classification of upper mississippi river pools based on contiguous aquatic/geomorphic habitats

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This article was downloaded by: [McMaster University] On: 25 November 2014, At: 15:07 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Freshwater Ecology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tjfe20 Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/ Geomorphic Habitats Todd M. Koel a a Minnesota Department of Natural Resources , Mississippi Monitoring Station , 1801 South Oak Street, Lake City, Minnesota, 55041, USA Published online: 06 Jan 2011. To cite this article: Todd M. Koel (2001) Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats, Journal of Freshwater Ecology, 16:2, 159-170, DOI: 10.1080/02705060.2001.9663801 To link to this article: http://dx.doi.org/10.1080/02705060.2001.9663801 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions

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Page 1: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

This article was downloaded by: [McMaster University]On: 25 November 2014, At: 15:07Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Freshwater EcologyPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/tjfe20

Classification of Upper Mississippi RiverPools Based on Contiguous Aquatic/Geomorphic HabitatsTodd M. Koel aa Minnesota Department of Natural Resources , Mississippi MonitoringStation , 1801 South Oak Street, Lake City, Minnesota, 55041, USAPublished online: 06 Jan 2011.

To cite this article: Todd M. Koel (2001) Classification of Upper Mississippi River Pools Based onContiguous Aquatic/Geomorphic Habitats, Journal of Freshwater Ecology, 16:2, 159-170, DOI:10.1080/02705060.2001.9663801

To link to this article: http://dx.doi.org/10.1080/02705060.2001.9663801

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

Todd M. Koela Minnesota Department of Natural Resources

Mississippi Monitoring Station 1801 South Oak Street

Lake City, Minnesota 5504 1 USA

ABSTRACT Navigation pools of the upper Mississippi River (UMR) vary greatly in terms

of available contiguous aquatic/geomorphic habitats. These habitats are critical for the biotic diversity and overall productivity of the floodplain corridor of each pool.. In this study, sirmlarities among pools 4-26 and an open river reach (river kilometer 47-129) of the UMR were determined from multivariate analysis of eleven habitat types that were hydrologically-contiguous (non-leveed). Isolated floodplain habitats were not included in frnal analyses because this isolation h i t s their contribution to overall riverine productivity, in part due to a lack of hydrologcal connectivity to the main channel during the flood pulse. Cluster analysis based on simple Euclidean distance was used to produce two major pool groups and five pool subgroups. Important habitat variables in defining pool groups, as interpreted from principal components analysis (PCA) axis 1, were contiguous floodplain shallow aquatic area and contiguous impounded area. The habitat variable most important in defining pool subgroups, as interpreted from PCA axis 2, was tertiary channel. Most notably, pool 6 was more similar to pools 14-24 than other upper pools, and pools 19 and 25 were more similar to pools 4-13 than other lower pools. These results were quite different fiom those of two previous investigators, primarily because only areas of non-isolated aquatic habitat were considered.

INTRODUCTION The upper Mississippi Rtver (UMR) includes that portion of the Mississippi

River fiom the confluence of the Ohio River (river kilometer (RKm) 0) upstream to Minneapolis and St. Paul, Minnesota and St. Anthony Falls (RKm 1374). The UMR includes a series of 26 navigation "pools" (the area between two successive locks and dams). In general, these pools are characterized by having an upper, riverine reach that is typical of the pre-dam Mississippi River, and a lower, lacustrine reach that is atypical of the pre-dam river. Only where tributaries created large alluvial fans do natural impoundments occur on the UMR system (Lake Pepin of the Mississippi River, and the Peoria Lakes of the Illinois River).

