\<~\.~\\ ! \,!I \, >\ i , """,{ \ ' \~ i " \-'i I"·i'a.. __ ._ l,' /'- \:/ .

.~ /--', /' ... \If~·;;·~;'::::''::..~=''~~l;:~~i='~-';;;-.ww::klo~~..::.~-:~/ .

2101980

310 441981

Time (Days)

144

Figure 4-6-Total suspended sediment budget in Twin Lakes, Colorado.

29

DISCUSSION

Sediment Trapping

One sediment trap was set near the south shoreof the lower lake. A road was built along thisshore in the spring of 1980 to allow access tothe Interlaken Hotel. Results showed an order­of-magnitude increase in sedimentation rate atthis time, with 96 percent being inorganic incomposition. This clearly was a direct result ofthe new road construction. The shoreline con­sisted of broken rock and cobblestone, suitablesubstrate for lake trout spawning. Visual exam­ination of nearshore littoral substrate in this areashowed most rock crevices necessary forspawning to be filled with silt. Since this sedi­ment trap was at a depth of 5 m, sediments gen­erated by the road construction were apparentlycarried to a considerable depth. The horizontalextent of these construction-caused sedimentsis unknown. Of the ripe lake trout netted byWalch (1979) [3], 33 percent were taken alongthis shoreline. Thus, construction activity asso­ciated with this road appears to have adverselyaffected, if not completely destroyed, a sub­stantial amount of lake trout spawning habitat.

Bergersen (1976) [9], using sediment cores,estimated the annual sedimentation rate in TwinLakes as 0.63 mm/a, with a possible range of0.42 to 0.83 mm/a. Results of sediment trap­ping in 1980-81 indicated that considerablyhigher rates may occur. Annual winter rateswere as low as 0.2 mm/a in the upper lake to0.3 mm/a in the lower lake. Spring and summerrates, however, were as high as 66.6 mm/a inthe upper lake during runoff to a more typicalvalue of 3.7 mm/a in the lower lake during sum­mer. A yearly sedimentation rate (fig. 4-5) of2.2 mm/a was estimated for the upper lake.This is considerably higher (3.5 times) than Ber­gersen's estimate. He did note however thatsedimentation rates near the Lake Creek Inletmight be as much as 2~ to 3 times greater thanelsewhere in the lake, which would be partially:onsistent with present estimates. It appearsthat the traps in this study were catching notmly sediments falling to the bottom for the firstime, but also bottom material which had be­:ome resuspended. This seems all the morekely when the ultrafine nature of the glaciallour bottom is considered. It has been reportedhat divers swimming 1 to 2 m above the bottom

30

would disturb the glacial flour, which would re­main suspended for several days (LaBounty,1976 [11] ). It is also believed bottom currentswere sufficiently strong to cause some resus­pension, leading to a higher sedimentation rate.

Resuspension of bottom sediments appears tooccur throughout the year and not just duringperiods of turnover. This becomes apparent bycomparing the trapping sedimentation rate tothe suspended sediment sampling sedimenta­tion rate (fig. 4-7). Traps set at various times ofthe year caught 10 to 150 times what would beexpected from suspended sediment sampling.Sartoris et al. (1977) [12] reported an occa­sional drop in transmissivity near the lake bot­tom and attributed this in part to the presence ofcold, sediment-laden water of Lake Creek plung­ing to the bottom. This would tend to lend fur­ther credibility to the supposition that near­bottom movements of sediments do occur. It isalso possible that the traps themselves tendedto collect a disproportionate amount of any hori­zontally moving sediments, which would resultin the higher rates recorded with this method.

Background sedimentation rates and probablesources have been established for comparativepurposes. The best time to mOQitor powerplantimpacts on sediment movement and depositionwill probably be during the winter months. Atthat time, background sedimentation rates arelow and fairly constant at 0.3 mm/a. Theorganic fraction is constant at 19 to 20 percent,lake current velocities are at their lowest levels,and vandalism or accidental disruption of sampl­ing devices is not likely.

