CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

RIVER CHANNEL DYNAMICS IN A DELTA ENVIRONMENT:

CHORRO CREEK, CALIFORNIA

A Thesis submitted in partial fulfillment of the requirements For the degree of Master of Arts in Geography

By

Aaron Marshall Davis

August 2006

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ACKNOWLEDGEMENTS

I would like to express appreciation to the following people who deserve recognition for helping me complete this thesis: my family, who have been so supportive over the years; Dr. Amalie Jo Orme, whose encouragement, dedication, and tireless efforts made the thesis possible; those who assisted me with the field work, especially my father, Marshall Davis, who literally trudged through the thick-of-it to help me gather data, and friends Tina White, Meredith Leonard, Alethea Steingisser, and the rest of the Morro Marsh Rats. A very special thanks to Linus Weigenstein, of the Metropolitan Water District, for running the analysis on my samples. Also, thanks to Dr. Julie Laity and Dr. Shawna Dark, who have supported me throughout my graduate studies and for their timely reading of my thesis. Thanks also go out to my grandfather, Arnold Davis, who also provided encouragement throughout this long process. Finally, to my wife Elena, who supported me on a day to day basis.

DEDICATION

Dedicated to my father, Marshall Davis; grandfather, Arnold Davis; and uncle, Mike White. I’ll always be your “campboy!”

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TABLE OF CONTENTS

SECTION PAGE

SIGNATURE PAGE ii

ACKNOWLEDGEMENTS iii

LIST OF FIGURES v

ABSTRACT x

CHAPTER 1 INTRODUCTION 1

CHAPTER 2 SCIENTIFIC BACKGROUND 4

CHAPTER 3 PHYSICAL SETTING 14

CHAPTER 4 METHODOLOGY 39

CHAPTER 5 DATA PRESENTATION 49

CHAPTER 6 DISCUSSION 97

CHAPTER 7 CONCLUSIONS 109

REFERENCES 112

APPENDIX 119

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LIST OF FIGURES

Figure Title Page

2.1 Evolutionary changes in tidal creek sinuosity 8

2.2 Cross-sectional profile of a tidal creek 13

3.1 Location of Morro Bay (map) 14

3.2 Morro Bay land use (map) 16

3.3 Morro Bay soil characteristics (map) 17

3.4 Morro Bay vegetation (map) 18

3.5 Morro Bay watershed (map) 19

3.6 Chorro Creek average daily discharge 21

3.7 Mean annual temperature along coastal cities 22

3.8 Precipitation along coastal cities 23

3.9 Inundation of summer coastal fog 24

3.10 Morro Bay average monthly temperatures 25

3.11 Morro Bay average monthly precipitation 27

3.12 Morro Bay borehole data (map) 29

3.13 Intertidal zonation of the Chorro Delta 32

3.14 Change in Chorro Creek channel form and flow 37

3.15 Current state of Chorro Creek and Chorro-D 38

4.1 Survey point locations (map) 40

4.2 Location of sample and cross-section sites 42

4.3 Typical middle marsh conditions 48

4.4 Typical Chorro Creek conditions 48

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5.1 Site A 50

5.2 Site A cross-section 51

5.3 Site A hydrological analysis 52

5.4 Site A cumulative sediment weights 53

5.5 Site A water sample results 56

5.6 Site B 58

5.7 Site B cross-section 59

5.8 Site B hydrological analysis 60

5.9 Site B cumulative sediment weights 61

5.10 Site B water sample results 63

5.11 Site C 65

5.12 Site C cross-section 66

5.13 Site C hydrological analysis 67

5.14 Site C bank collapse 67

5.15 Site C cumulative sediment weight 68

5.16 Site C water sample results 71

5.17 Site A-D 72

5.18 Site A-D cross-section 73

5.19 Site A-D hydrological analysis 74

5.20 Site A-D cumulative sediment weight 75

5.21 Site A-D water sample results 78

5.22 Site B-D 80

5.23 Site B-D at bankfull stage 81

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5.24 Site B-D cross-section 82

5.25 Site B-D hydrological analysis 83

5.26 Site B-D cumulative sediment weight 84

5.27 Site B-D water sample results 86

5.28 Site C-D 88

5.29 Site C-D cross-section 89

5.30 Site C-D hydrological analysis 90

5.31 Site C-D cumulative sediment weight 91

5.32 Site C-D water sample results 94

5.33 North-south delta surface slope 95

5.34 East-west delta surface slope 95

5.35 Slope of Chorro Creek thalweg 96

5.36 Slope of Chorro-D thalweg 96

6.1 Sedimentological summary for Chorro Creek 98

6.2 Sedimentological summary for Chorro-D 99

6.3 Silt-Clay index 102

6.4 Water characteristics for all six sites 103

6.5 Channel slope for Chorro Creek and Chorro-D 106

6.6 Sediment accumulation in Chorro Creek 108

A.1 Site A bank (after HHT) 119

A.2 Site A mid-channel (after HHT) 120

A.3 Site A thalweg (after HHT) 121

A.4 Site B bank (after HHT) 122

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A.5 Site B mid-channel (after HHT) 123

A.6 Site B thalweg (after HHT) 124

A.7 Site C bank (after HHT) 125

A.8 Site C mid-channel (after HHT) 126

A.9 Site C thalweg (after HHT) 127

A.10 Site A-D bank (after HHT) 128

A.11 Site A-D mid-channel (after HHT) 129

A.12 Site A-D thalweg (after HHT) 130

A.13 Site B-D bank (after HHT) 131

A.14 Site B-D mid-channel (after HHT) 132

A.15 Site B-D thalweg (after HHT) 133

A.16 Site C-D bank (after HHT) 134

A.17 Site C-D mid-channel (after HHT) 135

A.18 Site C-D thalweg (after HHT) 136

A.19 Site A bank (after LHT) 137

A.20 Site A mid-channel (after LHT) 138

A.21 Site A thalweg (after LHT) 139

A.22 Site B bank (after LHT) 140

A.23 Site B mid-channel (after LHT) 141

A.24 Site B thalweg (after LHT) 142

A.25 Site C bank (after LHT) 143

A.26 Site C mid-channel (after LHT) 144

A.27 Site C thalweg (after LHT) 145

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A.28 Site A-D bank (after LHT) 146

A.29 Site A-D mid-channel (after LHT) 147

A.30 Site A-D thalweg (after LHT) 148

A.31 Site B-D bank (after LHT) 149

A.32 Site B-D mid-channel (after LHT) 150

A.33 Site B-D thalweg (after LHT) 151

A.34 Site C-D bank (after LHT) 152

A.35 Site C-D mid-channel (after LHT) 153

A.36 Site C-D thalweg (after LHT) 154

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ABSTRACT

RIVER CHANNEL DYNAMICS IN

A DELTA ENVIRONMENT:

CHORRO CREEK, CALIFORNIA

by

Aaron Marshall Davis

Masters of Arts in Geography

Channel systems within deltaic salt marshes offer unique insight into the

transportation and deposition of sediment derived from terrestrial watersheds and from

neighboring beach and nearshore environments. Typically, within deltaic salt marshes

two types of channels form a network—active channels that conduct freshwater from

contributing watersheds and channels that actively conduct the daily bi-directional flow

of seawater associated with the ebb and flood of tidal surges.

Using the Morro Bay deltaic salt marsh as a field site, this study focuses on the

hydrodynamics and function of these channels and has three goals: (1) to describe the

nature of sediment and water transport in a tidal and a freshwater channel, (2) to examine

the differences and similarities in channel geometry between the two channels, (3) to

determine the role of each channel whether dominated by a fluvial or tidal flow regime.

Silt-clay indices and sediment analysis suggest that Chorro Creek, dominated by

tidal flow, is slowly infilling during the maximum flood tide, coupled with aggradation of

coarse sediments during infrequent freshwater flooding. The adjacent Chorro Creek

Distributary, now dominated by freshwater flow owing to shifts in its headwater

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morphology, is experiencing erosion and an increase in sediment size, especially along its

middle and lower reaches. While the changes are subtle, they reflect adjustments to

changing inputs of water and sediment both within the channels and across the marsh

surface.

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CHAPTER 1

INTRODUCTION

Channel systems within deltaic salt marshes offer unique insight into the

transportation and deposition of sediment derived from terrestrial watersheds and from

neighboring beach and nearshore environments. Deltaic salt marshes are initiated when

the rate of sediment supply exceeds the rate of sediment transport through and beyond the

delta by tidal currents, wave action, littoral drift, and flushing from freshwater channels

(Bearman, 1999). Typically, within deltaic salt marshes there are two types of channels

that form a network: (1) active channels that conduct freshwater from contributing

watersheds; and (2) channels that actively conduct the daily bi-directional flow of

seawater associated with the ebb (fall) and flood (rise) of tidal surges. The morphology

and function of these channels are highly dynamic. New distributary channels form when

sediment blocks the passage of water. As the network evolves, it re-distributes water and

sediment within the salt marsh. The evolution of channel systems within saltmarshes—

whether these channels are freshwater fluvial or tidal in nature—is, in turn, dependent on

the balance between the inherent erodibility of the salt marsh sediments, the rate of

freshwater and tidal input, and the vertical accretion of sediment. Ultimately, in the

broader context, the salt marsh and its channel systems, whether tidal or fluvial in nature,

reflect adjustments to sea level change and the tectonic and structural framework of the

region.

