CRREL REPORT 95-5

Initial Analyses of Eagle River Flats

Hydrology and Sedimentology,

Fort Richardson, Alaska

Daniel E. Lawson, Susan R. Bigl, John Bodette and

Patricia Weyrick March 1995
For conversion of SI metric units to U.S./British customary units of measurement

consult Standard Practice for Use of the International System of Units (SI), ASTM

Standard E380-89a, published by the American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa. 19103.

Abstract

The physical environment of Eagle River Flats (ERF), a subarctic tidal flat

and salt marsh, is progressively changing because of the interactions of

multiple physical processes, including a high tidal range, two primary sediment sources, cold climate and location within an active earthquake zone.

In addition, ERF has been used by the U.S. Army as an artillery range, where

high explosives or smoke-producing shells have been detonated, causing

cratering and disrupting drainage. The physical environment of ERF needs to

be understood to help remediate a problem of unusually high mortality rates

in migrating waterfowl. This high mortality of ducks is attributable to ingestion of elemental white phosphorus (P4) particles (from smoke-producing

devices), which are now distributed within near-surface sediments of the

ponds and marshes. The complexity of this dynamic environment makes it

extremely difficult to predict what physical effects remedial measures for the

P4 contamination will have and, conversely, what short- and long-term effects the physical system will have on the effectiveness and success of proposed remedies. Understanding both the system’s response and the effects

of remedial technologies is critical to deciding what measures are used. This

report presents the initial analysis of the physical processes of erosion, sedimentation and sediment transport and the factors controlling their activity

within a portion of ERF.

Cover: Peak tidal flooding inundates innermost pond areas of Eagle River Flats.

Area B, located in Figure 2 of text, is shown during peak flooding event

during May 1994.
CRREL Report 95-5

Initial Analyses of Eagle River Flats

Hydrology and Sedimentology,

Fort Richardson, Alaska

Daniel E. Lawson, Susan R. Bigl, John Bodette and

Patricia Weyrick March 1995

Prepared for

U.S. ARMY ENGINEER DISTRICT, ALASKA

Approved for public release; distribution is unlimited.

US Army Corps

of Engineers®

Cold Regions Research &

Engineering Laboratory
ii

PREFACE

This report was prepared by Dr. Daniel E. Lawson, Research Physical Scientist,

Geological Sciences Division, Susan R. Bigl, Research Physical Scientist, and John H.

Bodette, Engineering Technician, Civil and Geotechnical Engineering Research Division, and Patricia Weyrick, Physical Science Technician, Snow and Ice Division, Research and Engineering Directorate, U.S. Army Cold Regions Research and Engineering Laboratory. Funding and support for this research was provided through

the U.S. Army Engineer District, Alaska, Fort Richardson.

The authors thank William Gossweiler, Douglas Johnson, William Smith and

Laurie Angell for helpful discussions, logistical support and assistance in the field,

as well as their continuing support, and Dr. Charles Racine, Marianne Walsh and

Charles Collins for numerous discussions on the Eagle River Flats project and their

assistance in the field and laboratory. They also thank Lawrence Gatto and Lewis

Hunter for their critical review of the draft manuscript of this report.

The contents of this report are not to be used for advertising or promotional purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.
CONTENTS

Preface ........................................................................................................................................ ii

Executive Summary ................................................................................................................. v

Introduction ............................................................................................................................... 1

General environmental conditions......................................................................................... 2

Previous studies ........................................................................................................................ 3

Study area and methods .......................................................................................................... 4

Sedimentation and erosion ................................................................................................. 5

Gully erosion and recession ............................................................................................... 7

Water quality parameters ................................................................................................... 7

Process observations ................................................................................................................ 10

Glacier runoff and sediment yield ......................................................................................... 16

Tidal hydrology and inundation ............................................................................................ 16

Water quality parameters and tidal flats hydrology ........................................................... 19

Sediment transport ................................................................................................................... 23

Sediment sources ...................................................................................................................... 28

Erosion dynamics and gully hydraulics ................................................................................ 28

Erosion and recession rates ..................................................................................................... 31

Sedimentation rates .................................................................................................................. 32

System dynamics and remediation ........................................................................................ 34

Literature cited .......................................................................................................................... 34

Appendix A: Recession rate summary .................................................................................. 37

Abstract ...................................................................................................................................... 39

ILLUSTRATIONS

Figure

1. Upper Cook Inlet region showing ERF and several other estuarine salt marshes 1

2. General distribution of ponds and the primary drainage system of gullies and

vegetated channels within ERF ............................................................................... 2

3. Distribution of primary landform–vegetation units .................................................. 3

4. Location of sedimentation study sites ......................................................................... 4

5. Sedimentation stakes ...................................................................................................... 5

6. Paint layer within block of sediment as measured to define sedimentation rates 6

7. Sedimentation station in a pond site ............................................................................ 6

8. Location of gully headwall erosion study sites .......................................................... 7

9. Headwall erosion study site .......................................................................................... 7

10. Location of surveyed longitudinal cross sections of gullies ..................................... 8

11. Locations of instrumentation recording water quality parameters and water

depths ......................................................................................................................... 8

12. Platform and instrumentation measuring discharge characteristics at the

Bread Truck gully location ...................................................................................... 9

13. Aerial photographs of ERF ............................................................................................ 12

14. Drainage pattern analysis of 1993 aerial photograph in area east of Eagle River . 15

15. Water parameter data from Eagle River site just downstream of Route Bravo

bridge .......................................................................................................................... 17

iii
iv

Figure

16. Water elevation at three sites during the 19 August evening flood event ............. 18

17. Water elevations at three sites during a period of flooding cycles .......................... 18

18. Elevation of waters in Eagle River during September flooding cycles ................... 19

19. Variation of water quality parameters, Break Truck drainage site, 18 August ..... 20

20. Variation of water quality parameters, 20–21 August ............................................... 21

21. Variation of water quality parameters, 18–19 September ......................................... 22

22. Total suspended sediment concentrations measured at gully sites during

August and September flooding cycles ................................................................. 24

23. Total suspended sediment concentrations and water elevations at Parachute

drainage, 17–18 September ...................................................................................... 25

24. Optical backscatter measurements at gully sites during August and September

flooding cycles ........................................................................................................... 26

25. Comparison of OBS and TSS measurements at Parachute Pond ............................. 27

26. OBS measurements from Knik Arm during late September .................................... 28

27. Profiles of elevations along gully thalwegs of two drainages .................................. 29

28. Places where gully cross sections and peak flow were estimated ........................... 29

29. Idealized cross sections at the Bread Truck and Parachute drainages.................... 30

30. Examples of spatial variation in erosion rates measured at gully headwall sites . 32

TABLES

Table

1. Specifications of water quality parameter sensors ..................................................... 9

2. Summer erosional processes ......................................................................................... 16

3. Summer depositional processes .................................................................................... 17

4. Temperature and salinity of Eagle River and Knik Arm .......................................... 23

5. Gully parameters............................................................................................................. 30

6. Headwall recession rates in 1992 and 1993 ................................................................. 31

7. Gross sedimentation rates in 1992 and 1993 ............................................................... 33

8. Number of flooding events reaching critical heights to cover landforms in 1992

and 1993 ...................................................................................................................... 33
v

gullies and in the Eagle River at the head of the

Flats. These measurements are to define the timing and distribution of flood and ebb waters and

to qualitatively assess the contribution of the

Eagle River discharge to flooding in the study

area. Suspended sediment concentrations were

measured in the Eagle River, in gullies draining

the Flats, and in Knik Arm to define the relative

contribution of each source to pond and mudflat

sedimentation. Salinity, temperature, dissolved

oxygen, pH and turbidity were measured to

assess the relative contributions of tidal or riverine water masses to flood and ebb cycle sedimentation and erosion. Physical parameters, including ground elevation, drainage pattern, channel

gradients, flow velocity and gully dimensions,

are necessary to calculate ebb drainage and sediment transport rates.

Sedimentation in the C and Bread Truck

ponds complex is several millimeters per year on

levees, 10–15 mm per year on the mudflats, and

20–40 mm per year in the ponds. Limited data on

sediment concentrations in tidal flood waters of

Knik Arm indicate that they exceed those of Eagle

River discharge in August and September,

suggesting that sediments are predominantly tidal (i.e., water masses of the Knik Arm). Sedimentation rates of 20 to 40 mm/year, if characteristic,

are relatively high and would aid in infilling of

ponds remediated by dredging or temporary

draining.

Erosion and recession rates for the headwalls

and lateral walls of gullies are also relatively

high. Gullies are progressively extending into the

mudflats toward the C and Bread Truck ponds

complex. Headwalls and adjacent lateral walls receded at highly variable rates, ranging from 0.1–

4.9 m during the summer of 1992, to 0.4–6.3 m

during the winter of 1992–1993, and to 0.0–9.8 m

during the summer of 1993. Large tidal floods in

March, April, August and September of 1993

caused the greatest headwall recession. Two gully headwalls, one on the western side of Bread

Truck Pond and the other near the pond complex

between Bread Truck and C ponds, are advancing at rates sufficient to increase the drainage of

those ponds within the next 10–15 years, if the

EXECUTIVE SUMMARY

Eagle River Flats (ERF) is an estuarine salt

marsh with a physical environment that is in a

state of flux. Significant changes result from

numerous forces, including a high tidal range,

glacial river influences, large sediment influxes

from two distinct sources, and a subarctic coastal

climate. ERF is located within an active earthquake zone, further complicating process interactions and increasing the potential for rapid, but

probably unpredictable, physical changes in the

future. In addition, the U.S. Army has used ERF

as an artillery range since the early 1940s. Ground

explosions have caused physical changes to the

terrain, hydrology and surface drainage.