Suspended Sediment Sampling

Suspended sediments must be sampled for ayear before a sedimentation rate can be esti­mated. To illustrate this point, consider themonth of August 1980, when a negative sedi­mentation rate occurred (more sediment leavingeach lake than entering). This phenomenon maybe explained if the retention time of each lake isconsidered. In the fall, Lake Creek above the up­per lake cleared, while the upper lake still carrieda considerable sediment load and the lower lakehad an even higher sediment load. Gradually, thesediments worked through the lakes and clearerwater prevailed. Sampling over this time periodwould be meaningless without the rest of theyear's data.

10

8

'"......!'" 6.....'"'-c:0

.....'".....c:

'" 4.~'0

'"'"

2

r--l = Suspended sediments

n = Trapped sediments

6.62

1.68

1.15

1 I____-'-91-__

------ ·9_2S3 __ - .0077

2101980

310 441981

Time (Days)

144

Figure 4-7 - Comparison between sedimentation rates in the upper lake as estimated by sediment trapping and suspendedsediment sampling methods.

Also, it was interesting to note that the upperlake "trapped" 24.5 percent of the incoming in­organic suspended sediments. Organic material,however, was produced in the upper lake (22percent more left the upper lake than hadentered).

The sedimentation rate estimated by suspendedsediment sampling (0.091 mm/a) was only

31

one-seventh of the rate reported by Bergersen(1976) [9]. Several factors could account forthe present lower estimation. Current estimateswere based on the entire surface area of the lakeand did not allow for concentration of thesediments at deeper strata, which undoubtedlydoes occur. Also, the winter of 1980-81 hadbelow-average snowfall, resulting in decreasedspring runoff. These two factors could explainthe difference between the two estimates.

CHAPTER 5. ORGANIC CONTRIBUTION OF VEGETATIONINUNDATED BY NEW LAKE LEVELS

METHODS

INTRODUCTION

The cover dominance of each species was calcu­lated as follows:

• ',.r'-·-TWINU /LAKES. .. .

Known areas of each vegetation type were cut,weighed, dried at 103 0 C for 4 hours, andreweighed. Cover dominance of each species(divided by 100) was then multiplied by the totalarea sampled, its dry weight per square meter,and its respective caloric content (Cummins andWuycheck 1971 [14] ).

Figure 5-1. -Study area west of the upper lake which will beunundated by new lake levels. Dots indicate 20 randomsampling positions.

total intercept lengthspecies A

------- x 100%total transect length

Vegetated land areas will be inundated whenwater is backed up behind the new dam builtbelow the lower lake. The energy content of thisvegetation will become part of the lakes' energybudget and the potential for short-term stimula­tion of lake productivity exists. The objective ofthis investigation was to estimate the caloriccontent of this vegetation and to examine possi­ble consequences of its inundation on TwinLakes.

Cover dominance

The line intercept method (Smith, 1979 [13] )was used to sample vegetation below the pro­posed high-water line of the "new" lake. Twen­ty random stations were chosen between theelevations of 2799 and 2805 m (fig. 5-1). Ateach station, a 30-m tape was stretched to thesouth. Each plant species and the distance itcovered along the tape was recorded.

Table 5-1.-Cover dominance, weight, and caloric content of the major vegetation types found inthe study area above the upper lake.

Vegetationtype

Coverdominance

(%)

Wetweight

(kg)Calories

Mixed old field grass (Graminae)Willows (Salicaceae)Alpine sedges (Cyperaceae)Brushy cinquefoil (Rosaceae)Sagebrush (Carduaceae)Alders, birches (Betulaceae)Rushes (Juncaceae)Pine (Pinaceae)

Totals

44.237.910.83.21.91.80.40.1

100.3

2475033358588

8340081 59364603

159 5111 5949 116

4005908

5.734 X 10"9.5389 X 1012

1.6099 x 10"2.1810 X 10"1.9004 x 10"4.7202 x 10"3.6643 X 109

2.7371 X 10'0

1.1186 x 10'3

33

RESULTS

Cover dominance of the major vegetation types,their wet weight, and total calories in the soon­to-be inundated areas are shown in table 5-1.The major contribution of organic material willcome in the form of willows. The total caloriccontent of the inundated vegetation (4000metric tons) was estimated at 11.2 x 109 kilo­calories (64.9 x 109 kJ)