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1.1 STATEMENT OF PURPOSE

The purpose of this thesis is to describe and explain the nature of sediment

transport and deposition in two deltaic salt marsh channels—one dominated by tidal flow

(except during terrestrial flooding) and the other functioning largely as a freshwater

channel with secondary tidal influence. Using the Morro Bay deltaic salt marsh as a field

site, this study focuses on the hydrodynamics and function of these channels and has

three goals: (1) to describe the nature of sediment and water transport in a tidal and a

freshwater channel, (2) to examine the differences and similarities in channel width,

depth, slope, thalweg, and sinuosity between the two channels, (3) to determine the

present-day function of each channel, within the tidal or fluvial flow regime.

1.2 SIGNIFICANCE OF THE STUDY

This study contributes to the understanding of water and sediment flow in

channels that have dual and changing functionality. In an area that is experiencing rapid

sediment and water fluctuations owing, in part, to human interference, this study

examines some of the important principles underlying both salt marsh and freshwater

channel processes.

Channel processes in deltaic salt marsh environments have, in large part, focused

on passive-margin coasts such as those in the United Kingdom and the eastern United

States. Most process-oriented studies in these areas focus on tidal creeks (channels that

extend from the distal (seaward) edge of the delta into the salt marsh and which conduct

saltwater with the ebb and flow of the tide) and on channel expansion and hydraulics

under a bi-directional flow regime. These studies generally are of two types—those that

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incorporate data collected on sediment size and on marshland surveys and those that use

computer modeling to demonstrate delta development over a large area.

Within smaller areas, field studies have proven to be a rational measure of channel

processes. For example, Myrick and Leopold (1963) used survey data and bedload

sediment samples to describe channel forms and tidal creek development. Their work,

and the many projects that were to follow, focused on the development of tidal creek

networks and ignored, in large part, the relationship and the differences between the

effect of tidal and freshwater flow on channels in this environment

This study incorporates techniques that are inherently linked to research in

freshwater fluvial channels, yet set within a deltaic saltmarsh that is located on an active-

margin coast and experiencing rapid changes in channel function and form. While the

dynamics of the channels are partitioned into individual foci of interest (channel area and

slope, flow velocity, and sediment transportation and deposition), it is recognized that the

consolidation of these factors describe and explain channel form and function. Using this

knowledge, the inherent value of the research lies in its contribution to understanding

actual and potential changes in water and sediment distribution within and beyond the

salt marsh.

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CHAPTER 2

SCIENTIFIC BACKGROUND

Coastal salt marshes have been the focus of significant research over the past

three centuries. Understandably, much of the early research addressed the unique

relationship between salt marsh vegetation, periodic inundation from tidal fluxes, and the

spatial evolution of the marsh environment. During the latter part of the nineteenth and

early part of the twentieth centuries, the first studies that focused on the dynamics of tidal

creeks emerged from work on marshlands in the eastern United States (Shaler, 1885) and

the wetlands of western Wales (Yapp et al., 1917). Shaler addressed the important

relationship between tidal creek development and the significance of bidirectional flow

associated with daily tidal fluxes. Yapp’s work, while part of a broader investigation of

marshland ecology and salt pan development, addressed, in part, the evolution of tidal

channels.

Remarkably, the relationship between the dynamics of water sediment transport

and tidal channel changes remained largely untouched, with the exception of Gilbert’s

1917 work on the transportation of fluvial sediments into a deltaic environment (San

Francisco Bay). Though largely a study based on detailed experimentation in a flume,

coupled with field data from terrestrial channels impacted by the influx of debris from

hydraulic mining operations in California’s Sierra Nevada, Gilbert’s work addressed the

important relationship between peak velocity within tidal channels and its relationship

with low and high tide.

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While Gilbert’s work gained significant attention with regard to the geomorphic

relationship between channel slope, cross-section, discharge, and sediment size, its

importance in marshlands awaited much later research. Work on the dynamics of flow in

this environment eventually emerged with the application of the principles of hydraulic

geometry (Leopold and Maddock, 1953; Schumm 1977) derived from research on

terrestrial channels. The latter part of the twentieth and the early years of the twenty-first

centuries witnessed an important growth in studies of tidal creek development. Notable

are the work of Pestrong (1970) in the San Francisco Bay area, Frey (1985) in coastal

Georgia, Fagherazzi et al. (1999) on tidal channel networks in Italy and the eastern

United States, Allen (2000) on tidal channel sediment transport in England, Perillo (2003)

on tidal creek evolution in Argentina, and Hall (2004) on tidal channel restoration in San

Francisco Bay.

What has emerged from these and other studies is the unique relationship between

tidal channel form, channel patterns on the marsh surface, and the role of daily bi-

directional flow in the transport of sediment within and across the marsh. However, more

limited attention has been given to the dynamics of variable inflow from freshwater

fluvial systems and marshland channel changes. This thesis focuses on the critical

interface between these two linked systems (tidal and fluvial) and pays particular

attention to channel development and its relationship to sediment size, transport, and

deposition in this environment.

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2.1 CHANNEL DEVELOPMENT

The development of tidal creeks and their expansion within the marsh surface is

not without controversy. Conceptual models of marsh and tidal creek development

emphasize the importance of headward erosion, lateral migration, vertical aggradation of

levees, and channel incision (French and Stoddart, 1992). Pestrong (1965) noted that the

development of tidal channels is an erosive process that is associated with ebb tide

between over-marsh flow at slack high tide (the peak of flood tide associated with zero

flow velocities, marking the transition between the flood and ebb tide) and confinement

of the tide by channels. Ebbing during this period causes headward erosion, cutting

through the marsh substrate. Pestrong (1970) added that the headward expansion of tidal

channels was accompanied also by the extension of the marsh surface on the tidal flats.

Reed (2000) incorporated the vertical development of the marsh surface around the

channel owing to sediment trapping by vegetation in combination with the erosive power

from the ebbing of over-marsh sheetflow.

Further, Shi et al. (1995) found, using aerial photograph analysis in the Dyfi

Estuary (Wales), that tidal creek expansion was related to increased tidal and wave

energies from a rise in sea level and the form-process feedback between ‘deposition’ of

sediment on the salt marsh and ‘erosion’ within the tidal creeks. During the

developmental stages of tidal creek formation, scour of the channel is thought to be

related to the higher velocities associated with the ebb phase of the tidal cycle. Further,

deposition of sediment atop the salt marsh increases salt marsh elevation, ultimately

crossing the threshold for which halophytic vegetation, such as Salicornia, can become

established. The establishment of vegetation along the creek channels promotes soil

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cohesion, resulting in resistance to erosion along the channels and thus stabilizing the

channel forms.

Vegetation plays a dominant role in channel shape and morphology within the

saltmarsh. The rate of sediment accretion increases once vegetation is established by

slowing velocities, impeding flow, allowing sediments to settle, and increasing resistance

to erosion. This increases the incision of the tidal channel while also increasing the

hydraulic radius, ultimately advancing headward erosion within the saltmarsh and

extending tidal creek development (Pestrong, 1970).

Given the more turbulent flows created by the colonization of vegetation, tidal

creeks within the marsh are characterized by high sinuosity, channel bifurcation, and high

creek densities (Frey and Basan, 1985; Allen, 2000). Figure 2.1 shows, from youngest to

oldest, the four common stages of tidal creek maturity: linear, linear dendritic, dendritic,

and meandering dendritic. The tidal creek networks become increasingly sinuous and

dense with marsh maturity (Allen, 2000). Remarkably, these channels resemble drainage

patterns commonly associated with fluvial networks. However, the fundamental

difference between tidal and fluvial channel systems lies in the inherent erosional nature

of terrestrial watersheds. Nonetheless, while still poorly understood, certain variables

such as channel sinuosity and channel slope closely mirror one another in both

environments, although formed under different hydrological processes (fluvial and tidal).

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Erosional features that impede tidal creek expansion are bank undercutting as a

result of micro-wave erosion at bankfull capacity and bioturbation, both of which are

responsible for the slumping and sliding of consolidated vegetation blocks into the

channel and contribute to channel infill (Frey and Basan, 1985). Data presented in

Chapter 5 illustrate this phenomenon.

Research into the expansion of tidal creek networks generates significant

controversy. Much of the research in the United States suggests that tidal creek densities

are primarily ebb-influenced, while modified by the flood tide. In contrast, Pethick

(1992), using evidence from the North Norfolk salt marshes in England, concluded that

creek morphology and expansion is not related to the ebb tide but rather to the flood tide.

Given these differences of opinion, the dynamics of marsh channel development and

subsequent modification offer ideal areas of enquiry.

Figure 2.1.The evolutionary changes in creek sinuosity with salt marsh maturity (after Allen, 2000).