Understanding the processes controlling the

physical environment of Eagle River Flats is important because of our need to understand and

remediate a problem of unusually high mortality

rates in the migratory waterfowl that visit this

area each spring and fall. Previous studies have

shown that the high duck mortality is attributable

to ingestion of elemental white phosphorus (P4)

particles (Racine et al. 1992a,b) that come from

smoke-producing devices detonated either on or

above the ground (Racine et al. 1992a). White

phosphorus particles are now distributed within

near-surface sediments of the ponds and marshes

where dabbling ducks ingest it.

The inherent complexity of this dynamic environment makes it extremely difficult to predict

what effects potential remedial measures for

white phosphorus (WP) contamination will have

on the physical system and, conversely, how the

short- and long-term dynamics of the physical

system will influence proposed remediations.

Understanding both the system’s response and

the effects of such measures is critical to deciding

what remedial technologies should be used.

Studies begun in 1992 initially appraised sedimentation and erosion rates in the C and Bread

Truck pond areas. In 1993, we began to analyze

the factors and processes controlling erosion,

transport and sedimentation in this study area,

while continuing and expanding our sedimentation and erosion measurements.

Tidal height fluctuations have been measured

in concert with water level changes in two major
1993 rates persist. In the latter case, the headwall

eroded to depths of about 1 m, extending the gully nearly 40 m into the mudflats.

Surveyed longitudinal profiles of gully thalwegs revealed gradients indicating that they will

progressively grow into the mudflats by headwall erosion. Gradients near the Eagle River are

relatively steep (0.01 to 0.02 m/m) in response

to tidally controlled discharge variations in the

river. Up-gully of the steep section, the lower

reaches tend to range from about 0.003 to 0.007

m/m. With distance inland, channel gradients

steepen to the base of the headwall. Headwalls

have nearly vertical faces that vary from 1 to 2.5

m in height. Above them, drainage gradients

across the mudflats into the ponds range from

0.001 to 0.004 m/m, being slightly lower in slope

than within the gullies below the headwall. At

several points in each gully, however, there are

relatively sharp increases in gradient. These

“knickpoints” are places where erosion is causing a progressive up-channel deepening of the

gully thalweg, reducing the gradient in response

to headwall erosion, and extending the gully farther into the mudflats.

Tidal flooding of the Bread Truck and C pond

areas is enhanced by the discharge of Eagle River.

Both the timing and height of maximum inundation are affected. Flood height can exceed the

Anchorage datum by 0.5 m or more, the amount

being highly variable, depending upon the volume of water in the Eagle River that enters ERF.

A comparison of water level variations at the

heads of two gullies in the Bread Truck–C pond

areas and the tidal height variations in Knik Arm

clearly showed such effects on tidal flood and

ebb. In August, for example, seasonally high discharge from glacial meltwater in the Eagle River

supplemented tidal flood heights and significantly increased the areas inundated. In September,

precipitation in the Eagle River watershed of the

Chugach Mountains also increased the height of

two primary high tides, while the remaining

tides were equivalent to Anchorage predictions.

The Eagle River provides access for tidal

waters to inundate the innermost reaches of the

Flats. Water level and stream flow as far inland

as immediately upstream of the Flats proper can

vary with tidal flood and ebb cycles. Our preliminary analyses suggest, however, that sedimentation in the lower two-thirds of ERF is tidally

dominated, whereas the upper one-third appears

to be river dominated. Aerial photographs from

1950, 1967 and 1993 suggest that the river has deposited its sediments upon entering ERF proper,

forming an alluvial fan-like deposit, and distributing water flow in four radially spreading channels across the inner tidal flat. Remnants of

mudflat and tidal pond terrain (e.g., Racine Island) are now mostly buried beneath material deposited during overbank flooding of the river in

this area.

vi
Figure 1. Upper Cook Inlet region showing ERF and several other estuarine salt

marshes (shaded areas).

INTRODUCTION

Eagle River Flats (ERF) is a 1000-ha tidal flat

and salt marsh at the mouth of Eagle River on

Knik Arm, near Anchorage, Alaska (Fig. 1). Previous work by CRREL has shown that an unusually

high mortality of migratory waterfowl within

ERF, particularly ducks, is attributable to ingestion of elemental white phosphorus (P4) particles

(Racine et al. 1992a,b). The primary source of

white phosphorus particles is smoke-producing

shells detonated during training on this artillery

range, which has been active since the early 1940s

(Racine et al. 1992a, 1993). White phosphorus

(WP) particles are now present in near-surface

sediments at numerous locations within ERF, including the pond and marsh bottoms where dabbling ducks ingest them during feeding.

Various remedial measures are being considered to control duck mortality from WP ingestion

(Racine et al. 1993). Proposed remedies, which include dredging, draining ponds, placing geotexInitial Analyses of Eagle River Flats Hydrology and Sedimentology,

Fort Richardson, Alaska

DANIEL E. LAWSON, SUSAN R. BIGL, JOHN H. BODETTE AND PATRICIA WEYRICK
tiles or other ground covers, and enhancing sedimentation, will affect the physical ecosystem of

ERF in ways that are largely unknown. Likewise,

both the short- and long-term viability of these

treatments will be influenced by the physical system, where the occurrence, intensity and interaction of the physical processes remain poorly

understood. Baseline data are still required to define the sensitivity of the Flats to perturbations

and to evaluate its responses to potential remedies. Restoration strategies will depend upon the

methods chosen, but ultimately they must be

compatible with the physical system.

In this report, we present the initial analysis of

the physical system of Eagle River Flats, focusing

on the interrelationships of the hydrological and

sedimentological processes. Although preliminary, these initial results clearly demonstrate the

complexity of the Flats’ physical system and the

need for further quantitative studies.

2

GENERAL ENVIRONMENTAL

CONDITIONS

Eagle River Flats is one of several estuarine

tidal flats and salt marshes within the upper

Cook Inlet region of south-central Alaska (Fig. 1).

It lies at the mouth of Eagle River on the southeast side of Knik Arm and is about 2.75 km wide

at the coast, narrowing inland. This region has a

transitional maritime to continental climate, with

generally moderate annual temperatures (daily

mean 1.9°C; minimum mean –2.2°C) and precipitation (330 to 508 mm) (Evans et al. 1972).

A high tidal range with semi-diurnal fluctuations of 9.1 to 11 m and sediment-laden glacial

discharges are critical factors affecting ERF

hydrology and sedimentology (Lawson and

Brockett 1993). Inundation results from both the

tides in Knik Arm and the resultant overflow

from the Eagle River as it meets the rising tide.

Figure 2. General distribution of ponds (shaded areas) and the primary

drainage system of gullies and vegetated channels within ERF. The primary

study area was within the northeastern area marked by the rectangle.

KNIK ARM

uplands

uplands

Area

D

Pond

Beyond

Bread Truck

Pond

Area

C

EOD

Area

B

Area

A

Racine

Is.

0 1000 m

N
3

Eagle River drains a 497-km2 basin in the Chugach Mountains, 13% of which is covered by glaciers. Average discharge was about 14.7 m3/s

from 1966 to 1981 (USGS 1981). Because of the Eagle River’s glacial origin, its maximum and peak

discharges take place during the primary melt

season of July and August, with peak discharges

exceeding 100 m3/s when there are rainfallinduced floods during high melt periods.

The Eagle River cuts approximately through

the middle of ERF. The area primarily drains laterally from ponds and marshes through vegetated channels and unvegetated gullies into the

river channel (Fig. 2). About 20% of ERF is

drained along the coast by gullies that discharge

directly into Knik Arm. Its north and south boundaries are defined by uplands composed of glacial deposits covered by spruce and birch forests.

Sediments within both the Eagle River and the

tidal waters of Knik Arm and Cook Inlet enter

ERF, and both sources are derived mainly from

glacier runoff. Massive quantities of silt- and

clay-size particles are suspended within the numerous large rivers draining into the Inlet from

glacierized basins in the Alaska Range, Chugach

Mountains and Kenai Peninsula. These materials

can remain suspended for extended periods and

are transported into intertidal wetlands during

tidal flooding. The Eagle River also transports

glacially derived suspended sediment during the

melt season. The quantity of suspended sediment

entering ERF in transport by river and tidal waters is, however, unmeasured.

As with other estuarine salt marshes in Cook

Inlet (e.g., Hanson 1951, Vince and Snow 1984,

Rosenberg 1986), vegetation grows in zones that

are commonly determined by elevation and

related to the landforms of ERF (Fig. 3). This

relationship is believed to be a function of flooding frequency, drainage capacity and sedimentation rates. Levees, mudflats, marshes and shallow ponds (to 50-cm depth) are arranged in a pattern that approximately parallels the Eagle River

and coastline (Racine et al. 1992a). Freshwater

ponds or shrub bogs lie along the upland bordering ERF.

PREVIOUS STUDIES

The fundamental sedimentary and hydrological processes of subarctic and arctic tidal flats

and salt marshes are not well understood; few

quantitative analyses of these processes and the

factors controlling their activity have been made.