DISCUSSION

By using the 1977 to 1981 average for station2 of 6497.3 micrograms of carbon per squaremeter per hour fixed by plankton2 and expandingthis number, it was possible to estimate the an­nuallower lake primary productivity as 2.095 x108 g Cia. (This is equal to 78 mg C/(m2 'd),which is within Wetzel's (1975) [15] oligo­trophic lake range of 50 to 300 mg C/(m 2 ·d).)An estimate was made by Sartoris2 for the upperlake using an average plankton carbon fixationrate of 2025 ~g/(m2'h), which resulted in aprimary productivity estimate of 2.336 x107 g Cia. In total, 2.3295 x 108 g Cia arefixed in the Twin Lakes system. Grams of carbonwere then converted to kcal by the followingequations:

1 9 C = 3.205 9 O2 (Kajak, et a/., 1972 [16J )1 9 O2 = 3600 cal (Odum and Smalley, 1959 [17J )

:.1 9 C = 11.538 kcal

Rabinowitch and Govindjee (1969 [18] statethat all carbohydrates have approximately thesame energy content of about 112 kcal/g atom(12 g) of carbon contained in them. This resultsin a conversion factor of 9.33 kcal/g C, whichis slightly less but similar in magnitude to the11.538 figure given above. To accommodatethe differences in these conversion factors, bothare discussed. Thus, the grams of carbon fixedannually by plankton in both lakes is equal to2.173 x 109 kcal or 2.689 x 109 kcal, depend­ing on the conversion factor used. Potential in-

2J. J. Sartoris, Bureau of Reclamation, Division ofResearch, Denver, Colo., personal communication.

34

put from terrestrial vegetation is therefore equalto 4.2 or 5.1 times the average annual planktonprimary production, again depending on the con­version factor. Regardless of the factor used, itis evident that the potential exists for an in­crease in energy in the system when lake levelsrise. Although these estimates are admittedlycrude, they do reflect a very likely future energyscenario for the Twin Lakes system.

Not all the calories from terrestrial vegetationwill be available the first year. Temperature, ageof plant parts, flushing rate, and other factorswill undoubtedly play a role in this process.Woody plant parts may take more than severalyears to decompose. A large initial input,however, would be expected as easily decom­posable material (such as grasses, sedges,rushes, and leaves) breaks down.

Undoubtedly, changes will occur in the foodchain. Likely, these will be first noted as in­creases in the plankton population or changes intheir species composition. The addition oforganic material will also increase the lake'sbiological oxygen demand, which may becomeparticularly critical toward the end of the firstwinter. As a precaution, water level should bekept down to the 2800 m elevation throughoutthe first several winters. Through the summer,the large influx of organic material would con­tribute to decreased hypolimnetic oxygen. Sucha result could be particularly deleterious to thelake trout, which need cold, oxygenated water.

To minimize harmful effects, the water levelshould be raised half way to the maximum waterlevel the first spring to inundate and kill much ofthe vegetation. A summer drawdown wouldthen expose most of the killed vegetation, allow­ing it to decompose on dry ground. The secondspring, water could be raised to full pool, whichwould kill the remaining vegetation; but asummer-through-winter drawdown would stillbe advisable. Even with the above precautions inlake filling, nutrients will still be taken up by theplankton, recycled throughout the summer, andcontribute to under-ice anoxia.

BIBLIOGRAPHY

[1] Gregg, R.E., and E.P. Bergersen, "Mysisrelicta: Effects of Turbidity and Turbulence onShort Term Survival," Trans., Am. Fish. Soc.,No. 109, pp. 207-212,1980.