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2.2 CHANNEL HYDRAULICS

Bankfull discharge is the filling of a channel from bank to bank and is classified

as the dominant force in maintaining, shaping, and forming fluvial channels (Gordon, et

al., 1992; Leopold & Wolman, 1957). At bankfull, flow resistance is at a minimum,

allowing channels to transport water more efficiently than at other stages. Further,

Wolman and Miller (1960) added that the effectiveness of dominant flow is not just a

matter of discharge during a single event but also is related to the frequency of the events,

establishing an inverse relationship between the discharge rate and the frequency of

occurrence. In the eastern United States, bankfull capacity is reached on an average of

every 1.5 years in a fluvial system (Leopold & Wolman, 1957).

Unlike their fluvial counterparts, tidal channel form and function demonstrate

significant differences with respect to the work accomplished at bankfull stage. Myrick

and Leopold (1963) were two of the first researchers to compare the relationships

between channel width, depth, and discharge (Leopold & Wolman, 1957) in a fluvial

system to those variables in a tidal network. Their research on Wrecked Recorder Creek

along the Potomac River specifically analyzed tidal channel formation and maintenance

in relation to the variations in hydraulic factors along the length of the creek.

In contrast to a fluvial system, the frequency of bankfull stage in a salt marsh

environment is reached twice daily. Additionally, this stage of the flood tide is marked

with zero flow velocities and represents the transition between the flood and ebb tide.

Given this difference, Myrick and Leopold (1963) questioned the effectiveness of tidal

creek bankfull discharge in channel formation and maintenance.

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Pethick (1984) also addressed the hydraulic relationship between tidal and fluvial

channels and stated that similarities between the two are limited to the fact that they both

transport water. Besides that function, the hydraulic processes are markedly different,

mainly because tidal flows do not “go anywhere”. The filling of tidal channels only

occurs because the channel is present, linking discharge within the channel as dependent

on channel size – in other words, the channel is filled only because it is there. Fluvial

channels, by contrast, are independent of pre-existing channel size, with water eroding

the channel to fit the discharge from the contributing watershed.

In conclusion, whereas bankfull discharge is critical to the development and

maintenance of fluvial channels, it is the discharge that occurs during maximum flow

velocities that is most meaningful in a tidal setting (Myrick & Leopold, 1963).

Therefore, channel erosion and sediment transport of bed and bank materials occur

during the flood and ebb of the tide, when maximum tidal flow velocities are achieved.

By contrast, the settling of suspended sediments occurs during the high slack water and

undermarsh stages of the tide, in connection with the lowest flow velocities (Allen,

2000).

The timing of maximum tidal velocities differs both spatially and temporally,

dependent on where and at what stage the measurements are recorded. The ebb tide is

typically stronger, although the flood tide dominates near the head of the creek and

becomes progressively ebb-dominated seaward (French et al.¸ 1992; Allen, 2000).

Exceptions arise during storm conditions, where stronger flood flows are enhanced by

fluvial runoff.

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Maximum velocities during the early flood or late ebb tide may be a result of

channel constriction by smaller cross-sectional areas (Van Den Berg and Jeuken, 1996).

However, Ayles (1996) and Myrick & Leopold (1963) found that the highest velocities

are recorded during early mid-ebb or mid-to-late flood tides, which are associated with

the progressive confinement of the flow after the waning of over-marsh flow during the

ebb or before the bankfull threshold is overcome during the flood. Spatially, Bartholdy

(2000) observed in the Gradyb (Denmark) tidal area that the seaward portion of the tidal

channel is ebb-dominated while the landward tidal channel experiences a weak flood

dominance. While the above research proves that the highest tidal velocities are a

function of tidal confinement by creek channels, the timing, location, and intensity of

maximum velocities recorded are dependent on the size of the tidal prism (the change in

the volume of water covering an area, such as a wetland, between a low tide and the

subsequent high tide), the duration of the tide, and specific geomorphic characteristics of

each salt marsh (Myrick & Leopold, 1963).

The maximum velocity in a tidal channel can only occur during either the ebb or

flood tide, thus creating a “unidirectional” current that is dominant (Frey and Basan,

1985). This shared characteristic of both tidal and fluvial systems results in similar

depositional or erosional processes. Research by Ayles (1996) in the Dipper Harbour

Creek, New Brunswick, observed the migration of channel dunes as an indicator of the

dominant tidal process as well the interaction between tidal and intermittent seasonal

freshwater input. In this case, the high energy, low frequency runoff associated with

spring snowmelt reversed all headward dune migration during the year made by the

dominant flood tide.

http://www.biology-online.org/dictionary/Changehttp://www.biology-online.org/dictionary/Volumehttp://www.biology-online.org/dictionary/Areahttp://www.biology-online.org/dictionary/Low

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2.3 CHANNEL MORPHOLOGY

Although there are several distinct differences between tidal and fluvial channels,

they possess many similar sedimentary characteristics (Zeff, 1988). Figure 2.3 (Pestrong,

1970) illustrates the distribution and characteristics of sediment in a tidal channel cross-

section. The channel is composed of a combination of silt, clay, and shells, while the

levee is composed of silts and clays and area adjacent to the levee is composed of only

clays. The levee has the greatest density and shear strength when compared with the

other areas within and around the channel, having the capacity to carry the most weight.

The deposition of coarser material within the channel is due to the higher tidal velocities

associated with channel confinement. Once the channel banks are overtopped, the tidal

velocities decrease and the settling of the remaining silt material is deposited out of the

water column and onto the levees, while the finer clays are deposited on the salt marsh.

The increased shear strength associated with the levees is attributed to two

factors: larger sediment than the adjacent area (excluding the channel) and longer

exposure to atmospheric elements. In Figure 2.2, the levee is 12.7 cm higher than the

adjacent flats (Pestrong, 1970), equating to an additional 1 ½ to 2 hours of daily

exposure.

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Figure 2.2. The cross-sectional profile of sediment distribution and characteristics within a tidal flat creek. (after Pestrong, 1970).

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CHAPTER 3

PHYSICAL SETTING

The study site—Chorro Creek—is located within the Chorro Delta saltmarsh,16.5 km

west of San Luis Obispo, California (Figure 3.1). The Chorro Delta lies on the eastern

boundary of the bay and is dissected by Chorro Creek, Chorro-D (Distributary), and Los

Osos Creek. These creeks, together with the tidal creek network across the delta surface,

are the main arteries for interior sediment transport into the bay, linking the Morro Bay

watershed to the Pacific Ocean.

Morro Rock

Figure 3.1. Morro Bay and location of study site.

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With a population of 10,350 (U.S. Census, 2000), Morro Bay’s current economy

thrives on tourism and recreation but still houses some of the best sport fishing and

commercial fishing along the California coast. The most visible physical feature in

Morro Bay is Morro Rock, one of nine hypabyssal dacite plugs, sometimes referred to as

the “nine sisters” that formed during the late Oligocene. Once mined for material for the

Morro Bay break water and Port San Luis Harbor, Morro Rock is now a Historic State

Landmark (# 801) and is designated as a bird sanctuary.

Land use within the Morro Bay watershed (Figure 3.2) is 60% rangeland, 20%

brush land, and the remaining 20% is divided equally between agriculture, urban areas,

and woodlands (United States Environmental Protection Agency, 2001). Major land use

practices that have impacted sediment input into the bay include the trampling of creek

banks by cattle, removal of riparian vegetation, open and fallow agricultural fields, urban

run-off, and the construction of levees which reduced the natural flood plain area. Even

though Haltiner (1991) reported that Morro Bay is infilling at a rate ten times faster than

it would with no human disturbances, a sediment loading study by Tetra Tech in 1998

(Keese, 2001) indicated that the most significant source of sediment into Morro Bay

comes from the upper watershed, where periodic wildfires clear the chaparral vegetation,

promoting sediment erosion during the rainy season.

Soils and vegetation are typical of the central California Coast. Surface soils

(Figure 3.3) are generally fine in texture and include clay and silt loams, sands and sand

dune complexes, miscellaneous loams, and bare rock outcrops bounding the watershed

divides. Vegetation (Figure 3.4) is dominated by expanses of grasslands within the

valleys and hillsides, and chaparral and coastal sage along the coast line. Within the

16

valley lowlands, native grasses and riparian wetlands have been replaced in large part by

farmland and urban areas. Riparian woodlands are found along the main creeks and

tributaries within the valleys, including arroyo willow, mixed shrubs, grasses, cattails,

and several forbs (California Environmental Protection Agency, 2003).

Figure 3.2. Map showing Morro Bay land use.

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Figure 3.3. Soils of the Morro Bay watershed.

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Vegetation of the Morro Bay Watershed

Figure 3.4. Vegetation within the Morro Bay watershed (Gallagher, 1996).