Each UMR pool is relatively unique in terms of its hydrological r e p e and in terms of the number of aquatic/geomorphic habitat types that it contains ( U . S . Geological Survey 1999). Consequently, effects of river management actions, such as water level drawdown of a pool, may or may not evoke similar responses among UMR pools. For example, vegetation or fish communities in pools with relatively little contiguous shallow aquatic areas may not respond similarly to those

'Present address: National Park Service, Center for Resources, P.O. Box 168, Yellowstone National Park, WY 82 190 [email protected]

Journal of Freshwater Ecology, Volume 16. Number 2 -June 2001

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communities in pools with larger amounts of that habitat type. Analyses that compare pools based on available habitat types will allow for the generation and testing of hypotheses regarding the similarities of biotic response to ecosystem management practices of the UMR. T h s is significant because management of the UMR floodplain comdor is shifhng from a site-specific, single-species approach, to a more comprehensive, ecosystem-based approach involving whole pools. Because of thls change, there is a need for a better understanding of the structural and functional similarity of UMR pools. Two previous investigators have classified UMR pools based on enduring geomorphic characteristics (U.S. Geological Survey 1999, U.S. Army Corps of Engineers 2000a). However, these analyses were constrained because pools were classified into reaches (groups of pools) only in natural, longitudinal order.

I compared the UMR pools based on surface areas of several aquatic/geomorphic habitat types that are considered important for the success of plant and animal communities and the overall aquatic biotic productivity of the floodplain corridor (Bayley 1995, Johnson et al. 1995). Specific objectives were 1) to class* UMR pools based on hydrologically-contiguous habitats; 2) to determine which habitat types are most influential in classifying pool groups; and 3) to compare results to those of other investigators, including the three reaches of the UMR as described by U.S. Geological Survey (1999) and the ten natural geomorphlc reaches as described by U.S. Army Corps of Engheers (2000a).

METHODS AND MATERIALS Surface areas of eleven aquatic/geomorphic habitat types of the UMR

floodplain comdor were obtained from the Habitat Needs Assessment (HNA) query tool (DeHaan et al. 2000; Tables 1 and 2). Habitat surface areas were interpreted from 1: 15,000 scale aerial photographs taken in 1989. Data were avadable for RKm 47-129 (the open river reach) and for RKm 325-1283 (pool 26 - pool 4; there is no pool 23 but there is a pool 5a; Figure 1). For simplicity, the open river reach was referred to as pool 29, even though it is not impounded by a navigation dam. The analysis of UMR navigation pools was based on the aquatic habitat classification system developed by Wilcox (1993), in whlch the habitats of each pool were defined by geomorphic features, constructed features, and other physical and chemical characteristics (Figure 2).

Cluster analysis was used to class* UMR navigation pools with similar contiguous aquatic/geomorphlc habitats. Clustering methodology was an agglomerative, unweighted, average-linkage t e c h q u e using a simple Euclidean distance index. This method provided a balanced approach to creating pool groups (Van Tongeren 1995). Initially, analyses were conducted using all HNA habitat types. However, surface areas of isolated floodplain aquatic areas and of isolated terrestrial floodplain areas were not included in final analyses of pool similarity. This isolation 1irmt.s their contribution to overall riverine productivity, in part due to a lack of hydrological connectivity to the main channel during the spring flood pulse (Junk et al. 1989, Ward et al. 1999).

Because cluster analysis does not provide information regarding the nature of differences between pool clusters, principle components analysis (PCA; Ludwig and Reynolds 1988) was then used to determine the habitat variables that were most influential in classlfylng UMR pools. Habitat surface areas were normalized by

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loglox + I transformation prior to analysis. Following the suggestions of Pielou (1984), the PCA was centered because between-axes heterogeneity was low. The analysis was not standardized because an equal unit of measurement (hectares) was used throughout the habitat matrix. Sigmficance of PCA axes 1-3 was inferred using Fisher's (1958) proportion test. For purposes of defining relationships among pool groups, PCA loadings on axes 1-3 were considered high if their absolute values were >0.5.