Figure 3. Distribution of primary landform–vegetation units

(from Racine et al. 1993).
The magnitude and variability in factors such as

sediment supply, tidal regime, sea-level changes, local climate and storms, and wave and river

activity can alter the relative importance and interaction of the physical processes in ways that

remain poorly defined (e.g., Boon 1975, Frostick

and McCave 1979, Carling 1981, Collins et al.

1987, Dionne 1988, Stoddart et al. 1989, Reed

1990, Dalrymple et al. 1992, Andrew and Cooper

1993). The role of glacial rivers and glacial–marine sediment discharges in tidal flat and salt

marsh sedimentology have received only limited attention (Ovenshine et al. 1976a,b; McCann

et al. 1981; Bartsch-Winkler and Ovenshine

1984). Neither the sediment fluxes nor the sediment budget of tidal flats and salt marshes are

sufficiently defined to link their hydrology and

sedimentology (e.g., Frostick and McCave 1979,

Kohsiek et al. 1981).

Field studies and simulations of short-term

sediment accumulation have suggested that process and morphology are related to elevation,

and thus the annual frequency and depth of inundation and sediment supply, but that longterm rates may be more importantly controlled

by the eustatic rise in sea level (e.g., Krone 1987;

Allen 1990a,b; French 1991, 1993; French and

Spencer 1993; McLaren et al. 1993). Spatial variations in the texture and composition of sediment

deposited within tidal flats and salt marshes are

also expected to relate to the hydrological processes of tidal channels and gullies, as well as

tidal currents (e.g., Carling 1982, Allen 1992).

Sediment balance and the accretion and erosion of sediment in tidal flats and salt marshes

have been estimated using short-term vertical

sedimentation rates (e.g., Harrison and Bloom

1977, Richard 1978, Letzch and Frey 1980, Stoddart et al. 1989). Longer-term estimates have

been made by dating subsurface horizons (e.g.,

Hubbard and Stebbings 1968, Allen and Rae

1988) and creating radionuclide profiles (e.g.,

Bloom 1984, Keene 1971, Kearney and Ward

1986). Both estimates, however, may be misleading owing to compaction after deposition in the

former case and time-dependent accretion rates

in the latter (French 1993).

STUDY AREA AND METHODS

This study focused on the central area of ERF,

east of the Eagle River, extending from west of

the EOD pad to the outer coast (Fig. 2). The

morphological terrain units in this area are

4

Figure 4. Location of sedimentation study sites.

levees, gullies, upper vegetated mudflats and

lower unvegetated mudflats adjacent to ponds

and marshes.

Eleven transects across the representative

morphologic units of the study area were established in May 1992, with one additional added

in June 1993 (Fig. 4a) (Lawson and Brockett

1993). Surveying established ground surface elevations on each transect relative to local benchmarks and the UTM grid. Grab samples of the

surface sediment (to 5 cm depth) were taken at

each survey point (Fig. 4b) for analysis of the

grain size distribution using standard sieving

a. Surveyed transects within areas from the EOD pad

north to the Knik Arm coast.
5

Figure 5. Sedimentation stakes. The plate slides freely

on the rod. Wire flags mark an area of painted ground

that is now covered with sediment.

therefore used several methods to measure the

primary rates of sedimentation and related erosion, and the secondary rates of resedimentation

that result from resuspension of pond bottom

materials by various processes. Depositional and

erosional rates were also measured in the mudflats, levees and gullies. The various samplers

described subsequently were installed during 24–

28 May 1992, with several sets of measurements

taken between then and 28 September 1993.

“Sedimentation stakes” were used to measure

erosion and deposition at points along the surveyed transects (Fig. 5). These stakes consist of a

rod and a square, rigid plate (about 7 cm2) that

slides freely on the rod. Erosion depth is defined

by the increase in distance between the top of the

rod and the top of the plate, as measured periodically to the nearest 0.5 mm. The amount of accretion is the thickness of sediment deposited on the

plate surface. The difference between these two

readings defines the net sedimentation (or erosion) rate. We measured these rates in July, August and September 1992 and in May and September 1993, following the monthly period of tidal

flooding. A similar stake system was used previously in Turnagain Arm (Ovenshine et al. 1976a,

b).

We also monitored the amount of accretion at

transect survey points by spraying an area (30×

30 cm) on the ground surface with pavement

marking paint and locating the corners of this

area with wire survey flags. Net accumulated

sediment is measured by cutting and removing a

2-×2-in. (5-×5-cm) block of sediment with a putty

knife (Fig. 6). The thickness of sediment above the

paint layer is then measured to the nearest 0.5

Figure 4 (cont’d).

b. Surveyed locations of surface grab samples (open circles) on 12 transects east of Eagle River. Open circles

also mark the locations of sedimentation stakes. Filled

squares are locations of sedimentation stations (S-no.).

and hydrometer techniques. These data will be

used to delineate textural trends and evaluate the

relative importance of sediment sources.

Sedimentation and erosion

Sedimentation rates within tidal flats are difficult to measure accurately. Within ponds and

marshes, for example, only a slight disturbance

of the water column is needed to resuspend the

fine-grained bottom sediments. If these sediments are caught in sediment traps, erroneously

high depositional rates would be measured. We
6

mm with a millimeter scale. Paint was applied at

certain sites in June 1992, and others in midAugust 1992. In contrast to sedimentation stakes,

which are broken or removed by various processes during winter, the wire flags and the

persistence of buried paint horizons suggests this

method may provide a continuous record of net

sedimentation rates.

Sedimentation rates in ponds were also measured at 10 sites following tidal inundations in

June, August and September 1992 and May and

September 1993 (Fig. 4). We expected these rates

to be extremely low and possibly to be affected

by resuspension of bottom sediments during the

process of obtaining data. We, therefore, used

three methods at each site initially, but have

found two to be relatively reliable.

Gross deposition is measured in a sediment

trap consisting of a 4-in.-diameter (10.2-cm-diameter) PVC “cup” glued to one end of a short

length of 2-in.-diameter (5-cm-diameter) PVC

pipe. The pipe was inserted into the pond until

the bottom of the cup was in contact with the bed

(b on Fig. 7). Sediments held within the cup include new sediment brought into the ponds by

tidal inundation and river currents, and materials

resuspended by wind waves, dabbling ducks or

other mechanisms. The quantity of accumulated

sediment is measured to the nearest 0.5 mm by

inserting a millimeter rule into the sediments at

three places. After measurement, the sediment in

the cup is cleared away.

We measured the net sedimentation rate in

ponds using a thin, rigid plastic plate of about 30

cm square that is pushed gently into the pond

bottom until it is flush with the surface. The corners of the plate are secured by aluminum tent

pegs (c on Fig. 7). Sediment in suspension settles

onto this plate, but this sediment can also be reworked and resuspended by wind or other mechanisms, thereby delineating a net rate. In addition, by measuring the sediment thickness at

three random locations, the plate’s dimension

provides some measure of the spatial variability

in sedimentation or erosion rates. Plates are covered by sediment, and later by aquatic vegetation,

within one to two weeks or less of placement; this

cover negated the possible effects of their smooth

surfaces on their sediment trapping efficiency.

Initially in 1992, a stake with a millimeterFigure 7. Sedimentation station in a pond site. A

meter stake (a), cup sampler (b) and plate sampler (c)

are shown in position. The plate sampler is located between the two survey flags.

Figure 6. Paint layer within block of sediment as

measured to define sedimentation rates.
7

Figure 9. Headwall erosion study site. The line of stakes

approximately parallels the gully crest. The hub stake is to the

right of center behind the stake line. * The terms gully and drainage are used interchangably.

graduated scale on its face was inserted into

the pond bottom, and the height of the pond

bottom and the water column were measured

to provide a second measure of the net sedimentation rate (a on Fig. 7). Any wind waves

or water surface glare on sunny days, however, commonly made this a difficult measurement to obtain. In addition, aquatic vegetation on the bed commonly covered the base of

the scale. For these reasons, only water level

measurements are now recorded.

Gully erosion and recession

Gullies* draining the ponds and mudflats

are actively extending inland by headwall

and lateral wall erosion and recession. We established 14 sites to evaluate retreat rates (Fig.

8); 11 sites were established in June 1992, 3 in

June 1993. At each site, stakes were driven

into the ground along a straight line at known

distances from one another and from the crest

of the gully (Fig. 9). A “hub” stake was set at

a known distance from this stake line. The

distance between the hub and the crest of the

gully scarp was measured across the top of

each line stake in succession with a tape measure. The position of the gully scarp was

identified with a plumb bob suspended from

the tape measure. Flagged wire stakes were

then placed at each point of measurement

along the headwall.

Measurements were repeated in September

1992, June 1993 and September 1993 after

tidal inundation. On the basis of repeated

measurements at points without any retreat

(as indicated by the continuing presence of

wire flags), we consider these measurements

reproducible at ±2–5 cm. The accuracy, however, is limited by how well the crest of the

gully scarp can be defined, the shape of

which is highly irregular, and therefore accuracy is probably limited to ±10 cm.

The elevations of the bed along the length

of several gullies were surveyed, beginning at

or near the Eagle River and continuing into

the ponds (Fig. 10). These gradients were calculated from these data for characteristic

reaches in each gully. Gradients were incorporated into estimates of gully discharge and

current velocity at surveyed cross sections

during maximum or bankfull ebb.