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3.1 MORRO BAY WATERSHED

The Morro Bay watershed covers (Figure 3.5) an approximate area of 190 km2

and transports sediment through two primary creeks, Chorro and Los Osos. Chorro

Creek originates from the southern slope of the Cuesta Ridge, which has a maximum

elevation of 822 m. This portion of the watershed, referred to as the Chorro watershed,

has a drainage basin of approximately 120 km2. Los Osos Creek originates from the

northern Irish Hills and has a maximum elevation of 446 m and a watershed basin of 76

km2. These watersheds yield over 71,000 tonnes of sediment annually, with 77 percent

derived from the Chorro Valley. Ten percent of this load is composed of sand and gravel

while the remaining 90 percent is clay and silt (California Regional Water Quality

Control Board, 2002).

Figure 3.5. Morro Bay watershed.

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Under normal conditions, Chorro Creek and Los Osos Creek were classified as

naturally intermittent streams by Ford (1997) and the USEPA (2001). However, 13 years

of data from local stream gauging stations show that Chorro Creek is perennial, except

two dry years, 1989 and 1990. The flow of Chorro Creek is perennial owing to the

anthropogenic inputs, including the release of water from Chorro Reservoir, the

California Mens’ Colony waste treatment plant, agricultural irrigation, and urban runoff.

In 1994, the California Water Resources Control Board placed water release requirements

on the reservoir. If the upper creek flows at more than 0.06 m3s-1 into the reservoir, 0.03

m3s-1 must be released from the dam. If less than 0.06 m3s-1 is being discharged into the

reservoir, one-half of the flow must be discharged. Los Osos Creek, with fewer

anthropogenic contributions, remains intermittent, usually recording no discharge during

the summer months. The remainder of watershed analysis in this thesis will focus on

Chorro Creek and exclude Los Osos.

Average annual stream discharge (1990-1999) in Chorro Creek is highly variable

and ranges from below 2.47 x 106 m3 to just over 35.45 x 106 m3 and averages 6.91 x 106

m3 per year. It is cautioned that these data, although higher than reported by the USEPA

(2001), are still lower than projected owing to missing data from the gauging station

during high-flow events. The daily rate of stream discharge is seasonally controlled and

varies from month to month. Chorro Creek discharge shows that the average annual

daily flow is 0.40 m3s-1, with a maximum rate of 1.35 m3s-1 in March and a minimum rate

of 0.047 m3s-1 in October (Figure 3.6).

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Chorro Creek Stream Discharge(1990-1999) Canet Road Gauge Station

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Avg

. Dai

ly D

isch

arge

(cm

s)

Average Annual Daily Flow

Stream flow data were collected by the County of San Luis Obispo at the Canet

Road gauging station from 1988 through 2000 and again from August 2003 to November

of 2004. From 2001 to 2003, the County was in the process of building a new gauging

station at the same location. Although there are no data for this period, it can be inferred

by the moderate rainfall received and the mandated release of water from Chorro

Reservoir that Chorro Creek probably did not experience days of zero discharge. Data

provided from August of 2003 to November 2004 did not include calculations of stream

flow or stream volume because the wetted perimeter of the new station was unknown at

the time. Instead, staging depths were available which monitored a change in water level,

indicating that the channel experienced some flow.

Figure 3.6. Chorro Creek average daily discharge (County of San Luis Obispo, 1999, unpublished data).

22

3.2 CLIMATE

California Coastal Climate: The central California coast has a cool Mediterranean

climate (Miller and Hyslop, 1983) with the majority of heavy precipitation occurring

during the winter months. The littoral influence of the cold California Current moderates

air temperature and creates coastal fog during the summer months, contributing to the

cool summer days and relatively warm winter temperatures. Compared with other

California cities, ranging from Crescent City at 41.76° N latitude to San Diego at 32.73°

N latitude, Morro Bay experiences typical central coast temperatures moderated by

coastal fog. (Figure 3.7)

11.8 12.3

14.513.7 13.5

16.7 16.4

18.2 18.1

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Mean Annual Temperature for California Coastal Cities(2004)

Figure 3.7. Comparison of annual temperatures for California Coastal Cities, 2004 (National Climatic Data Center, 2005).

23

Coastal precipitation varies widely and generally increases in latitude. In 2004,

for example, the northernmost city, Crescent City, received 159.4 cm of rain, while San

Diego, the southernmost city, received the lowest (33.8 cm) (Figure 3.8). These data are

measurements of total rainfall and do not include precipitation in the form of fog drip.

Coastal fog cannot be overlooked

Precipitation for California Coastal Cities (2004)

159.4

96.7

49.0

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51.2 46.8 44.737.9 37.9 33.8

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20

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60

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100

120

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as an important contributor towards total precipitation, although how much fog

contributes is controversial.

Coastal fog, also known as advection fog, is common in areas that are influenced

by cold ocean currents, such as Namibia (Benguela Current, Chile and Peru (Humboldt

Figure 4.1

Figure 3.8. A comparison of annual precipitation for California Coastal Cities, 2004. (NCDC, 2005).

24

Current), and the California/Oregon Coastline (California Current). Advection fog

requires some turbulence, usually winds between 10 to 30 km hr-1, and is formed when

warm air masses come in contact with a cold land or water surface. In California, the

Pacific (Hawaiian) High-Pressure system is dominant during the summer months and

pushes warm ocean air towards the coast from the west-north-west at an average speed of

12 km hr-1 (NCDC, 2004). When these warm air masses come in contact with the cold

California Current, a thick layer of coastal fog (Figure 3.9) is formed, inundating the

coastline (Lutgens and Tarbuck, 1998).

Morro Bay Climate:

Morro Bay’s climate, like the other coastal cities, reflects its location adjacent to

the ocean. With an average annual mean temperature of 13.5°C, Morro Bay is one of the

Figure 3.9. The inundation of summer fog along the Central California Coast. Photo by Steve Wolf (2000).

25

coolest coastal cities in California. The annual temperature varies by roughly 4°C

between winter and summer months, registering 11.7°C and 15.6°C respectively. The

1994-2004 Average Monthly Temperature chart (Figure 3.10) illustrates slight seasonal

variations in mean temperature, while the maximum and minimum record a wider range.

Although the average minimum and extreme minimum temperatures follow the same

trend as the mean temperature, the average maximum and extreme maximum are

bimodal. While the mean temperature increases during the summer months, reaching a

high of 15.6°C in September, the maximum temperatures decrease during this same

period.

1994-2004 Average Monthly TemperaturesMorro Bay, CA

-5

0

5

10

15

20

25

30

35

40

45

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Deg

rees

(C)

Extreme MaxAve. MaxiumumMeanAve. MinimumExtreme Min

Figure 3.10. Seasonal temperature variations in Morro Bay. The decrease in the maximum temperatures from late spring through summer is due to the cooling effect of summer coastal fog. (NCDC, 2005)

26

The highest maximum temperatures recorded in Morro Bay are during October,

with an average maximum of 29.5°C and extreme maximum of 36.1°C, and during April,

averaging a maximum temperature of 26.4°C and an extreme maximum of 34.4°C –

coinciding with the end of the fog season in October and the beginning in April.

Compared to the mean temperature curve, maximum temperatures decline during the

summer months while the mean slowly climbs to its September peak. This decline in

maximum temperatures (Figure 3.10) is attributed to the presence of coastal fog.

Summer fog generally develops from late April to October, with the greatest number of

fog days recorded in early August (Burgess et al., 2004). Research by Xu et al. (2005)

over the subtropical southeast Pacific observed similar results as stratocumulus clouds

produced a cooling effect on atmospheric temperatures. Although the 11.0 percent drop

in average maximum temperatures from April to August does not offset the increase in

mean temperature, it does moderate it. The mean temperature only increased by 3.0°C

from April to August compared to the 5.3°C increase in neighboring San Luis Obispo,

16.5 km inland from Morro Bay.

Morro Bay receives an annual average of 46.8 cm of precipitation per year,

mainly falling from December to March (Figure 3.11). The southern migration of the

Pacific High Pressure system during the winter months allows storms driven by the

southward expansion of the Aleutian Low to track over California. These cool moisture-

bearing storms are strongly influenced by orographic lifting, increasing precipitation

inland with an increase in elevation. For example, San Luis Obispo is at 70 meters and

receives 67 cm precipitation, 20.2 cm more than Morro Bay at 15 meters of elevation.

27

Monthly Average Precipitation for Morro Bay(1994-2004)

9.6

11.1

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2.9

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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(cm

)

3.3 GEOLOGY

Morro Bay is situated within the Southern Coastal Range province, which extends

from San Francisco in the north to the western-most extension of the Transverse Range in

the south. This area is composed, in large part, of the Franciscan subduction zone

complex and is characterized by crustal motion that includes tectonic deformation,

clockwise and counter-clockwise rotation, slip and thrust faulting, uplift and subsidence,

erosion and deposition, and volcanism (Page, 1981; Sedlock et al. 1991). Bounded by

the Santa Lucia Range to the north and the San Luis Range to the south, Morro Bay

occupies a subsiding structural block within the larger Santa Maria Basin and is wedged

Figure 3.11. Seasonal precipitation variation, Morro Bay (1994 – 2004). (NCDC, 2004)

28

between two thrust faults, the Cambria fault to the north and the Los Osos reverse fault to

the south (Orme, 1998), forming the Morro basin.