To determine if pool groups as defined by cluster analysis and PCA were significantly different fiom one another, the groups were tested statistically using a multi-response permutation procedure (Biondini et al. 1988) with PCA loadings for axes 1-3 used as the response variables for each pool. Cluster analysis and PCA were completed using the computer program MVSP version 3.0 (Kovach 1998). Finally, Kruskal-Wallis ANOVAs were used to determine differences among pool groups based on each aquatic/geornorphlc habitat type (SAS 1997).

Table 1. Aquatidgeomorphic classification descriptions for habitats used to determine similarity of upper Mississippi River navigation pools (Modified with permission from DeHaan et al. 2000).

Aquatic/Geomorphic Area Classification Description Main Navigation Channel Designated navigation comdor area of the main channel

marked by channel buoys.

Main Channel Border Area between the navigation channel and the river bank.

Tailwater

Secondary Channel

Tertiary Channel

Areas downstream of the navigation dams with deep scour holes, high velocity and turbulent flow.

Large channels that cany less flow than the main channel.

Small channels less than 30 meters wide.

Tributary Channel Channels of tributary saeams and rivers.

Contiguous Floodplain Lake Distinct lakes formed by fluvial processes or are manmade.

Contiguous Floodplain Shallow Aquatic Area Portions of floodplain inundated by the navigation dams that are not part of any channels or floodplain lakes.

Contiguous Impounded Area

Terrestrial Island

Large, mostly open water areas located in the downstream sections of the navigation pools.

Terrestrial areas (at reference river discharge) that are not connected to the floodplain.

Contiguous Terrestrial Floodplain Terrestrial floodplain areas (at reference river discharge) that are not protected by flood control levees.

RESULTS Cluster analysis separated navigation pools into two well-defined groups that

included 12 pools each (Figure 3). Group A included predominantly pools classified by U.S. Geological Survey (1999) as lower UMR pools, with the exception of pool 6 , and group B included predominantly pools classified by U.S. Geological Survey (1999) as upper UMR pools, with the exception of pools 19 and 25. The multi-response permutation procedure revealed significant differences between pool groups A and B (P=O. 13 x lo-').

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Page 5: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

Tab

le 2

. S

urk

ce a

rea

(hec

tare

s) o

f aqu

ntic

/gco

mor

phic

hab

itats

as

defin

ed b

y th

e Iln

bita

t Nee

ds A

sses

smen

t for

upp

er M

issi

ssip

pi R

iver

nav

igat

ion

pool

s 4-

26 a

nd th

e op

en r

iver

(29)

. G

roup

s (A

and

B) a

nd s

ub-g

roup

s (A

I-U

2)

are

resu

lts o

f clu

ster

ana

lyse

s ba

sed

on c

ontig

uous

hab

itats

. H

abita

t typ

es

wcr

c th

e nl

nin

~lo

vig

atio

n ch

nntic

l (M

NC

), m

nin

chan

nel h

ortlc

r (M

CB

), t

nilw

ntcr

('TW

Z),

acco

ndnr

y ch

nnnc

l (S

CI I

), tc

rlin

ry c

hnnn

cl (T

CI!)

, tr

ihut

nry

chan

nel (

TRC

), co

ntig

uous

floo

dpla

in la

ke (C

FL)

, co

ntig

uous

floo

dpla

in s

hallo

w a

quat

ic a

rea

(CFS

), co

ntig

uous

impo

unde

d ar

ea (C

IM),

iso

late

d flo

odpl

ain

aqua

tic a

rea

(IFP

), te

rres

tria

l isl

and

(TIS

), co

ntig

uous

terr

estr

ial f

lood

plai

n (C

TF

), is

olat

ed te

rres

tria

l flo

odpl

ain

(ITF

), an

d to

tal c

ontig

uous

ha

bita

t are

a (T

OC

).