Figure 8. Location of gully headwall erosion study

sites.
Figure 10. Location of surveyed longitudinal cross sections of gullies.

Figure 11. Locations of instrumentation recording water quality parameters and water depths.

K

T

P

R

EOD

RI

A

D

PB

BT

C

B

8

Instrumentation sites:

K – Knik Arm

P – Parachute drainage

R – Eagle River

T – Bread Truck drainage

Other site abbreviations:

BT – Bread Truck pond

PB – Pond Beyond

RI – Racine Island

Ponded areas labeled A through D
9

a. Platform.

Table 1. Specifications of water quality parameter sensors.

Instrument type Sensor Accuracy Resolution

Hydrolab Temperature ±0.15°C 0.01°C

(H2O multiprobe) pH ± 0.2 units 0.01 units

Specific conductance

Fresh water ±0.0015 to 0.1 mS/cm* 0.001 mS/cm

Salt water ±0.15 to 1.0 mS/cm* 0.01 mS/cm

Salinity ±0.2 ppt 0.1 ppt

Dissolved oxygen ±0.2 ppm 0.01 ppm

Redox ±20 mV 1 mV

Depth ±0.45 m water 0.1 m water

CRREL thermistor Temperature ±0.02°C 0.01°C

Druck (PDCR 950) Pressure (water depth) ±0.008 m water 0.001 m water

OBS–3 Turbidity ±100 mV 1 mV

5 V = 2000 FTU

Seagauge Wave/Tide Pressure (water depth) ±0.003 m water 0.0015 m water

(model SBE 26–03) Temperature ±0.02°C 0.01°C

Seacat CTD Pressure (water depth) ±0.75 m water 0.045 m water

(model SBE 19–03) Temperature ±0.01°C 0.001°C

Conductivity ±0.001 S/m 0.0001 S/m

OBS [5V = 2000 FTU] ±100 mV 3 mV

* Depends on which of three auto-adjusting ranges is employed.

Water quality parameters

Water quality characteristics were analyzed

during the August and September 1993 flood and

ebb cycles at two gully sites—one in a gully

draining C Pond (called Parachute gully or drainage) and one in a gully draining the Bread Truck

Pond area. The same measurements were made

in the Eagle River, just downstream of the Route

Bravo access road bridge (Fig. 11).

A Campbell CR10 datalogger was used to

record data from a variety of sensors. A Hydrolab Model H20 Multiprobe Transmitter provided

data on a suite of parameters related to WP particle and sediment transport: temperature, pH, salinity, dissolved oxygen, redox and water depth

(Table 1). We also used a Druck pressure transducer to measure water level variations to ±8

mm, an individually calibrated thermistor to

measure temperatures to ±0.02°C, and a D&A

OBS–3 optical backscatter sensor to measure turbidity.

Data loggers and other instrumentation were

positioned on a floating platform next to gullies,

while the various sensors were mounted on

stakes set into the small plunge pools at their

heads (Fig. 12). Sensors were placed in the

plunge pools to keep them wet at all times.

Each platform was constructed of a 4- × 4-ft

(1.2- × 1.2-m) plywood deck mounted on a 2- × 6-

in. (5- × 15-cm) frame. Large metal eyehooks were

screwed into the frame on each corner (Fig 12a);

Figure 12. Platform and instrumentation measuring

discharge characteristics at the Bread Truck gully

location.

b. Sensor detail.
12,000 years ago may be affecting processes as

well.

The glacial sources for the Eagle River modify

the discharge seasonally, mainly in response to

variations in climate, and daily, mainly in response to diurnal changes in ablation and meltwater runoff. Maximum discharge occurs during

and following the spring runoff of snowmelt, but

may remain high throughout the summer, depending upon meteorological factors affecting

meltwater production on the glacier surface (e.g.,

Röthlisberger and Lang 1987, Lawson 1993).

Rapid, hourly fluctuations in discharge may result from heavy rain in the river’s watershed.

The glacially derived sediment within runoff

from the watershed also varies seasonally and

annually; within ERF, this component mainly

affects the amount of sediment in suspension.

Sediment in suspended transport from the Eagle

River is one potential source of material deposited within the ponds and mudflats.

Tidal flood waters are another major source of

suspended sediment. These sediments are likewise of glacial origin, being derived from glacierized watersheds feeding major rivers that enter

Knik Arm and Cook Inlet, including the Susitna,

Knik and Matanuska rivers. It is not known

whether the seasonal and annual variability of

sediment discharge from these glacial rivers is

evident within the tidal waters of Knik Arm. Limited measurements of Knik Arm in November

1993 and March 1994 indicated that sediment is in

suspension through the winter, but in smaller

quantities than during the summer. The continued presence of suspended sediment during winter is important, as it would then be the most

important if not the only source of sediment for

deposition during that time. The Eagle River, as

with other glacial rivers, transports an insignificant amount of material in suspension during the

fall through the early spring when glacial discharge is limited to groundwater sources (e.g.,

Lawson 1993).

The interaction and combination of tidal and

river sources of water and sediment control the

amount of material available for deposition in

ERF, and may also affect the locations and rates

of erosion. The amount of sediment within flood

waters is a primary factor determining their capacity to erode and transport additional sediments

during ebb.

A preliminary analysis of aerial photographs

of ERF suggests that sedimentation of the landward third of the area (southern one-third) is

10

5-ft (1.5-m) sections of 0.75-in. (2-cm) diameter

steel pipe were mounted vertically in large cans

filled with concrete, and the eyehooks mounted

on the platform’s wood frame were lowered over

them. Foam filled the inner space of the deck

framework and provided flotation so that the

platform could move vertically up or down the

steel pipes during tidal inundation and ebb. The

platforms allowed us to establish monitoring stations without setting stakes or foundations in the

ordinance-bearing mudflat sediments and, at the

same time, keep the data loggers and related devices dry even under the highest flood levels.

An ISCO Model 3700 water sampler, also

mounted on each platform, obtained 500-mL water samples at specific intervals through the flood

and ebb cycle. The Total Suspended Sediment

concentration (TSS) of each sample was analyzed

using standard vacuum techniques with 45-mm

glass microfiber filters. The TSS data were later

used to calibrate the D&A Optical Backscatter

Sensor (OBS) turbidity readings for sediment

concentrations measured at each site.

Water quality data were recorded from midAugust to the end of September using 2-minute

sampling intervals at the gully sites and 10-

minute intervals at the river site. A Seabird

SBE19 Profiler or CTD (Conductivity–Temperature–Depth) profiler was moored at the coast in

Knik Arm from mid- to late September to measure salinity, temperature, depth and turbidity

during several of the tidal cycles (Fig. 11). At the

same location, a Seabird SBE26 Wave and Tide

Gauge measured tidal height variations and water temperature from mid-August to late September (Table 1). ISCO water samples were acquired

at 1/2-hour intervals through the flood and ebb

cycles of August, but this was changed in September to a 10-minute interval about peak tide.

Water samples were also taken by hand from

Knik Arm at the wave and tide gauge recorder

during the September inundation cycle.

PROCESS OBSERVATIONS

The activity and relative importance of erosional and depositional processes depend upon a

number of factors, including those that are external to the immediate Flats environment. External

factors include tidal forces, glacially derived runoff and sediment, climate, and tectonic and earthquake activity. In addition, a slowly rising sea

level and isostatic rebound following occupation

of the region by glaciers that retreated about
11

dominated by the Eagle River. Landform types

and distribution differ here, being mainly levees

and vegetated marsh floodplains. These riverine

landforms overlie and cover older marshes, a

part of which is apparently exposed in the center

of Racine Island (Fig. 13c). In the northern twothirds of ERF, landforms are similar to tidal flats

near river mouths elsewhere in this region (e.g.,

Ovenshine et al. 1976b).

Historical aerial photographs and ground observations also indicate that ERF is physically

changing over both short- and long-time scales.

Large-scale changes resulting from riverine processes over the last 40 years include abandonment of a primary channel within the southern

part of the Flats, and channel migration of various amounts from near the Route Bravo bridge

north to the river mouth at Knik Arm.

Aerial photographs from 1950, 1967 and 1993

reveal that the primary river channel (of three)

was abandoned sometime between 1950 and

1967 (Fig. 13). By 1967, two channels diverged

from Eagle River north of the Route Bravo

bridge, the primary avenue of discharge now appearing to be the eastern course. Our initial review of these photographs does not reveal the

cause of this change; however, it is clearly important to understanding the hydrology and sedimentology of the Flats.

The two primary channels entering the Flats

today are partially braided and have a divergent

pattern characteristic of an alluvial fan (Fig. 13c).

Both suggest that the channel gradient significantly decreases near the bridge, leading to a reduction in carrying capacity, deposition of sediments in transport, and a fundamental change in

the channel configuration (e.g., Leopold et al.

1964). In addition, the 1993 color infrared image

shows a well-defined change in texture and color

where these channels merge into a single meandering channel (Fig. 13c). This textural change

may indicate a physical change, such as in elevation, or in the grain size and mineralogy of the

substrate materials. The predominant types of

vegetation reflect these physical differences and

thus the apparent change from river dominated

to tidally dominated physical processes.