The origin of the basin-ridge blocks within the Santa Maria Basin began during

the late Cenozoic with the convergence of the Pacific, North American, and the

subducted Farallon plates. With the North American plate overriding the Pacific plate,

the San Andreas transform boundary was formed, separating the Mendocino and Rivera

triple junction. The eastward migration of the San Andreas transform boundary across

what is now California caused transtensional and later, transpressional forces, generating

several sedimentary basins and adjacent ridges (Orme, 1998).

The subsidence of Morro Bay is evident by observing the 80 ka marine terrace

between the southwest margin of the Santa Lucia Range (SLR) and the western end of

the Casmalia Range (CR). Orme’s (1998) use of emergent marine terraces to calculate

tectonic uplift along the Southern California coast has shown that the southwest margin

of the SLR and the western end of the CR have experienced relatively uniform uplift

rates. The 80 ka SLR shoreline lies at 5 meters (above present sea-level) and the CR 80

ka shoreline is measured between 7 – 10 meters, indicating uplift rates of roughly 0.06

meters per 1000 years and 0.09 - 0.125 meters per 1000 years, respectively. However,

the lack of the 80 ka shoreline within the Morro basin indicates probable subsidence.

Although fluvial erosion could account for the lack of marine terraces in this region, the

sudden termination of the 80 ka shoreline against the northern Los Osos Fault suggests

tectonic subsidence.

Lettis and Hall (1994) provide further evidence that the Morro basin is tectonic in

origin rather than erosional. Borehole and gravity data collected around the basin

29

indicate the presence of Quaternary sediments at least as deep as 189 meters below sea

level that overlie Pliocene bedrock. Figure 3.12 illustrates the location of wells and

contours relative to the Los Osos Fault zone. Given that the lowest sea level during the

mid-to-late Pleistocene was 130 to 150 meters below present, fluvial erosion can only

account for less than 150 meters of erosion at best (Lettis & Hall, 1994).

Figure 3.12. Borehole data collected around the Morro Basin (Lettis & Hall, 1994).

30

Morro Bay proper is classified as a mix between a “drowned-river valley” and a

“bar-built” estuary. Emmett et al. (2000) claims that a “drowned-river valley” estuary is

created as the result of river valleys flooding owing to the rise in relative sea level (RSL)

during the late Pleistocene through the mid-Holocene. This is the most common type of

estuarine system found along the west coast of the United States. A “bar-built” estuary is

created when a barrier beach is formed, protecting a semi-enclosed lagoon from littoral

drift and allowing the accumulation of estuarine sediments.

Morro Bay was formed by the flooding of the Chorro Creek and Los Osos Creek

confluence during the peak of the mid-Holocene rise in RSL around 5 to 6 ka (Haltiner,

1991, Wilson et al.,2000). During this period, the rate of RSL rise had slowed,

subsequently promoting the accumulation of aeolian and littoral material, forming the

barrier beach (Haltiner, 1991; Gallagher, 1996). Radiocarbon dating of material from the

center of the barrier beach, as reported by Gallagher (1996), establishes that the barrier

was in place by 5.5 ka, while the marsh itself is as old as 4 ka (Ford, 1997).

At present, Morro Bay itself is a shallow lagoon, approximately 6.5 km long and

3 km at its maximum width. The lagoon occupies approximately 930 ha of open water

during high tide, reducing to 340 ha during low tide and exposing the remaining 590 ha

as mudflats (Haltiner, 1991). The barrier-beach to the west of the lagoon is 6.5 km long,

350 to 600 m wide, and has a maximum elevation of 30 m (Ford, 1997).

3.4 TIDAL ENVIRONMENT

Morro Bay experiences a mixed tidal regime (2 unequal high and 2 unequal low

tides) that is dominantly microtidal (average spring tides < 2.0 m). However, the region

31

also approaches a mesotidal regime (2.0-4.0 m) where, at Mean Lower Low Water

(MLLW), average high water spring tides approximate +1.7 and where average low water

spring tides approximate -0.2 m (National Oceanic and Atmospheric Administration,

2006).

Though Morro Bay experiences microtidal conditions, the strong influence of

tidal processes over those of freshwater (fluvial) inputs is very apparent. Haltiner (1988)

estimated that 1.2 x 107 m3 of tidally-generated water cycles daily through the delta area.

By contrast, on an annual basis, overall daily discharge of freshwater from the Chorro

and Los Osos watersheds is approximately 2.65 x 104 m3. Clearly, the study area is

tidally dominated.

3.5 CHORRO DELTA

The Chorro Delta was formed by the coastal accumulation of fluviatile sediments

where the mouth of Chorro Creek enters the bay. Under normal coastal conditions, river-

borne sediments only accumulate when the rate of sedimentation is greater than the rate

of coastal erosion by littoral, wave, and tidal action. For the Chorro Delta, the presence

of the barrier beach protects the bay from wave and littoral erosion and provides the low

energy environment necessary for a fluvial system, such as Chorro Creek, to deposit

sediments and prograde into the back-barrier lowland.

The delta has three main zones (Figure 3.13): (1) the sub-tidal delta front, (2)

inter-tidal flats, and (3) the salt-marsh. The sub-tidal delta front is located at the distal

end of the delta and remains submerged even during low tide, although some portions are

exposed during low spring tides. Sediment in this area consists of sands and some small

32

gravels within the sub-tidal channels while vegetation is limited to submersible plants,

primarily eel grass (Zostera marina).

The second zone, the inter-tidal flats, is the widest zone and is exposed during low

tide and submerged during the mid-to-high tide. The upper end of this zone, sometimes

referred to as the high-tidal flat, is reached by the peak of the mean high-tide when tidal

velocities slow to zero. This decrease in velocity allows the settling of clays and

eventually forms mud flats. Vegetation growth in this zone is non-existent owing to the

duration of submergence — too much to establish pickleweed and not enough for eel

grass.

Figure 3.13. Intertidal zonation of the Chorro Delta. (USGS, 1989).

33

The third zone, the salt-marsh, is the most diverse vegetation zone. The lower

portion is submerged once daily by the highest of the high tides while the middle region

is only submerged approximately once every lunar month (Ford, 1997), although

saturation of the surface still takes place through subcutaneous piping during the high

tide. Sediment in this zone ranges widely, from clays and silts, fine sands and gravel size

in the lower marsh to coarser sands and pebble-sized material in the upper marsh.

The colonization of pickleweed (Salicornia spp.) and jaumea (Jaumea carnosa)

along the lower salt-marsh defines its distal boundary and is critical to salt-marsh

longevity. The development of plant root systems help bind sediments, preventing

erosion, while the plant stems and leaves act as sediment traps and encourage deposition

(Pethick, 1984). The middle portion of the marsh contains more diverse halophytic

vegetation, expanding to arrow grass (Triclochin, spp.), alkali heath (Frankenia salina),

sea lavender (Limonium, californicum), and saltgrass (Distichli spicata). The upper salt-

marsh is a transition zone and contains some perennial glycophytes, plants that are non-

salt tolerant, such as coyote brush (Baccharis pilularis), cress (Cardamine draba spp.)

and several annual grasses.

Drainage on the delta surface occurs through the fluvial channels and a network

of tidal creeks. The fluvial channels are incised into the delta surface and carry terrestrial

runoff from the local watershed as well as groundwater seepage. The channels are

slightly sinuous with vertical banks in the upper marsh and become more sinuous with a

decrease in gradient further downstream. Channel sediments are classified as estuarine

by Buller and McManus (1979) and are a mix of fluvial sediments in the channel bed

blanketed by clay and silt materials associated with the tidal inundation.

34

The tidal creek networks provide two main functions within the estuarine

environment: (1) to dissipate flood tide energy and (2) to drain sheet flow from the marsh

surface. Unlike a fluvial system which transports terrestrial flow from the local

watershed through the deltaic system into the bay, tidal creeks are dependent on tidal

flooding. Pethick (1992) argues that the initiation of tidal creeks is dependent on the tidal

prism and that tidal creeks are an extension of the mud flats into the salt marsh,

possessing many of the characteristics of the intertidal zone. Morphologically, the width

of tidal creeks decreases exponentially inland. This forces the flood tide to flow into

progressively inefficient channels causing the dissipation of tidal energy through

increased channel friction (Pethick, 1992). Once the ebbing of the tide commences the

sheet flow from the marsh surface drains through these channel networks. Expansion of

these tidal creeks are through the continual headward erosion and bifurcation of the first

order tributaries toward areas of high sheet flow velocities (Pye, 1992).

Additional features in the delta include salt ponds and salt pans within the lower

and upper salt-marsh. Salt ponds are small, unvegetated water-filled catchments along

the lower marsh that fluctuate in volume through marsh sheet flow, piping, and changes

in the water table associated with ebb and flood tide fluctuations. Salinity levels vary,

increasing with drainage and evaporation while decreasing during the flood tide (White,

2002).

Salt pans in the upper marsh are usually dry and fill with water in response to

nodal high tides, precipitation, and over-bank flow from fluvial runoff. The lack of

vegetation is the result of high levels of salinity that blanket the surface. These pans are

35

most likely situated above the water table and are disconnected from the everyday ebb

and flood of the tide.