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Page 6: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

The nature of major pool groups (A and B) formed by cluster analysis and PCA was inferred from loadings of habitat variables for PCA axis 1 (Table 3). Axis 1 of the PCA explained 71% of variation in the habitat data for all pools and clearly defined differences between pool groups A and B (Figure 4). The cumulative variance explained by PCA axes 1-3 was 88% (Table 3). PCA axis 1 was significantly (P=0.50 x 1 0 3 different from a random axis according to the Fisher's proportion test. PCA axes 2 and 3 were not significantly different from random axes (P>0.1 in both cases).

Aquatic/geomorphic habitat. with hlgh positive loadings on axis 1 were contiguous impounded area and contiguous floodplain shallow aquatic area. It is these two habitat types that defined the two major pool groups A and B (Figures 2 and 3). In general, group A pools have less contiguous impounded area and contiguous floodplain shallow aquatic area than group B pools. The average size of contiguous impounded and contiguous floodplain shallow aquatic areas for group A was 20 (SE = 20) and 29 (SE = 22) hectares, respectively. The average size of contiguous impounded and contiguous floodplain shallow aquatic areas for group B was 1990 (SE = 474) and 121 1 (SE = 241) hectares, respectively.

R i v a Km 1374

%

I River Km 0

Figure 1. Upper Mississippi hve r floodplain boundary and locations of locks and dams (3-26) whlch defrned navigation pools. The reach of the open river, also included in analyses of pool similarity, was UMR RKm 47- 129 (29).

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Page 7: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

Five sub-groups containing 2-7 pools each were defined by choosing a maximum Euclidean distance of 2.2 on the cluster analysis dendrogram (Figure 3). Similarity of pools based on aquatic/geomorphic habitats was hlgh for pools 17 and 15 (sub-group Al); for pools 24, 18,20,22,21, and 14 (sub-group A2); for pools 16 and 6 (sub-group A3); for pools 12, 11, 25, 10, 13, 9, and 5a (sub-group Bl); and for pools 19, 8,7, 5, and 4 (sub-group B2).

The nature of pool sub-groups (Al-B2) formed by cluster analysis and PCA was inferred from loadings of habitat variables for PCA axes 2 and 3 (Table 3). The aquatic/geomorphic habitat with a hlgh positive loading on axis 2 was tertiary channel. The aquatic/geomorphic habitat with a hlgh positive loading on axis 3 was contiguous floodplain shallow aquatic area. Contiguous impounded area had a hgh negative loading on axis 3.

Pools of sub-group A1 were characterized by no contiguous impounded area, contiguous floodplain shallow aquatic area, or tertiary channel (Table 3, Figure 4). Overall, contiguous floodplain area in these pools was small, having an average total area of 3901 hectares of contiguous aquatic/geomorphic habitat. Pools of sub- group A2 were also characterized by having no contiguous impounded area or contiguous floodplain shallow aquatic area, and only an average of 19 hectares of tertiary channel. With an average total area of 10,356 hectares, subgroup A2 pools occupied a larger floodplain area than other group A pools. Much of this habitat was terrestrial island, main channel border, and (especially) contiguous terrestrial

Figure 2. Example of upper Mississippi River navigation pool aquatic/geomorphic areas used to determine similarity among pools 4-26 and the open river (RKm 47- 129).

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Page 8: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

Tab

le 3

. L

oadi

ngs

of a

quat

ic/g

eom

orph

ic ha

bita

t var

iabl

es o

n pr

inci

pal c

ompo

nent

axe

s 1-

3 w

ith m

ean

(SE

) hec

tare

s fo

r fiv

e po

ol g

roup

s fo

nned

by

clus

ter a

naly

sis.

PC

A

axis

load

ings

with

abs

olut

e va

lue

>0.5

, use

d to

exp

lain

rela

tions

hips

amon

g po

ol g

roup

s, a

re in

bol

d ty

pe.

Cum

ulat

ive

(%) v

aria

nce

expl

aine

d ar

e in

par

enth

eses

fo

r eac

h ax

is (1

-3).