Both the tributary channel and the meandering main channel progressively deepen downstream, essentially being incised over 5 m during

low tide. Near-vertical scarps and evidence of recent slumping indicate that the river banks are

actively eroding. A qualitative assessment of the

1950, 1967 and 1993 aerial photography suggests

that, historically, banks have receded several

tens of meters at some locations.

Significant changes in the relatively tight meander loops in the lower section of the river

have taken place over the last 40 years. Landforms, such as meander scars and abandoned

point bars, are common along the length of the

active channel and are especially evident on

photographs of the southern reaches of the Flats

(Fig. 13c). These landforms result from changes

in the channel location over time. Channel

changes are a natural progression resulting from

the erosion and recession of the outer banks of

meander bends, deposition of sediments as

point bars within the inner parts of each bend,

and a general downstream migration of the

channel (e.g., Allen 1982). At some locations, a

breaching of tight meander loops causes their

abandonment (Fig. 13c).

Erosion associated with channel changes

remobilizes bank sediments, which may be

transported either downstream or upstream, depending upon the capacity of the river and tidal

forces. In the idealized river model, they would

move to the next point bar and be redeposited.

Redeposition will also be controlled by tidal and

river processes and by the materials’ fine grain

size, and therefore they may also be redeposited

within the ponds and mudflats, as well as within Knik Arm.

Gullies have also progressively lengthened

and deepened during the last 40 years, in some

cases extending over an estimated 200 to 300 m.

A significant extension of gullies toward the

ponds and marshes is particularly evident on

photographs covering the period of 1967–1993

(Fig. 13b and c), but the amount of recession

cannot yet be quantified. Recession is in response to erosion during ebb flow from the

ponds and mudflats into the gullies (Table 2).

Erosion measurements in 1992 indicated that

this recession is continuing at variable rates of

up to 5 to 10 m per year (Lawson and Brockett

1993).

The cause of gully expansion is unclear, but it

may be a response to external forces such as

earthquake activity, including the 1964 Alaskan

earthquake when this area subsided about 0.6 m

(Small and Wharton 1969), the eustatic rise in

sea level, or large-scale river channel changes

caused by flooding. Further quantitative analyses of historical aerial photographs and sedimentological analyses of deposits within the

alluvial landforms are required to determine
12

a. 1950.

Figure 13. Aerial photographs of ERF.
13

b. 1967.

Figure 13 (cont’d).
14

c. 1993.

Figure 13 (cont’d). Aerial photographs of ERF.
15

Figure 14. Drainage pattern analysis of 1993 aerial photograph in the study area east of Eagle River.
16

and minimum volume of hourly, daily and seasonal discharges, the lag between precipitation

and the resultant increase in runoff, and longterm trends in annual discharge of the basin (e.g.,

Gurnell and Clark 1987, Lawson 1993). Water

quality parameters, including suspended solids,

show similar, but not directly related, variations

with time.

The USGS recorded discharge from the Eagle

River between 1966 and 1981 (USGS 1981). During this period, daily discharge averaged 14.7

m3/s. The maximum mean discharge during the

height of the glacial melt season (July–August)

averaged 42.5 m3/s, with peaks ranging from

about 65 to 76 m3/s. Rain-induced floods during

peak glacial runoff exceeded 105 m3/s. Unfortunately, a new station to record discharge of the

Eagle River could not be installed in 1993, and we

therefore do not have current hydrologic records

for the Eagle River.

We were, however, able to establish instrumentation in mid-August to measure basic water

quality parameters, suspended sediment concentrations and water level changes through September. These data show diurnal variations in

water height, conductivity and temperature typical of a glacially fed river (Fig. 15) (e.g., Lawson

1993). Sediment concentrations were generally

low from 25 August to 20 September, ranging

from 23 to 275 mg/L and decreasing steadily

through September. Additional water quality and

sediment transport data will be acquired during

1994.

TIDAL HYDROLOGY AND INUNDATION

Erosional, transport and depositional processes are complexly related to tide height, Eagle River

discharge, and the length of time of inundation.

Although sufficiently high tides can flood all of

ERF, lower high tides may be supplemented by

the river’s discharge and flood more area to a

greater depth than the predicted tide height indicates. During spring snowmelt runoff, precipitation-induced flooding or other high discharge

events, tidal waters moving up the channel block

the flow of the river water, causing it to slow and

pool, and eventually can reverse flow within the

Flats proper. Some overflow of the tidal water by

fresh river water may also take place. The flooded area is increased and runoff during ebb is prolonged. Similarly, increased flooding can result

from storm-driven waters moving into the Flats

from Knik Arm and Cook Inlet.

Pond sedimentation results mainly from set

Table 2. Summer erosional processes.

Morphological

unit Processes

Ponds Currents Tidal currents

—wind Debris impacts

—tidal (e.g., logs)

Wind waves

Wind currents

Ducks and other

bottom-feeding

organisms

Gullies Currents Groundwater

—tidal —piping

—runoff —sapping

Overland flow Slope mechanisms

—sheet —slump

—rill —block collapse

Wind waves —sediment flow

Mudflats Currents Wind waves

—wind Debris impacts

—tidal (e.g., logs)

Overland flow Rain drop impact

—sheet

—rill

Marshes Currents (rare)

Levees Currents Debris impacts

—tidal Wind waves

—river

whether the gullies are expanding in response to

such external forces.

Tributary channels with a dendritic pattern

feed water into the gullies (Fig. 14). These shallow, vegetated channels cross the mudflats and

head in the ponds. A second set of channels that

is intercepted by the active tributary system is

also common (Fig. 14). These channels, however,

unconformably cross other landforms, including

ponds (such as C and Bread Truck) where they lie

below the water surface. Their pattern is irregular

and unrelated to the dendritic channel and gully

system. These secondary channels are probably

relict drainage and indicate significant changes to

the drainage in the past. The causes of such

changes are unknown, but may be related to larger scale forces such as flood-induced channel migration or earthquake-induced subsidence.

GLACIER RUNOFF AND SEDIMENT YIELD

Runoff from the Eagle River is characteristic of

a watershed occupied partly by glaciers. Glaciers

cover about 13% of the watershed area, yet it

takes only a few percent glacier cover to significantly modify the runoff and sediment yield of a

drainage basin (e.g., Lawson 1993). Glaciers modify peak discharges, the timing and maximum
17

5.2

5.4

5.6

5.8

18 Aug '93 20 22 24 26 28 30 1 Sep 3 5

Elevation (m)

a. Water elevation.

4

6

8

10

18 Aug '93 20 22 24 26 28 30 1 Sep 3 5

Temperature (°C)

b. Temperature.

0.04

0.08

0.12

18 Aug '93 20 22 24 26 28 30 1 Sep 3 5

Specific Conductance (mS/cm)

c. Specific conductance.

Figure 15. Water parameter data from Eagle River site just downstream of Route Bravo bridge.

Morphological

unit Processes

Ponds Suspension sedimentation

—settling-out

—vegetation trapping

Gullies Suspension sedimentation

—settling-out

Bedload deposition

Sediment flows

Slumping

Table 3. Summer depositional processes.

Morphological

unit Processes

Mudflats Suspension sedimentation

—settling-out

—vegetation trapping

Marshes Suspension sedimentation

—vegetation trapping

Levees Suspension sedimentation

—settling-out

—vegetation trapping
18

0

2

4

6

18:00 20:00 22:00 04:00

Elevation (m)

Bread Truck Dr.

Parachute Dr.

Knik Arm

(coast)

19 Aug '93 20 Aug

Tide

Chart

02:00

Parachute Dr.

Knik Arm

(coast)

Bread Truck Dr.

Time of Flooding

Figure 16. Water elevation at three

sites during the 19 August evening

flood event. The base level shown on

the Knik Arm plot is equal to atmospheric pressure measured when the

sensor is exposed in air.

Levee

Mudflat/Pond

Parachute Dr.

Knik Arm

Bread Truck Dr.

Tide Chart

8

6

4

2

0

14 Sep '93 16 18 20 22 24 Elevation (m)

6

4

2

0

17 Aug '93 19 21 23 25 Elevation (m)

Tide Chart

Levee

Mudflat/Pond

Parachute Dr.

Bread Truck Dr. Knik Arm

8

b. September.

a. August.

Figure 17. Water elevations at three sites

during a period of flooding cycles. Knik

Arm represents the tidal cycle external to

ERF proper. In August, Parachute and

Bread Truck data show peak heights

greater than predicted due to various factors including high river flow, but little

augmentation over predicted heights in

September.
5.0

5.2

5.4

5.6

5.8

Elevation (m)

15 Sep '93 17 19 21 23 25

Rain

(0.96 in)

Rain

(0.18 in)

tling-out of sediment suspended in the water column (Table 3) and, presumably, sediments are

deposited mainly following slack high tide. Thus,

the longer the period of inundation, the greater

the time for sediment in suspension to be deposited. In addition, the mixing and exchange of

flood waters with the original pond and marsh

waters increases the amount of sediment suspended in the ponds and marshes, and provides

the primary source of newly deposited materials.

The timing of flooding is a function of both the

elevation of the levees and mudflats surrounding

the gullies, and of the distance of the gully headwall from the coast. Water moving down the river can also alter the timing of flooding inland

from the coast, depending upon its volume relative to the volume of tidal waters. A quantitative

relationship for these factors is not yet developed, and thus the relative duration of inundation per tidal flooding cannot be predicted.