3.7 CHORRO CREEK – TWIN BRIDGES TO MORRO BAY

There are two channels that are investigated in this thesis, Chorro Creek and

Chorro Distributary (known as Chorro-D). Chorro Creek is a former fluvial channel, that

presently conveys, and is being modified by, bi-daily tidal flows. During exceptional

precipitation events, it carries storm flow from the watershed, which introduces coarse

sediment (pebbles and granules) into the channel. Daily fluvial flow is not found in

Chorro Creek. By contrast, Chorro-D is a fluvial channel that conveys both daily and

storm runoff to the ocean. In its lower reaches, it also conveys tidal water twice daily.

Air photographs taken from 1963 to 1990 show that Chorro Creek was delivering

the primary fluvial flow to the delta, whereas Chorro-D was a lower-order distributary

channel. By 1995, photos showed that sediment aggradation within Chorro Creek forced

the development of a new channel, linking Chorro Creek to Chorro-D north of the Twin

Bridges (Figure 3.14) (Ford, 1997). Currently, under normal conditions, Chorro Creek

(near the Twin Bridges) has become infilled with sediment and choked with vegetation,

only holding stagnant water, while Chorro-D continues to deliver the daily fluvial flow

(Figure 3.15).

Channel avulsion from Chorro Creek to Chorro-D at the location of the Twin

Bridges (Figure 3.1) is a response to sediment aggradation from natural processes (such

as fires along the watershed ridges increasing erosion and ultimately sediment into the

system) and anthropogenic alterations to the natural environment, including the

36

abandonment of the Chorro Creek floodplain (north of the delta) as a result of levee

construction, increases in urban runoff associated with economic growth in the region,

and impediment of creek flow from Chorro Reservoir (until the 1994 release

requirements were placed by the CWRCB). With the daily freshwater fluvial flow

diverted to Chorro-D, Chorro Creek has been abandoned as the primary fluvial channel

and is subject to tidal processes except during large storm events, where large-sized

material is transported during infrequent flooding.

37

New Channel

Chorro Creek

Chorro-D

Figure 3.14. Changes in channel form and creek flow in Chorro Creek (Ford, 1997).

Channel Changes at the apex of the Chorro-Los Osos delta (1963-1995)

38

Chorro Creek

Chorro-D

Figure 3.15. Current state of Chorro Creek and Chorro-D just below the Twin Bridges at the apex of the Chorro Delta. Chorro Creek is choked with vegetation and the flow is stagnant, symptomatic of an abandoned fluvial channel. By contrast, Chorro-D is an active fluvial channel, carrying the daily flow from the Twin Bridges into Morro Bay.

39

CHAPTER 4

METHODOLOGY

This study examines the morphologic characteristics of two stream channels in a

complex environment where fluvial processes are influenced by both freshwater and tidal

water input. The study assesses the differences in stream morphology, sediment

distribution, and the nature of channel shape under these two different physical processes,

tidal and fluvial. The methodology is presented in this chapter, which is divided into two

sections: field/laboratory and technical. Field and laboratory methodologies include the

longitudinal and at-a-station surveys of the creeks and delta surface. Additionally, the

collection and laboratory analysis of channel bed and water samples (measuring total

suspended solids, turbidity, and conductivity) is included. The technical methodology

includes the interpretation of aerial photographs and the application of ArcGIS to assist

with map registration.

4.1 PHYSICAL METHODOLOGY

A longitudinal survey of the stream channels, stream cross-sections, and the delta

surface was accomplished using a Nikon Total Station and Automatic Level. In order to

determine channel length and depth, both creek channels were surveyed by placing the

rod at points along the thalweg and recording the Height of Rod (HR), Vertical Angle

(VA), Slope Distance (SD), Horizontal Distance (HD), and Vertical Distance (D). Cross-

sectional points were collected also by placing the rod from bank, to mid-channel, to

thalweg, to the opposite bank (Figure 4.2). The areas surveyed are illustrated on the map

40

in Figure 4.1. In all, 455 survey points were collected and used to calculate the cross-

sectional area, wetted perimeter, hydraulic radius, slope, mean velocity (ms-1), discharge

(m3s-1), and width/depth ratios of the channels. Surveying the delta surface slope

required collecting points in a straight line across the delta in all cardinal directions, north

to south and west to east.

Surveyed Areas of Chorro Creek and Chorro DeltaCreek SurveyDelta Survey

Surveyed Areas of Chorro Creek and Chorro DeltaCreek SurveyDelta Survey

Figure 4.1. Survey points along the Chorro Delta were taken in both creek channels and along the delta surface.

41

The estimation of the mean flow velocity (ms-1) was accomplished using

Manning’s Equation:

S0.50 R0.65 V = K

n Where: V = mean velocity K = 1 (metric units) OR 1.486 (English/Imperial units). S = slope of reach (energy gradient) R = hydraulic radius of channel = A/P where A is the cross-sectional area and P is the wetted perimeter.

while discharge (m3s-1) was calculated using:

Q = A*V

Where Q = Discharge A = Cross-sectional area V = Velocity of flow

4.1.1 SEDIMENT COLLECTION AND ANALYSIS

Sediment samples were collected and examined for six different locations, with

three locations per creek. Chorro Creek and Chorro-D were split into three different

sample locations, lower (Sites A & A-D), middle (Sites B & B-D), and upper (Sites C &

C-D), where the hyphenated suffix of “D” denotes “Distributary.” Each sample location

was divided into three sections: bank, mid-channel, and thalweg (Figure 4.2). Two sets

of samples were collected at the surface of each section using a bulb planter. One set was

collected after the higher-high tide (1.44 m, 4/8/2005) and the other after the lower-high

tide (1.03 m, 4/9/2005).

42

Figure 4.2. Location of sample sites (A through C-D) and channel cross-sectios and area of channel where sediment samples were collected.

43

Most samples (90 percent) were dry sieved using ½ phi intervals, except where

there was a high concentration of clays and silts – those samples were wet sieved.

Recommended sample size of Buller and McManus (1979) is 100-250 g for coarse-sized

sand and 10-100 g for finer sands. Given the diversity of sediment size between the

bank, mid-channel, and thalweg, the average sample size collected was 300 (dry weight)

to provide adequate representation of coarse to fine materials.

To ensure that the sediment was disaggregated, a mortar and pestle were used to

break-up any material before it was added to the nest of sieves. The sieves were set on a

vibrapad and timed for 12 minutes, allowing enough material to fall through the sieves to

ensure reproducible results.

Given the cohesive nature of clay and silt material, wet sieving was used to

separate the clays and silts from the larger sand and pebble sized sediment for several of

the bank samples. Prior to wet sieving, the samples were soaked over night in a Calgon

(sodium hexametaphosphate) solution to disperse the clay material that had adhered to

other sediments. Once the clays and silts were separated, the remaining material was dry

sieved using the methods discussed above.

After sieving, the grain size data were characterized by using the graphic method

of statistical analysis with a phi-based (φ) formulae. The method chosen was based on

the best technique to provide the most accurate information within an estuarine

environment (Dyer, 1979). The phi (φ) system was best suited for analyzing estuarine

sediments, versus method of moments, owing to the open-ended distribution associated

with silt and clay material. In this study, any material that passed through a 4φ (#200)

44

sieve was classified as silt and clay, therefore rendering any smaller size classification

open-ended because additional detail in sediment size was not necessary for this study.

Using Folk’s (1974) methods of analysis, the graphic mean (Mz), sorting (σI), and

skewness (SkI) were calculated and the cumulative frequency curves and sample weight

distributions were graphed. The graphic mean was used for the measure of central

tendency, around which other values will cluster. Compared with using the mode or the

median, the graphic mean is superior because it is based on three points of measure and

gives an overall picture of the sediment distribution. The formula for the graphic mean

is:

Sorting (σI) measures the uniformity of the sediment sample by measuring the

spread of data about the graphical mean. This method covers 90% of the sample

distribution, including the “tails” of the distribution curve. It is the average of the

standard deviation derived from φ16 and φ84 and the standard deviation derived from the

φ5 and φ95. The formula to calculate the sorting value, also called the inclusive graphic

standard deviation (σI), is:

The verbal classification for sorting (σI) is:

under 0.35φ = very well sorted0.35 to 0.50φ = well sorted0.50 to 0.71φ = moderately well sorted0.71 to 1.0φ = moderately sorted1.0 to 2.0φ = poorly sorted2.0 to 4.0φ = very poorly sortedover 4.0φ = extremely poorly sorted

45

Skewness measures the degree of symmetry and determines the tendency of data

to spread to one side of the average, essentially defining the “tails” of the curve. The

“tails” of the curve are thought to be the prevailing indicator that describes the

depositional environment. Symmetrical curves that have a tail to the right have a positive

skewness and excess fine material. Conversely, those curves that have a tail to the left

have a negative skewness and excess coarse material. The formula for skewness is:

The verbal classification for skewness is:

1.00 to 0.30 strongly fine-skewed0.30 to 0.10 fine-skewed0.10 to -0.10 near-symmetrical-0.10 to -0.30 coarse-skewed-0.30 to -1.00 strongly coarse-skewed

4.1.2 WATER CHARACTERISTICS AND SUSPENDED SEDIMENT

Water and suspended sediment samples were collected to measure the tidal

impact of salt water intrusion at each study site. Using a DH-48 suspended sediment

sampler, water was collected approximately at mid-channel depth with the water intake

facing topographically upstream at a depth of 0.5*D (D = Depth). Two samples were

collected at each location, one at high tide (1.03m) and the other at low tide (0.10m).