P-v

alue

s are

resu

lts o

f K

rusk

al-W

allis

AN

OV

As

test

ing

hypo

thes

es o

f no

diff

eren

ce a

mon

g po

ol g

roup

s fo

r eac

h ha

bita

t typ

e.

PCA

Axi

s Po

ol G

roup

H

abita

t l(

71.1

) 2(

81.4

) 3

(88.

1)

Al

A2

A3

B 1

B2

P

Mai

n N

avig

atio

n C

hann

el

0.O

Ol

0.14

9 -0

.128

50

6 (2

70)

727

(60)

47

5 (1

70)

803

(162

) 72

7 (2

16)

0.75

46

Mai

n C

hann

el B

orde

r A

-0.0

30

0.15

1 -0

.132

88

0 (1

01)

1715

(141

) 11

81 (6

25)

1437

(260

) 14

93 (9

45)

0.27

83

o,

Tai

lwat

er

ul

0.01

7 0.

065

0.09

0 31

(7)

28

(3)

21

(5)

23 (2

) 21

(3)

0.

3004

Se

cond

ary

Cha

nnel

0,

000

0.21

2 0.

001

486

(320

) 85

6 (1

69)

1281

(312

) 83

5 (2

09)

663

(221

) 0.

3561

T

ertia

ry C

hann

el

0.lO

l 0.

834

0.22

4 0

(0)

19 (1

0)

20 (1

9)

94 (

55)

1 (0

) 0.

0053

T

ribu

tary

Cha

nnel

0.

040

0.26

4 -0

.170

2

(2)

80 (3

0)

30 (7

) 41

(18

) 62

(14)

0.

1445

C

ontig

uous

Flo

odpl

ain

Lak

e 0.

166

0.23

3 -0

.103

12

4 (1

17)

26

1 (9

8)

200

(4)

735

(206

) 25

85 (1

94 1)

0.

0776

C

ontig

uous

Flo

odpl

ain

Shal

low

Aqu

atic

Are

a 0.

662

-0.1

33

0.66

6 0

(0)

0 (0

) 17

6 (7

1)

1289

(390

) 11

01 (2

42)

0.00

16

Con

tiguo

us Im

poun

ded

Are

a 0.

718

-0.0

82

-0.6

08

0 (0

) 0

(0)

0 (0

) 18

96(7

10)

2121

(645

) 0.

0016

T

erre

stri

al Is

land

0.

075

0.20

0 -0

.067

66

4 (4

84)

1477

(286

) 94

0 (2

76)

2421

(34

9)

1907

(353

) 0.

0447

C

ontig

uous

Ter

rest

rial

Flo

odpl

ain

0.01

7 0.

148

-0.2

22

1210

(48)

51

93 (1

647)

31

36 (6

00)

4316

(950

) 69

28 (2

045)

0.

1420

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Page 9: Classification of Upper Mississippi River Pools Based on Contiguous Aquatic/Geomorphic Habitats

floodplain. Pools of sub-group A3 were characterized by having no contiguous impounded area, a small contiguous floodplain shallow aquatic area (average of 176 hectares), and only an average of 20 hectares of tertiary channel. Sub-group A3 pools had more secondary channel (average of 128 1 hectares) than all other pool sub-groups. Pool 26 and the open river reach (29) were outliers in pool group A and were not included in any pool subgroups.

Pools of sub-group B 1 were characterized by large contiguous impounded area and large contiguous floodplain shallow aquatic area (1896 and 1289 hectares, respectively; Table 3, Figure 4). These pools had the hghest tertiary channel area (average of 94 hectares) of all other pool sub-groups. Pools of sub-group B2 were also characterized by large contiguous impounded area (average of 2 12 1 hectares) and large contiguous floodplain shallow aquatic area (average of 1101 hectares). These pools had relatively little tertiary channel habitat (average of 1 hectare); however, they had the highest average contiguous terrestrial floodplain of all other pool sub-groups. Overall, contiguous floodplain area in pools of both sub-groups B1 and B2 was large, having an average total area of 13,891 and 17,609 hectares of aquatic/geomorphic habitaf respectively.