Water level measurements at the river and

gully sites (Fig. 11) verified our visual observations that the flood cycle normally results in a

progressive rise in the river channel and gully

water levels before a gradual flooding of the tidal

flats takes place. This progression is illustrated

by water height changes at the three sites during

a tidal inundation in August (Fig. 16). Flooding

begins first within the coastal mudflats on Knik

Arm, while water is progressively moving inland

up the Eagle River channel and the tributary gullies before it spills onto the inner mudflats first

at the Bread Truck gully and then at Parachute

gully. Water levels decline first in the coastal

zone, actually while tidal flood waters are still

moving up Otter Creek and into the southwestern corner of the Flats.

Water levels were also greater than predicted

for the Anchorage datum from 17–22 August,

showing primarily the effects of a high glacial

meltwater discharge in the Eagle River (Fig.

17a). Precipitation and wind were not factors

during this time.

Water heights measured at these same sites

during tidal flooding in August and September

illustrate the seasonal shift from higher to lower

discharge as melting of glacial river sources is

reduced (Fig. 17b). Flood heights generally exceed the tide chart predictions for Anchorage in

August, until the discharge decreased around 22

August. In September, flood heights generally

match predicted tide levels; whenever they exceeded those levels, river discharge had been increased by rainfall in the Eagle River watershed

(Fig. 18).

WATER QUALITY PARAMETERS AND

TIDAL FLATS HYDROLOGY

Water quality during tidal influx and efflux is

critical to understanding the interplay of tidal

flood waters and river discharge and to examining their role in the erosion, transport and deposition of sediment within the Flats. Water quality parameters were only measured during two

summer months (August and September), so

that we can only speculate on the overall role of

tidal and riverine water sources. The relative importance of these parameters as a data source

can, however, be illustrated by examining variations in selected parameters during a flood and

ebb cycle at the Bread Truck Pond gully on 18

August 1993, at the Bread Truck and Parachute

gully sites on 19–20 August, and at both sites on

18–19 September.

Data during the flood and ebb cycle of 18

August show the basic relationships of water

quality parameters to inundation (Fig. 19). As

Figure 18. Elevation of waters

in Eagle River during September flooding cycles. Rainfall in

the watershed affected elevation

twice during the September tidal

cycles.

19
20

3

4

5

6

7

06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00

0

400

800

1200

1600

Salinity (‰)

Salinity

OBS

Bread Truck Drainage

Elevation

8 2000

6

5

4

3

Water Elevation (m)

3

4

5

6

06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00

10

12

14

16

Water Elevation (m)

Bread Truck Drainage

Depth to flooding

Elevation

Temperature

Figure 19. Variation of water quality parameters, Bread Truck drainage site, 18 August.

a. Water elevation, salinity

and optical backscatter measurement.

b. Water elevation, temperature

and depth to flooding of the

mudflats.

dominated waters therefore takes place more

gradually. In part, the more gradual change in

temperature and salinity, for example, results

from mixing and homogenization in the ponds

and mudflats.

The sharp, transient rises in turbidity are,

however, related to some other process. They

may be caused by vegetation or fish covering

the optic sensor, but are most likely actual increases in turbidity from sediment in transport.

The increased turbidity may be caused by scour

of the gully headwall or lateral walls, or perhaps

sudden bank failures, causing a rapid, but local,

increase in the amount of sediment suspended

in the water column. Until ebb waters are completely contained in the gully channel during the

latter stages of runoff, water may flow into the

gullies at numerous locations along their

lengths.

Data from the heads of the Bread Truck and

Parachute drainages during a flood and ebb

cycle on 20–21 August of a predicted high tide of

10.0 m and on 18–19 September of a predicted

the flood tide progresses into ERF, it moves up

the Eagle River, backing river waters up each

gully, and causing them to spill onto the mudflats. The water level rises initially at a steady

rate, but rapidly decreases as the area being

flooded increases when the water crests the gully headwall and spills into the vegetated drainageway and onto the mudflats above it. Water

temperature, salinity and turbidity change in the

headwater pool in which the sensors are submerged as its water is exchanged with the more

saline tidal flood waters. Just before the headwater pool attains conditions that reflect the character of Knik Arm waters, parameters may fluctuate rapidly while the mixing front of the two

water masses passes.

During ebb, the drop in water level is slower

than the rise during flooding, indicating temporary storage of tidal waters within the Flats.

The stored water may include river influx and

possibly may involve temporary storage in the

sediments as groundwater. The transition from

more saline tidal flood waters to fresh or pondTemperature (°C) OBS (mV)
21

3.8

4.2

4.6

5.0

5.4

0

500

1000

1500

2000

2500

Salinity (‰)

Bread Truck Drainage

Salinity

OBS

Elevation

( )

6

5

4

3

OBS (mV)

3

4

5

6

20:00 21:00 22:00 23:00 24:00 01:00 02:00 03:00

12

13

14

Water Elevation (m)

Bread Truck Drainage

Depth to Flooding

Temperature

Elevation

Temperature (°C)

Water Elevation (m)

a. Bread Truck drainage. Water Elevation (m)

3

4

5

6

20:00 21:00 22:00 23:00 24:00 01:00 02:00 03:00

10

12

14

Elevation (m)

Parachute Drainage

Depth to flooding

Temperature

Elevation

b. Parachute drainage.

Figure 20. Variation of water quality parameters, 20–21 August.

1000

2000

0

2

4

6

0

Salinity (‰)

Parachute Drainage

Salinity

OBS

Elevation

6

5

4

3

OBS (mV) Temperature (°C)
22

4

5

6

7

0

200

400

600

800

Salinity (‰)

Parachute Drainage

Salinity

OBS

Elevation

6

5

4

3

OBS ( V)

3

4

5

6

19:00 20:00 21:00 22:00 23:00 24:00 01:00 02:00

7

9

11

13

Water Elevation (m)

Parachute Drainage

Depth to Flooding

Water Elevation (m)

Temperature (°C)

5.4

5.8

6.2

6.6

0

400

800

1200

Salinity (‰)

Salinity

OBS

OBS ( V)

6

5

4

3

Elevation

3

4

5

6

19:00 20:00 21:00 22:00 23:00 24:00 01:00 02:00

8

9

10

11

Water Elevation (m)

Bread Truck Drainage

Depth to Flooding

Temperature

Elevation

Bread Truck Drainage

Water Elevation (m)

Temperature (°C)

OBS (mV)

OBS (mV)

a. Bread Truck drainage.

b. Parachute drainage.

Figure 21. Variation of water quality parameters, 18–19 September.
23

Table 4. Temperature and salinity of Eagle

River and Knik Arm.

20 August 18 September

Temperature Salinity Temperature Salinity

Site (°C) (ppt) (°C) (ppt)

Eagle River 6.0 0 4.0 0

Knik Arm 12.9 — 10.3 6.8

sharply increase as the tidally dominated water

mass subsequently fills the gully (Table 4).

As ebb occurs, the decrease in turbidity takes

place more gradually than at the Bread Truck site

until the water elevation drops below bank

height. Then it decreases rapidly, as does temperature and salinity, to levels less than Knik Arm,

suggesting that river water is a larger component

of the ebb discharge at the Parachuate site.

In contrast, the 18–19 September tidal event is

characterized by lower suspended sediment

concentrations, higher salinities and lower water

temperatures (Fig. 21). Several other important

aspects are also evident. At Bread Truck, the temperature does not decrease rapidly as the tidal

waters rise. An initially lower temperature within the plunge pool during September than in

August may account for a temperature similar to

that of water in Knik Arm (Table 4). Both turbidity and salinity increase rapidly, however, as

flooding takes place. The rapid drop in salinity

just before peak flood level may indicate incomplete mixing of Knik Arm and Eagle River waters

within the flood. During ebb, water temperature

and salinity gradually decline as the freshwater

component becomes more important, but the turbidity values exhibit a series of rapid fluctuations

that nearly equal those during peak flood.

The Parachute gully record has a more gradual and sustained fall in turbidity beginning at

about the same time as at the Bread Truck site.

The source of this difference from Bread Truck is

unknown, but we speculate that the sustained

values at Parachute represent discharge of sediment transported from within the Flats. The

pulses of suspended sediment in the water column may result from local erosion, failure and

collapse of the gully walls. But this argument is

less convincing for the Bread Truck site; the highly erratic peaks are more likely caused by vegetation that caught on the probe.

Of further note are the significantly higher salinities in the Parachute flood and ebb waters in

September than in August. These values probably represent the seasonally reduced contribution

of Eagle River water to flooding of the Flats.

Similarly, turbidity values at both sites were less

than in August and correlate with reduced concentrations of suspended sediment in both the

river and Knik Arm. This reduction is also a response to the seasonal drop in sediment discharge from the glacierized basins that flow into

the Eagle River and the rivers entering Knik

Arm.

high tide of 10.33 m illustrate several important

aspects of tidal flats hydrology (Fig. 20 and 21).

As described for 18 August at the Bread

Truck site, the water level on 20–21 August rises

rapidly as flooding begins, but slows as water

spills into the vegetated drainageway above the

gully headwall and floods the mudflats. Coincident with this rise, water temperature, salinity

and turbidity change but in different ways at

the Bread Truck than at the Parachute site.