46

Laboratory analysis included tests for conductivity, turbidity, and total suspended

solids. Conductivity (μmho/cm: microohms per centimeter) measures the amount of salt

water intrusion by analyzing the ability of an aqueous solution to carry an electric

current. The ability to carry an electric current is directly proportional to the presence of

ions and the temperature within the water. All samples were tested using Standard

Methods for the Examination of Water and Wastewater, method 2510B. To ensure

consistency between tests, all samples were tested at 25°C. Holding temperature

constant, the factor for conductivity was the number of ions in the water solution, which

is directly related to the salinity level. Therefore, the greater the level of salinity within

the water, the higher the conductivity.

Turbidity describes the clarity of water and is caused by suspended and colloidal

material such as clay, silt, inorganic and organic matter. Specifically, this test monitors

light that is scattered and absorbed at a 90° angle rather than transmitted with no change

in direction through the sample and is measured in nephelometric turbidity units (NTU).

Therefore, as the quantity of suspended matter increases within the water column, the

turbidity also increases. Within an estuarine environment, turbidity would change

significantly between tidal cycles, increasing with the high tide and decreasing with the

low tide.

The last water sample test was for total suspended solids. Total suspended solids

refer to material that is suspended in a column of water that is filterable up to 2μm. This

is a direct test of the tide’s ability to transport suspended material such as clays and silts

throughout the Chorro Creek channels. Each sample was weighed on a glass-fiber filter

47

(Whatman 934-AH) and baked at 103°C (Standard Methods 2540D). The data results

were calculated as:

(A-B)*1000 / C

A = weight of filter + dried residue B = weight of filter C = sample volume (mL)

4.2 TECHNICAL METHODOLOGY

Technical methods include the analysis of aerial photograph to calculate channel

characteristics. Aerial photograph interpretation was used to calculate channel sinuosity

and mapping sample locations. The aerial photograph (2000) was registered to an

ArcGIS 8.0 road layer, and a concurrent scale was set. Once this was completed,

sinuosity was calculated by dividing the creek channel distance by the valley distance.

48

4.3 OVERVIEW

In summary, many challenges were encountered while surveying and collecting

samples in the field, including the need to work quickly between tidal cycles, forging

through soft mud that encouraged instant sinking (Figure 4.3), and endurance to walk and

map through 10 km of thick vegetation and channel crossings (Figure 4.4).

Figure 4.3. Typical conditions within the middle marsh.

Figure 4.4. Typical conditions within Chorro Creek

49

CHAPTER 5

DATA PRESENTATION

This chapter evaluates the results of data collected for Chorro Creek and Chorro-

D. Chorro Creek was once the primary drainage path for freshwater fluvial flow from the

Chorro watershed. Current channel morphologies, such as channel shape, sediment

distribution, and entrenchment are fluvial in character, ruling out tidal inception.

The avulsion from Chorro Creek to Chorro-D is evident through the analysis of aerial

photograph interpretation, showing the infill of sediment in Chorro Creek just below the

Twin Bridges has isolated Chorro Creek from the daily fluvial flow.

Data were collected to monitor the changing dynamics within the Chorro Creek

channel and to analyze the impact of tidal processes on channel modification.

Additionally, Chorro-D was evaluated to further knowledge on the impact of bi-daily

tidal fluctuations on a fluvial system and to provide a comparison between the current

state of Chorro Creek (tidally dominated) and its prior conditions (fluvially dominated).

The remainder of this chapter will focus on a detailed description of each of the six site,

A through C-D.

50

5.1 SITE A

Site A was located at the southernmost section of the main channel of Chorro

Creek along the lower margin of the salt marsh (Figure 5.1). This site is located on the

second bend (meander) from the confluence of Chorro Creek and Morro Bay and has the

widest cross-section of all the channel study sites. The channel profile in this area is

asymmetric with the steep-wall cutting bank on the west side. Bankfull capacity is

reached twice daily and over-bank flooding is associated with the less frequent

maximum-high tides. Vegetation is limited to saline adapted halophytes, dominantly

Salicornia and Jaumen.

Figure 5.1. Site A is along the lower Chorro Creek and is tidally dominated. During most low tides, the point bars are exposed while the steep-wall bank pools with estuarine water.

51

5.1.1 STREAM GEOMETRY AND DISCHARGE

Site A is tidally-dominated though the channel shape reflects the freshwater

fluvial processes that initially formed it. This site has a cross-sectional width of 32.9

meters and an average depth of 0.65 m (Figure 5.2). A maximum depth of 1.02 m was

recorded adjacent to the steep-wall bank in the thalweg, an area that is characterized by

higher flow velocities, coarser sediments, a steeper bank slope (28%) and bank erosion.

The east side of the channel is characterized with a lower slope (2%), finer material, and

sediment accumulation as evident by the establishment of a point bar.

Site A Cross-Sectional Profile

-3

-2.5

-2

-1.5

-1

-0.5

0

33

31.530

28.527

25.524

22.521

19.518

16.515

13.512

10.597.56

4.53

1.50

Distance (m)

Dep

th (m

)

West East

Figure 5.2. A cross-sectional profile of Site A looking upstream.

52

The channel survey data (Figure 5.3) demonstrate the bankfull mean flow velocity

is 0.87 ms-1 and would be capable of entraining, transporting and eroding any material

smaller than -3.00φ (Summerfield, 1991). The maximum discharge of 18.6 m3s-1 was

estimated at bankfull. Being that Chorro Creek is devoid of fluvial flow, the above

estimates only show past fluvial potential – Chorro Creek only reaches bankfull capacity

during the flood tide. Haltiner (1991) estimated that the tidal prism is 6.17 x 106 m3, but

as stated by Pethick (1992), estuaries receive ‘an amount of water not in proportion to a

fixed drainage [as in a fluvial system] but determined by the capacity of the channel

supplying the water.’ Chorro Creek’s capacity at bankfull for the surveyed area (Sites A

– C) was calculated, given the distance of the channel and the average cross-sectional

area, to be 1.23 x 104 m3. A mean tidal velocity of 0.02 ms-1 was estimated using

Manning’s Equation and corresponds well with the value of 0.01 ms-1 recorded in Morro

Bay tidal creeks by CSUN students (2002). The difference of 0.01 ms-1 between Chorro

Creek and the connecting tidal creeks is not uncommon given the nature of tidal creeks is

to dissipate tidal energy.

Site A Hydrological Analysis Channel Width (m) 32.90 Average Depth (m) 0.65 Cross-Sectional Area (m2) 21.37 Wetted Perimeter (m) 33.03 Hydraulic Radius 0.65 Channel Slope 0.0012 Mean Velocity – Bankfull (ms-1) 0.87 Mean Velocity – Estimated Tidal (ms-1) 0.02 Discharge (m3/s-1) 18.59 Width/depth ratio 50.95

Figure 5.3. Calculations of Site A channel characteristics. The mean velocity and discharge represent conditions at maximum bankfull discharge.

53

5.1.2 SEDIMENT CHARACTERISTICS

Sediment size distribution was diverse between the three sample locations (bank,

mid-channel, thalweg) and ranged in mean size from granules (-1.34φ) in the thalweg to

fine sands (1.81φ) at the bank. All samples were poorly to very poorly sorted and most

were coarsely skewed, excluding the Low High Tide thalweg sample. Figure 5.4 shows

the cumulative frequency curve for samples collected after the Higher-High Tide (HHT)

and the Lower-High Tide (LHT) and their corresponding locations: bank, mid-channel,

and thalweg.

Cumulative Sediment Weight for Site A

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-5.25 -4.25 -3.25 -2.25 -1.75 -1.25 -0.75 -0.25 0.25 0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.00

Pebble & Granule Coarse Sand MediumSand

Fine Sand Silt&

ClayPhi Size

Bank - HHT

Bank - LHT

Mid-Channel - HHT

Mid-Channel - LHT

Thalweg - HHT

Thalweg - LHT

Figure 5.4. Cumulative weights for samples collected at Site A.

54

Bank Samples

Sediments collected after the HHT and LHT at the bank ranged in mean size from

medium sands at 1.41φ to fine sands at 1.81φ respectively. The HHT sample consisted of

46.6% sand size sediments while 33.3% of the sample was clays & silts. The remaining

20.4% fell into the pebble to granule size range with 13.5% granule and 6.9% pebble.

The LHT sample had a higher percentage of silts and clays compared to the HHT sample,

at 39.1%, and the remaining material was distributed with 45.5% as sand and 15.5% and

granule or larger.