Two Five Cluster Cluster

Distance Distance

UMR U.S. U.S. Army Navigation Geological Corps of

Pool Survey Engineers

Euclidean Distance

Figure 3. Similarity of upper Mississippi fiver navigation pools based on eleven contiguous aquatic/geomorphc habitats. Pool groups (clusters) A and B were formed using an average Euclidean distance of 3.6. Pool groups A1 B2 were formed using an average Euclidean distance of 2.2. Results are also provided for previous studies by the U.S. Geological Survey (1999; L=Lower Impounded Reach, U=Upper Impounded Reach) and the U.S. Army Corps of Engineers (2000a; Natural Geomorphic Reaches 2-9 of the UMR, James Knox, University of Wisconsin, Madison).

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Initial analyses that included the HNA isolated floodplain aquatic areas provided identical pool groups (A and B) and subgroups (Al-B2). Analyses that included the HNA isolated terrestrial floodplain areas provided identical pool groups A and B, but a somewhat different arrangement of pool subgroups than presented in Figure 3. As stated previously, the final results highlight pool sirmlarity based on connected habitats. This classification is based on the importance of the annual flood pulse.

Figure 4. Three-dimensional scatter plot of upper Mississippi hve r navigation pools (4-29) on principal components analysis axes 1 (PC l), 2 (PC2) and 3 (PC3). Symbol shadings are results of cluster analysis, where gray represents pool groups A1 (circles), A2 (triangles), and A3 (squares), and black represents pool groups B 1 (triangles) and B2 (circles). Unshaded symbols represent pool 26 and the open river reach (29), which were outliers in group A of the cluster analysis.

DISCUSSION Multivariate methods allowed a means for classifying and describing UMR

navigation pools based on 11 aquatic/geomorphc habitats as defined by the HNA. The pool groups and sub-groups that were created provide an additional tool with a systemic focus, and should complement the observations of other investigators. Pool assemblages of groups A and B were similar respectively to the Lower Impounded Reach (pools 14-26) and the Upper Impounded Reach (pools 1-13) of U.S. Geological Survey (1999), a classification which was based on floodplain width, impounded area, and area of non-channel aquatic habitats and marshes. Exceptions noted fiom the present study included the similarity of pool 6 to Lower Impounded Reach pools, and the simrlarity of pools 19 and 25 to Upper Impounded Reach pools. Also of interest was the greater similarity of the unimpounded open river reach (29) to Lower Impounded Pools than that of pool 26. Both pool 26 and the open river study reach are among the largest in terms of total contiguous

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aquatic/geomorpluc habitat area (27,764 and 20,099 hectares, respectively). Pool 26 has 18,663 hectares of contiguous tenestrial floodplain, no contiguous floodplain shallow aquatic area, and a moderate amount of contiguous impounded area (245 hectares), a combination unlike that of any other pool.

U.S. Army Corps of Engineers (2000a; James Knox, University of Wisconsin, Madison) divided the UMR into ten natural Geomorpluc Reaches based on valley and floodplain morphology, geologic factors, longitudinal gradient, and sediment transport characteristics. In a few cases, pool sub-groups defined by the present study followed the Geomorphic Reach designations. Since the large-scale factors used by U.S. Army Corps of Engineers (2000a) to define Geomorpluc Reaches were likely correlated to some extent with area of aquatic/geomorphic habitats w i h pools, some similarity between the two studies was expected. The results of the present study are unique because they take into account human- induced disconnection of floodplain habitats; pool similarities were based solely on contiguous aquatic habitats that are (at least seasonally) available to large river biota. Moreover, analyses were not constrained by any longitudinal arrangement of pools. Emphasis was on the importance of hydrologic connectivity and habitat diversity for purposes of explaining variability in spatial trends of riverine fish, macroinvertebrates, submersed aquatic vegetation, or other biota among pools. It is possible that if human impacts to pools were reduced and isolated floodplain areas were reclaimed, resulting habitat changes would move pools into groupings more sirmlar to that of U.S. Geological Survey (1999) andlor U.S. Army Corps of Engineers (2000a).