[2] Carruthers, J. N., "A Leaning-Tube CurrentIndicator ('Pisa'}," Bull. Inst. Oceanogr., No.1126 (Sept. 15, 1958), Monaco, 1958.

[3] Walch, L. A., "Movements of 'Lake Trout inTwin Lakes, Colorado, in Relation to a Pumped­Storage Powerplant," M.S. thesis, Colo. StateUniv., Fort Collins, 129 pp., 1979.

[4] Fultz, D., " Experimental Analogies to At­mospheric Motions," In: Compendium ofMeteorology, T. F. Malone (ed.), Am. Meteorol.Soc., Boston, pp. 1235-1248, 1951.

[5] __, "Experimental Models of RotatingFluids and Possible Avenues for FutureResearch." In: Dynamics of Climate, R. L. Pfef­fer (ed.), Pergamon Press, N.Y., pp. 71-77,1960.

[6] Likens, G. E., and R. A. Ragotzkie, "RotaryCirculation of Water in an Ice-covered Lake,"Verh. Int. Ver. Limnol., No. 16, pp. 126-133,1966.

[7] Verber, J. L., "Initial Current Studies in LakeMichigan," Limnol. Oceanogr., No.9, pp.426-430, 1964.

[8] Keefe, T. J., "Statistical Review of the Lim­nological Data Collected at Twin Lakes, Col­orado, 1974-1979," EnviroStat Associates,Fort Collins, Colo., 1980.

[9] Bergersen, E. P., "Aging and Evaluation ofTwin Lakes Sediments." In: Studies of the Ben­thic Environment of Twin Lakes, Colorado, J. F.LaBounty (ed.), Bur. Reclam. Rep. REC­ERC-76-12, 1976.

35

[10] American Public Health Association,American Waterworks Association, and WaterPollution Control Federation, "StandardMethods for the Examination of Water andWastewater," 14th ed., APHA, Washington,D.C., 1193 pp., 1975.

[11] LaBounty, J. F., and J. J. Sartoris, "TheProfundal Benthic Environment of Twin Lakes,Colorado." In: Studies of the Benthic Environ­ment of Twin Lakes, Colorado, J. F. LaBounty(ed.), Bur. Reclam. Rep. REC-ERC-76-12,1976.

[12] Sartoris, J. J., J. F. LaBounty, and H. D.Newkirk, "Historical, Physical, and ChemicalLimnology of Twin Lakes, Colorado," Bur.Reclam. Rep. REC-ERC-77-13, 1977.

[13] Smith, R. L., Ecology and Field Biology, 2nded., Harper and Row, N. Y., 850 pp., 1979.

[14] Cummins, K. W., and J. C. Wuycheck,"Caloric Equivalents for Investigations inEcological Energetics," Int. Ver. Theor. Angew.Limnol., No. 18, pp. 1-151, 1971.

[15] Wetzel, R. G., Limnology, W. B. SaundersCo., Philadelphia, Pa., 743 pp., 1975.

[16] Kajak, A., AI Hillbricht-lIIkowska, and E.Pieczynska, "The Production Process in SeveralPolish Lakes." In: A Manual on Methods forMeasuring Primary Production in Aquatic En­vironments, R. A. Volenweider (ed.), BlackwellScientific Publications, London, p. 141, 1972.

[17] Odum, E. P., and A. E. Smalley, "Com­parison of Population Energy Flow of an Her­bivorous and Deposit-feeding Invertebrate in aSalt Marsh Ecosystem," Proc., Nat. Acad. Sci.,India, vol. 45, pp. 617-622, 1959.

[18] Rabinowitch, E., and Govindjee, Photosyn­thesis, John Wiley and Sons, Inc., N. Y., 273pp., 1969.

APPENDIX

Surface, bottom, and under-ice current maps forTwin Lakes, Colo., including corresponding winddata. Surface and bottom current velocities aregiven in cm/s. Under-ice current velocities are

given in em/min. Where measured, averagewindspeeds (m/s) are shown in the center circleof each wind rose. Arrows indicate direction ofcurrent.

500 M

SCALE

~O~5

---.-!:!..7.'