At the Bread Truck gully site, where the

flooding threshold is lower, water temperature

shows little variation until the approximate

time when water height is sufficient to overflow

the gully. Temperature then rapidly declines to

near that of water in Knik Arm, signaling the

arrival of tidal flood waters (Table 4). Small

temperature variations (tenths of a degree Celsius) during the remainder of the tidal cycle

may result from local mixing of tidal and fresher water masses. Coincident with the temperature drop, turbidity increases sharply and

remains high through flooding; however, just

after this rise, OBS values decrease sharply, perhaps reflecting an initial override of fresher

water by the intruding more saline tidal floodwaters.

As ebb takes place, turbidity values drop

rapidly within about 30 minutes, indicating either that waters inundating the Flats have lost

their suspended sediment through deposition

in ponds and mudflats, or that the mixing of

river and tidal water masses has diluted the

concentration of solids. With time, the turbidity

continues to decrease as water elevation declines; both sediment concentration and salinity

decrease to levels present before the flood.

At the Parachute gully site, the drop in temperature accompanying the initial encroachment of flood waters is greater than at Bread

Truck. This reduction is believed to be attributable to a higher proportion of river water in the

first flood waters rising to the gully sensors because of the more inland location of the gully

mouth. Salinity, turbidity and temperature
Figure 22. Total suspended

sediment (TSS) concentrations measured at gully sites

during August and September flooding cycles.

0

500

1000

1500

2000

Parachute Drainage

0

500

1000

1500

2000

14 Sep 16 Sep 18 Sep 20 Sep 22 Sep 24 Sep

Bread Truck Drainage

TSS (mg/L) TSS (mg/L)

0

500

1000

1500

Parachute Drainage

0

500

1000

1500

17-Aug 19-Aug 21-Aug 23-Aug 25-Aug

Bread Truck Drainage

TSS (mg/L) TSS (mg/L)
25

3

4

5

6

17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00 01:00 02:00 03:00

0

400

800

1200

1600

Water Elevation (m)

Depth to Flooding

TSS (mg/L)

Figure 23. Total suspended sediment concentrations and water

elevations at Parachute drainage, 17–18 September.

TSS (mg/L)

SEDIMENT TRANSPORT

Sediments are deposited within ponds and

mudflats mainly through settling-out from

waters flooding ERF (Table 3). New materials are

in suspended transport by river and tidal flood

waters, while a second source is sediment eroded

and remobilized from the pond bottoms and

mudflats by wind currents, bottom-feeding waterfowl and other mechanisms. River currents

may also erode sediment from riverbanks; gullies

likewise can be eroded by flood or ebb currents.

Total Suspended Sediment (TSS) in discrete samples and the continuous recording of turbidity by

an Optical Backscatter Sensor (OBS) provide data

on suspended sediment transport and the relative importance of sediment sources.

Samples for TSS analysis were obtained during the August and September flooding cycles at

the Parachute and Bread Truck gully sites (Fig.

22). During the August cycle, water samples

were taken every half hour over 12 hours running from low tide to low tide. Relatively low

values were recorded at the Bread Truck drainage in three of four recorded cycles; the fourth

event produced a peak value of 1000 mg/L (Fig.

22b). In one cycle recorded at Parachute drainage, the samples captured a peak of 1500 mg/L

(Fig. 22a).

This sampling scheme did not sufficiently represent the variability in suspended sediment concentration over time and therefore the sampling

interval was altered in September. Samples were

then acquired every 10 minutes, beginning near

the predicted peak high tide at Anchorage and

continuing through the initial phase of the ebb.

The time interval was progressively lengthened

to a half hour and then one hour during the latter

stages of ebb, as well as for several hours prior

to high tide.

In the first sampled event of 15 September,

flood levels were barely high enough to spill

onto the mudflats above the gullies and low TSS

peaks were recorded at both sites (170 mg/L)

(Fig. 22). Subsequently, the morning tide of 16

September flooded most of the Flats up to about

0.2 m depth; both Parachute and Bread Truck

had similar TSS peaks of approximately 1250

mg/L. Flooding cycles sampled on 17, 18 and 19

September were characterized by deeper inundation (about 0.4 m) and higher peak TSS values. TSS peak values in the Parachute drainage

were 1700 and 1900 mg/L, exceeding the Bread

Truck values by 300 mg/L. In general, low suspended sediment concentrations characterized

the rising tide while flood waters moved into

each gully. A sharp rise in TSS occurred as the

rising flood waters from Knik Arm began to

spill onto the mudflat. Suspended sediment concentrations gradually declined only after ebb began (Fig. 23).

OBS turbidity levels peak during flooding of

the gully sites in August, generally ranging from

1300 to 1500 mV (500 to 600 FTU) at Parachute

gully and 1200 to 2300 mV (480 to 920 FTU) in

the Break Truck gully (Fig. 24a and b). In both

cases, the OBS values indicate that sediment

concentrations progreasively increase as tidal

waters enter and flood each site, gradually falling at a slower rate as ebb takes place and gully

drainage becomes well established. In September both sites had lower ranges in turbidity

(about 700 to 1300 mV [280 to 520 FTU] at Bread

Truck and about 800 mV [320 FTU] at Parachute

[Fig. 24b, c and d]). The OBS records for a SBE19

CTD temporarily established at the tide gauge
26

OBS (mV)

0

1000

2000

3000

17 Aug '93 19 21 23 25

V)

Bread Truck Drainage

OBS (mV)

0

1000

2000

3000

14-Sep '93 16 18 20 22 24

Parachute Drainage

OBS (mV)

Figure 24. Optical backscatter (OBS) measurements at gully sites during August and

September flooding cycles.

0

1000

2000

3000

17 Aug '93 19 21 23 25

Parachute Drainage

a.

c.

b.
27

on Knik Arm in September recorded peak values ranging from 1400 to 1800 mV (560 to 720

FTU). OBS readings were not obtained at Eagle

River, however, owing to a lack of equipment.

As stated previously, the sporadic peaks in

OBS turbidity at both gully sites may be important indicators of erosion in the gully headwall

region, but vegetation or other objects in the ebb

discharge can cover the sensor and cause a false

indication of higher turbidity. It should be noted

that because the withdrawal point for the suction sampler and the view of the OBS sensor are

not precisely the same location, local increases in

suspended sediment by scour or bank failure

may not be sampled by the suction technique. In

addition, the optical character of the water may

OBS (mv)

b.

Figure 24 (cont’d).

0

1000

2000

3000

14 Sep '93 16 18 20 22 24

Bread Truck Drainage

V) OBS (mV)

d.

0

400

800

1200

1600

0

400

800

1200

1600

Parachute Drainage

20 Aug '93 21 Aug 

18:00 20:00 22:00 02:00 04:00 06:00

(

TSS

OBS

OBS (mv)

a.

0

200

400

600

800

16:00 18:00 20:00 22:00 02:00 04:00

0

400

800

1200

1600

2000

2400

Parachute Drainage

18 Sep '93 19 Sep

OBS

TSS

TSS (mg/L) TSS (mg/L)

Figure 25. Comparison of OBS and TSS measurements at Parachute Pond.
28

Figure 26. OBS measurements from Knik Arm during late September.

0

1000

2000

S

)

Knik Arm

20 Sep '93 21 22 23 24 25

OBS (mV)

not always relate directly to the weight of solids

present.

We attempted to calibrate OBS sensor readings of backscatter to suspended sediment concentrations using the TSS measurements. In a

few events, correlations were good (Fig. 25a).

However, in others, there is not a simple relationship between the two values. For example,

secondary peaks in OBS readings were observed

during ebb tide at the Parachute site without

correspondingly high TSS values (Fig. 25b). Regression analysis of this relationship produced

low correlation coefficients. A possible explanation for the secondary peaks is that, if extremely

fine material is in transport during ebb, it can

cause a large amount of optical backscatter, but

add little to the total weight of suspended solids.

This problem remains under investigation.

SEDIMENT SOURCES

The suspended sediment concentrations

measured during August and September suggest that the tidal flood waters of Knik Arm

have greater concentrations of sediment in suspension than do those in the Eagle River. TSS

measurements preceding, during and following

tidal flooding cycles at both the Bread Truck and

Parachute gully sites are consistently higher

than those in the Eagle River and are similar in

range to those of Knik Arm. TSS data for the Eagle River reveal concentrations ranging from 23

to 275 mg/L for 25 August to 20 September.

Concentrations in two samples from Knik Arm

at the coastal site were much higher: 1116 and

1273 mg/L. OBS turbidity data for 20 to 24 September at the coast also were consistently high,

while the TSS samples from the river yielded

much lower values (less than 100 mg/L) (Fig.

26).

Further samples are clearly needed to determine if the relationship of sediment concentration for the river and tidal sources is representative of the entire year, especially because of the

inherent seasonal variability in the Eagle River’s

glacial sources. Sediments in Knik Arm waters

are, however, also derived from glacierized

basins feeding the Susitna, Knik, Matanuska and

other rivers that drain into Cook Inlet and thus

the tidal source may also vary seasonally.

EROSION DYNAMICS AND

GULLY HYDRAULICS

Longitudinal cross sections of the gullies, surveyed along their central channel or thalweg,

have several characteristics in common. Each of

the nine gullies in the study area has moderate

to relatively steep slopes downstream of an

eroding headwall (Fig. 27). The headwall region

is characterized by a nearly vertical face ranging

from 1.2 to over 2.0 m in height, at the base of

which is a depression or plunge pool scoured by

water flowing over the headwall (Fig. 27). This

plunge pool ranges in depth from about 0.5 to

over 2.0 m.