A skewness value of -0.47 and -0.57 (HHT and LHT respectively) indicates that

the material along the bank is strongly coarse-skewed and that the sample has a coarse

tail. Sediment was also poorly-sorted, testing 2.57φ (HHT) and 2.29φ (LHT). This

asymmetry of the sample is not uncommon in estuarine environments and implies that the

presence, even though minimal, of coarse material is beyond the normal distribution of

sediment range in relationship to the mean size of the sample.

Mid-Channel Samples

Mid-channel sediments collected after the HHT and LHT were dominantly coarse

sands and ranged in mean size from -0.94φ to -0.53φ respectively. Unlike the bank

samples, the mid-channel only contained 1.1% clay and silt sized material for both HHT

and LHT samples. Sand sized material registered at 57.9% and 68.6% of the sample

weight and 41.1% and 30.3% for granule size material or larger.

The skewness values for this section of the channel were near symmetrical, with

-0.01 for the HHT and -0.13 for the LHT, indicating that all the material in these samples

fall within a normal distribution from the mean. The sorting values of 1.39 φ for the

55

HHT and 1.48φ for the LHT were the lowest of all the samples collected at this site but

still indicated poorly sorted material.

Thalweg

Thalweg sediments collected after the HHT and LHT ranged in mean size from

granule (-1.34φ) to very coarse sand (-0.34φ). This region contained the lowest portion

of clay and silt material with 0.7% and 0.9% for the HHT and LHT samples respectively.

The dominant material for the HHT sample was pebble to granule in size (55.8%) and

was distributed as 15% pebble and 40.8% granule. The remaining 43.5% of the sample

contained sand size material.

The sample collected after the LHT differed drastically with 72% of the sample

being sand, 48% coarse and 24% medium in size. The remaining 27.1% included granule

size or larger sediments.

Skewness values for the thalweg samples reflected the differing sample size

distribution. The HHT sample was almost near symmetrical, leaning towards being

finely-skewed at 0.12. This implies the presence of excess fine material, although in a

very small fraction of the total sample. The LHT sample provided an opposite result with

a skewness value of -0.23, indicating excess coarse material. This sample was poorly-

sorted, testing 1.83φ for the HHT and 1.50φ for the LHT, owing to a wide range of

sediment sizes within an estuarine environment.

5.1.3 WATER CHARACTERISTICS AND SUSPENDED SEDIMENT

Water samples were collected in the thalweg during the high and low tide. The

data in Figure 5.5 suggest that Site A is tidally dominated and receives no fluvial input.

56

Conductivity tests demonstrated that Site A had the highest levels of salinity compared to

the other five sites, with the high tide sample value of 38,320 µmho cm-1 (micro-ohms per

centimeter) and with low tide sample 10% less (34,640 µmho cm-1). Compared to average

ocean water, with 53,000 µmho cm-1, the tidal currents appear to experience some

freshwater mixing. The decrease in conductivity at low tide is attributed to the denser

saline water coming into contact with the less-dense saline water.

Tidal Phase

Conductivity (µmho/cm)

Turbidity (NTU*)

Total Suspended

Solids (mg/L) LHT (1.11 m) 38,320 16 47.2 HLT (0.14 m) 34,640 5.6 26 *NTU - nephelometric turbidity unit.

The second battery of tests analyzed water turbidity, or clarity. High tide turbidity

levels were recorded at 16 NTUs, indicating very poor water visibility. Unlike the

conductivity levels, turbidity levels were greatly reduced with the ebbing of the tide,

registering at 5.6 NTUs. The increase in turbidity associated with the incoming tide

indicates that there is a significant amount of suspended and colloidal material carried by

the flow. Even though the NTU value of the low tide sample was 65% less than the high

tide sample, it is still greater than turbidity levels from freshwater reservoirs, such as

Castaic Lake, at 0.6 NTU or Lake Matthews, at 1.6 NTU (Metropolitan Water District,

2005). This indicates that there is still a significant source of suspended sediments in the

water subject to deposition during this slack tide period.

The last test, total suspended solids (TSS), confirmed the turbidity tests. TSS

levels were measured at 47.2 mg/L during the high tide and dropped by 44.9% to 26.0

Figure 5.5. Results from the Site A water samples collection during the Lower-High Tide (HT) and Higher-Low Tide (LT).

57

mg/L during the low tide. These results suggest that the incoming tide carries a

significant amount of suspended sediment which is injected into the channels during the

flood and the remainder that does not drop out of suspension during slack tide is flushed

back into the bay. These values were the highest of all the sample sites under each tidal

condition and significantly greater than those measured in the distributary.

58

5.2 SITE B

Site B was located in the main channel of Chorro Creek, approximately 0.5 km

north of Site A (Figure 5.6). This area occupies the northwest portion of the salt marsh

and is adjacent to State Park Road, also making it the most accessible site. The channel

profile is largely symmetric with the thalweg adjacent to the eastern bank. Like Site A,

bankfull capacity is also reached twice daily with the flood and ebb flow of the tide, but

unlike Site A over-bank flooding is rare. Saline tolerant vegetation still dominates this

region of the salt-marsh although plant species are more diverse.

Figure 5.6. Site B during low tide, looking upstream.

59

5.2.1 STREAM GEOMETRY AND DISCHARGE

Site B is an entrenched channel confined by steep banks on both sides of a slightly

convex channel bed (Figure 5.7). The cross-sectional width is 22.3 m with an average

depth of 0.47 m – the shallowest of all six sites profiles. The east bank of the creek has a

58% slope and a thalweg depth of 0.62 m. The west bank is characterized with a 34%

slope and the presence of a sub-tidal channel superimposed on the main channel.

Site B Cross-Sectional Profile

-3

-2.5

-2

-1.5

-1

-0.5

0

0 1.5 3 4.5 6 7.5 9 10.5 12 13.5 15 16.5 18 19.5 21 22.5

Distance (m)

Dep

th (m

)

West East

The channel survey data (Figure 5.8.) demonstrates the mean velocity for this

reach of Chorro Creek is 0.70 ms-1 and the maximum bankfull discharge is 7.32 m3s-1.

Like the previous site, this portion of the channel is tidally-dominated and does not

Figure 5.7. A cross-sectional profile of Site B looking upstream.

Superimposed sub-tidal channel

60

receive daily fluvial flow. Sediment transport and deposition are limited to that carried

by the flood and ebb of the tide.

Site B Hydrological Analysis Channel Width (m) 22.3Average Depth (m) 0.46Cross-Sectional Area (m2) 10.44Wetted Perimeter (m) 22.51Hydraulic Radius .46Channel Slope 0.0012Mean Velocity (ms-1) 0.70Discharge (m3s-1) 7.31Width/depth ratio 47.64

5.2.2 SEDIMENT CHARACTERISTICS

Sediment size distribution was diverse between the three sample locations and

ranged in mean size from granule sized material (-2.55φ) to coarse sands (-1.11φ).

Unlike Site A, this region did not show an increase in sediments from the bank to the

thalweg. Mid-channel sediments were the largest, followed by the thalweg samples and

finally by the bank samples. Figure 5.9 shows the cumulative frequency chart curve for

samples collected after Higher-High Tide (HHT) and Lower-High Tide (LHT) and their

corresponding locations: bank, mid-channel, and thalweg.

Bank

Sediments collected along the bank were classified as coarse sands and ranged

from -1.06φ for the HHT and -1.12φ for the LHT. Overall, the HHT sample contained

54.7% sand size material with 51.7% sorted as coarse sand while the remaining 3% were

Figure 5.8. Calculations of Site B channel characteristics. The mean velocity and discharge represent conditions at maximum

61

Cumulative Sediment Weight for Site B

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-5.25 -4.25 -3.25 -2.25 -1.75 -1.25 -0.75 -0.25 0.25 0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.00

Pebble & Granule Coarse Sand MediumSand

Fine Sand Silt&

ClayPhi Size

Bank - HHTBank - LHTMid-Channel - HHTMid-Channel - LHTThalweg - HHTThalweg - LHT

medium to fine sand sizes. The pebble and granule material made up 40.4% of the HHT

sample while the remaining 5% were silts and clays.

The LHT sample plotted very close to the HHT sample on the cumulative curve

chart but contained more granule and pebble sized material, measuring at 43.0% of the

total weight. Sand size material made up the majority of this sample with 51.4% while

silt and clay sediments lagged with 5.6%.

A skewness value of +0.27 and +0.22 for the HHT and LHT indicates that the

material along the bank is finely-skewed with both samples containing fine tails.

Figure 5.9. Cumulative weights for sample Site B.

62

Additionally, both samples were poorly sorted, with values of 1.34σ and 1.43σ for the

HHT and LHT samples, respectively.

Mid-Channel

Sediments collected along the mid-channel of this site had a bi-modal distribution

with a value of -2.55φ (HHT) and -2.16φ (LHT), in the granule range. The HHT sample

contained 69.8% pebble and granule sized material, with 60.9% classified as pebble.

Sand sized material on