. 8~·~.1 ~ ./-::-:j?o~ 20

:0- 0 o~82Yo ~

°o~2}0 O~.83.0/ 4.0/ o__o~

~.o/· 3.1 8.~ -)2.'j") o________

~o...£ tl.2 o~ 11.0

6.7 \ 8.0&.4 4.2

~I. 0

• . 7.3 .~; 1.9'.A.4

4.'J

Figure A-1.-Surface-the upper lake-Aug. 11, 1980.

N

1

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-2.-Windspeed and direction-Aug. 11. 1980.

37

500 M

SCALE

4.6-.

• !US 2.5---.~/.-O

--.....-.1.62.2

7·21

Y·o42--.~.....-- . ~.

.~.----J

Figure A-3. -Surface-the upper lake-Sept. 4, 1980.

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-4. - Windspeed and direction -Sept. 4, 1980.

38

"II

\:~

.~.45.8 ____..~

.~ .~.~~7.~.

500 MSCALE

N

f

~..~

.~.

Figure A-5. -Surface-the lower lake-June 20, 1980.

EARLY AM

N

r

LATE AM

EARLY PM LATE PM

Figure A-6.-Windspeed and direction-June 20, 1980.

39

N

f~2.1 •./

......... • S I

2.1./3.8 ~

10.8.~ •

• 3.0 3.2 • a·~

.~~~.~~':-o.!.~2~'4\ 1.0 1.0

4.4--...... • $.11

~: ---..

500 M

SCALE

"II"----------

Figure A-7.-Surface-the lower lake-July 22, 1980.

N

i

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-8.-Windspeed and direction-July 22, 1980.

40

N

f

500 MSCALE

~4

12 ~.~

\~:c~·~~~. .~>·~~.2~37\ ""-'.:.~ '\ '{A

..~.~ .• jZI. \,.,

3.3 3.1. 29 \___·-~-""'4·~.

""II---------

Figure A-9.-Surface-the lower lake.-Aug. 13, 1980.

N

1

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-10.-Windspeed and direction-Aug. 13,1980.

41

",.,.--.-----------\ 1,7

/<200~4 /0O/~'

F0.9--'1.~4.0

~3

;°11.8

2.9 •

-"1.4-\2.4

'.6

500 MSCALE

.~ 4.10--0

.~.

N

f

Figure A-11. -Surface-the lower lake. -Sept. 8, 1980.

EARLY AM

N

I

LATE AM

EARLY PM LATE PM

Figure A-12.-Windspeed and direction-Sept. 8,1980.

42

""11.__- __----__

500 MSCALE

N

f

Figure A-13.-Surface-the lower lake-Oct. 24,1980.

N

t

EARLY AM

EARLY PM

LATE AM

LATE PM

Figure A-14.-Windspeed and direction-Oct. 24, 1980.

43

/.0 e( 3.0

~2...--• <3.0

'.0 \.0--...................

• < 3.0

J /.0 500 M

SCALE

Figure A-15.-Bottom-the upper lake-June 26,1980.

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-16.-Windspeed and direction-June 25,1980.

44

EARLY AM LATE AM

Figure A-17.-Windspeed and direction-June 26. 1980.

500 M

SCALE

-< 3.0

-< 3.0

.-<.

/.6

-< 3.0

/ •.5

J

Figure A-18.-Bottom-the upper lake.-July 24. 1980.

45

EARLY AM

EARLY PM

N

1~

LATE AM

LATE PM

Figure A-19.-Windspeed and direction-July 23,1980.

N

1

EARLY AM LATE AM

Figure A-20.-Windspeed and direction-July 24, 1980.

46

I"~

/3.2

• <3.0

-< 3.0

/.6 -< 3.0

..y

500 M

SCALE

Figure A-21.-Bottom-the upper lake.-Aug. 15, 1980.

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-22.-Windspeed and direction-Aug. 14, 1980.

47

• {3.0

.{ 3.0

Figure A-23.-Bottom~the upper lake.-Sept. 11, 1980.