Analyses of pool similarity were based on hectares of habitats estimated fiom aerial photographs taken in 1989, which was the most recent systemic information available. There are several potential limitations of this analysis to be aware of because of this, including: 1) the UMR in the late 1980's was in a severe drought, 2) the estimated habitat areas are based on photographs from a single year, and 3) there was no photographic coverage for a small portion of a few pools. Even though locks and dams have tended to stabilize the river, regardless of climatic conditions, it is possible that the drought may have caused an underestimation of some aquatic areas, especially off-channel habitats in the upper UMR pools. Additionally, use of a photographic snapshot of the UMR should only be considered an "index" of the actual habitat areas that are available seasonally and annually. The areas provided by the HNA and means calculated for pool groups in this study are only a broad estimate of actual conditions. In reality, large rivers are highly dynamic and habitats change seasonally and annually as discharge varies, water levels vary, and habitats are altered. The pool means for habitat types should also be considered as estimates because some pools lacked complete photographic coverage. Most areas not photographed were isolated terrestrial floodplain, a habitat class not used for analyses of pool similarity. However, there were a few cases where contiguous terrestrial floodplain, primarily, was underestimated by this study. Extreme cases were pools 18 and 26, where respectively 25% and 3 1% of floodplains had no photographic coverage, most of whlch was llkely contiguous terrestrial floodplain. Since loadings of thls habitat on PCA axes 1-3 was very low, it was not considered a "dnving" variable in thls analysis and &s lack of data should not have greatly affected overall results.

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In summary, pool similarities based on floodplain habitats available to rivenhe biota should provide managers a basis for extrapolating results of large- scale management strategies to other reaches of the UMR. For example, hydrologic manipulation to restore summer low-water conditions has been used on pools 24, 25, and 26 (U.S. Army Corps of Engineers 2000b) to stimulate vegetative growth and improve habitat for fish and waterfowl populations. The results of these studies could be used to predict the outcomes of similar manipulations on other UMR pools, especially the results fiom pool 25, which was highly similar to pools 9-13 in terms of its areas of aquatic/geomorphic habitat. Pool groups defined by tlus study and UMR reaches defined by U.S. Geological Survey (1999) and U.S. Army Corps of Engineers (2000a) provide a basis for development and testing of hypotheses regardmg the function of large rivers. Naturalization of the hydrologcal regime by mimicking pre-dam hydrographs (Galat and Lipkin 1999, Koel2000) could be attempted in a series of pools with similar habitat structure to improve statistical sigudicance of results. Finally, the pool groups defined by this study will also be useful as the Long Term Resource Monitoring Program (U.S. Geological Survey 1999) expands its spatial coverage of fish, vegetation, macroinvertebrate, and water quality monitoring (Koel 1999).

ACKNOWLEDGMENTS Funding for this study was provided by the Minnesota Department of Natural

Resources, Division of Ecological Services; the U.S. Geologcal Survey, Upper Midwest Environmental Sciences Center; and the U.S. Army Corps of Engineers; through the Long Term Resource Monitoring Program and Environmental Management Program for the Upper Mississippi River System. The author thanks Jeff Arnold, Thad Cook, and Kevin Irons of the Illinois Natural History Survey; Tim Cross of the Minnesota Department of Natural Resources; and Barry Johnson, Hank DeHaan, and Chuck Theiling of the U.S. Geological Survey for reviewing earlier versions of this manuscript.

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Rece~ved: 5 September 2000 Accepted 22 December 2000

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