• {3.0

.{ 3.0

.{3.0

13.8 I'· 1. '\'0

~.{ 3.0

...·{3.0 ---',1----.

.{ 3.0

Figure A-24.-Bottom-the upper lake-Nov. 19, 1980.

48

"IIII----------

• <3.0

/..• ( 3.0

/.2

/'07.0---.( 3.0

500 MSCALE

-< 3.0

N

f

Figure A-25.-Bottom-the lower lake-June 25,1980.

EARLY AM LATE AM

EARLY PM LATE PM

Figure A-26.-Windspeed and direction-June 24, 1980.

49

IIII NII

f~4 ...~

/3. -< 3.0

•<3.0 ./-..( 3.0

-< 3.0

-< 3.0

V· ·(3.0/0.•

.< 3.0

/3. • ( 3.0 • <3.0

-< 3.0 -<3.0-< 3.0

-< 3.0 /...-< 3.0

500 M -< 3.0 --...:.SCALE

Figure A-27.-Bottom-the lower lake-July, 28, 1980.

N

r

EARLY AM LATE AM

LATE PM

Figure A-28.-Windspeed and direction-July 27, 1980. ..50

N

1

EARLY AM LATE AM

Figure A-29.-Windspeed and direction-July 28, 1980.

~o

.,IIII, -----_

500 MSCALE

N

f.( 3.0

/46

\4'

186 \;.6

Figure A-30.-Bottom-the lower lake-Sept. 23, 1980.

51

N

r

EARLY AM LATE AM

Figure A-31.-Windspeed and direction-Sept. 23, 1980.

EARLY AM

N

i

LATE AM

EARLY PM LATE PM

Figure A-32.-Windspeed and direction-Aug. 24,1980.

52

EARLY AM LATE AM

Figure A-33. - Windspeed and direction-Aug. 21, 1980.

EARLY EV:::N I N G

EARLY PRE-DAWN

LATE EVENI NG

LATE PRE-DAWN

Figure A-34.-Windspeed and direction-Oct. 21-22, 1980.

53

EARLY AM

EARLY PM

N

i

LATE AM

LATE PM

Figure A-35.-Windspeed and direction-Oct. 22, 1980.

EARLY AM

EARLY PM

LATE AM

LATE PM

Figure A-36.-Windspeed and direction-Oct. 27,1980.

54

EARLY EVENI NG

EARLY PRE-DAWN

LATE EVENING

LATE PRE- DAWN

Figure A-37.-Windspeed and direction-Oct. 27-28, 1980.

EARLY AM

EARLY PM

LATE AM

LATE PM

Figure A-38.-Windspeed and direction-Oct. 28, 1980.

55

N

I

1.2

EARLY AM

EARLY PM

LATE AM

LATE PM

Figure A-39.-Windspeed and direction-Noy. 13, 1980.

N

i

EARLY AM

EARLY PM

LATE AM

LATE PM

Figure A-40.-Windspeed and direction-Noy. 14, 1980.

56

Figure A-41.-Under-ice currents-the upper lake-April 17, 1980. Currentvelocities are given in em/min.

Figure A-42.-Under-ice currents-the upper lake.-Jan. 19, 1981. Currentvelocities are given in em/min.

57

17

-.,. '0

~-24

--­0-30

40--­•

~

0-12

20

'..--'3

II II

\!

500 M

"-.,.0-35

SCALE

12-21

----

",."------------

Figure A-43.-Under-ice currents-the lower lake-April 16, 1980. Currentvelocities are given in em/min.

" N"II

f---<'

~o3?'-

~24

0-30 30 30

< --- ---25"""'-'25

~40

~.

-3 -40--......

13

15+15

40""'-40IS

500 MSCALE

Figure A-44.-Under-ice currents-the lower lake-April 8-10, 1980. Currentvelocities are given in em/min.

58

II1\1\----------- N

Figure A-45.-Under-ice currents-the lower lake-Feb. 18, 1981. Currentvelocities are given in em/min.

59

GPO 835 -584