Channel gradients are steepest near the Eagle

River (0.01 to 0.02 m/m). The steep bed slope

probably results from the large range in tidally

controlled variations in water level and discharge in the river proper. Upstream of this

steep section, the gradient becomes shallower

(0.003 to 0.007 m/m), but then progressively

steepens with distance to the base of the head
29

Figure 27. Profiles of elevations along gully thalwegs of two drainages with calculated

gradients indicated (see Fig. 10). Top–Gully C, bottom–Gully E.

Distance (m)

–2

–1

0

1

2

3

4

5

0 200 400 600 800

Elevation (m)

Headwall Plunge Pool

To Eagle R.

Gradients

(m/m)

0.0099 0.0061 0.0044 0.0027 0.0045

0.0063

0.0038

0.010

– 0.0014

E0 1 F F 0

Distance (m)

1

2

3

4

5

0 50 100 150 200 250 300 350

Elevation (m)

Headwall

Knik Pt.

Plunge Pool

Gradients

(m/m)

0.0165 0.0070 0.0081 0.0042 0.0143 0.0059 – 0.0044 0.0014

To Eagle R.

C0 C2

Transect 5

Transect 1

Transect 2

Transect 4

Transect 3

Bread Truck

Drainage

Parachute

Drainage

Channel

Cross-sections

Figure 28. Places where gully cross sections and peak

flow were estimated (intersection of gully and transect 1). Figure 4 shows their location within the study

area.

wall (Fig. 27). At one or more locations within

the gully, short segments of the channel have

sharp increases in gradient. These locations,

which we call knickpoints, may indicate that the

bed profile is not in equilibrium with the existing hydrological conditions and that the bed

slope and elevation are actively being modified

by scour during flood or ebb. Such a progressive

deepening of the gully thalweg and reduction in

the channel gradient may be in response to the

on-going headwall erosion and extension of

each gully into the mudflats.

Channels above each headwall are lined by

vegetation and their bed slopes range from 0.001

to 0.004 m/m, generally being less than the gradients within the gully proper. With distance,

slopes gradually reverse in direction and dip upstream across the unvegetated mudflats next to

the ponds.

Both the relatively rapid erosion and extension of the gully system and their relatively
30

Figure 29. Idealized cross sections at the

Bread Truck and Parachute drainages.

42.40 m

6.55 m

4.09 m

Elev.

5.05 m

Irregular

Channel Cross-section Approximate

Channel Cross-section

Bread Truck Drainage

Parachute Drainage

33.02 m

15.90 m

2.44 m

Approximate

Cross-section

Elev

4.95 m

Table 5. Gully parameters.

A Pw R S

Location (m2) (m) (m) (m/m) N

Bread Truck drainage 86.7 43.8 1.98 0.0044 0.018

Parachute drainage 40.3 33.4 1.21 0.0062 0.018

steep gradient suggest that the drainage system

is actively changing. The reasons for this activity

are not yet clear. Hydraulic and geotechnical

forces are clearly causing such changes, but their

relationship to the dynamics of the entire system

remains unknown. Basic parameters such as current velocity, discharge and channel geometry

need to be measured accurately to define the gullies’ roles in tidal flat hydrology.

To evaluate the potential magnitude and relative importance of forces causing erosion and

sediment transport within the gullies, empirical

equations were used to estimate hydraulic parameters for peak flow during ebb. Gully profiles, bank elevations and calculated slopes for

the Bread Truck and Parachute drainages were

used to approximate the shape of the drainage

cross section and estimate maximum velocities

and discharge (Fig. 28 and 29).

Sediment transport and gully erosion are primarily related to the flow velocity. Gully longitudinal profiles and cross sections were measured

for the Bread Truck and Parachute drainages in

an attempt to determine flow velocities in these

channels. Figures 28 and 29 are representative

sectional shapes that expand in each gully as one

proceeds downstream.

The Manning equation was used to calculate

the average flow velocity V in the gully.

V = 1.0/N R2/3 S1/2 (1)

where N = Manning coefficient (0.018 for clean

earth)

R = hydraulic radius

S = slope.

Table 5 gives values of the above parameters, as

well as area (A) and wetted perimeter (Pw). We

assumed that the depth of flow was at bankfull

stage and that the channel slope in the gully was

in uniform flow. These calculations produced

very high flow velocities (more than 4.5 m/s) and

Froude numbers in excess of 1.0. Since these values indicate supercritical flow, the flow must undergo a hydraulic jump to a greater depth, which

is not possible for the assumed bankfull conditions. Thus, simplifying the hydraulic conditions

to a uniform flow assumption is not feasible for

currents within the gullies.

Obviously, these flow velocity calculations are

not representative of the actual situation and

more detailed information on the hydraulic controls (downstream and upstream) or actual velocity measurements are needed. Our observations

suggest flow velocities are actually in the 1–3 m/s

range during ebb.

EROSION AND RECESSION RATES

Erosion of gully headwalls is causing their progressive elongation into the mudflats and presumably into the ponds. Rates of recession of the

gully headwall and adjacent lateral walls were
31

measured at 11 sites established in 1992 and at 3

additional sites established in 1993 (Fig. 8). Net

recession rates were measured between 28 May

and 22 September 1992, 22 September 1992 and 2

June 1993, and 2 June and 22 September 1993,

while measurements on 26 August and on 22

September 1993 permit comparison of erosion

and recession resulting from a single month’s

cycle of flooding tides (Appendix A).

The amount of recession of the scarp crests

was highly variable within a gully, as well as

among the sites (Table 6). Maximum recession

rates ranged from 0.1–4.9 m during May to September 1992 , 0.4–6.3 m during September 1992

to June 1993, and 0.0–9.8 m during June to September 1993 (Table 6).

At each site, however, some parts of the scarp

crests did not retreat at all. This spatial variability in recessional rates is evident in plots of the

scarp crest location with time (Fig. 30). Recession rates for the September 1993 flooding cycle

were a maximum of 1.4 m. The extreme rate of

change of 45 m was generated by the development of a new network of shallow (0.5 m) gullies

in the mudflats between C and Bread Truck

ponds during the 1992–93 winter (Fig. 30c).

The variability in recession rate reflects the

nature of the erosional processes in causing rapid

short-term changes (Table 2). Recession appears

to be caused mostly by current erosion of the

lower, unvegetated part of the near-vertical gully walls during ebb. Because the uppermost 20

to 30 cm of material is consolidated and rootbound, this soil and root mat are undermined by

current erosion and only fail after an erosional

niche of approximately 0.5 m or deeper is cut below it.

Eroded sediments form deposits within the

gullies and are eventually transported down-gully into the Eagle River. Along steep scarps, blocks

of consolidated sediments bound by roots fall

and roll into the gully bottoms where currents

eventually may break them down by scour or by

rolling them along the gully bed. The lateral

walls of gullies, which tend to develop low angle

slopes as headwalls recede inland, fail mainly by

retrogressive slump flow, the lower half or more

of the slope creeping slowly as a mudflow into

the gully channel. On gentler slopes, blocks of

rootbound material remain intact as they are

transported downslope by slow-moving mudflows that are active in the latter stages of the ebb

cycle. Because this sequence of events happens at

different times and rates, recession varies within

as well as among the study sites (Table 6).

If our measurements of recession rate are characteristic, erosion will cause increased drainage of

ponds within the next 10 to 15 years. Two ponds

in particular will likely be affected: one is Bread

Truck, on its northern side, and the other is the

pond complex between the Bread Truck and C

ponds (Fig. 5). Further analyses of recession rates

are needed to determine if the 1992 to 1993 rates

are representative of more than this 1-year

period.

Table 6. Headwall recession rates in 1992 and 1993.

Recession range (m)

Erosion Location May–Sept 92 Sept 92–June 93 June-Sept 93

site Drainage Position* Min Max Min Max Min Max

1 B† L 0.0 2.1 0.3 3.2 0.1 3.9

2 B L 0.0 4.9 0.0 2.0 0.0 1.2

3 C L 0.0 0.6 0.0 0.9 0.0 0.0

4 C H 0.1 3.1 0.5 1.5 0.0 3.8

5 C L 0.0 1.3 0.0 0.8 0.0 0.2

6 D—S. Fork L 0.1 2.4 0.0 0.6 0.0 1.1

7 D—S. Fork H 0.0 1.1 0.0 0.4 0.0 0.8

8 D—Cent. Fork L 0.0 0.1 0.0 1.0 0.0 0.4

9 D—Cent. Fork H 0.0 0.5 0.1 6.3** 0.0 9.8

10 D—Cent. Fork L 0.0 0.2 0.0 0.5 0.0 0.0

11 D—N. Fork L 0.0 0.5 0.0 0.4 0.0 1.0

12 F L — — — — 0.0 3.1

13 F L — — — — 0.1 1.8

14 F L — — — — 0.0 1.2

* L = lateral wall, H = headwall.

† See Figure 8.

** 45-m shallow erosion extending into mudflats (Fig. 30).
32

b. Sites 3–5.

Scarp Profile

Gully

Thalweg

Stakes

Hub

0 10 m

Summer 1 (22 Se 92)

Winter ( 2 Jun '93)

Summer 2 (22 Sep '93)

c. Sites 8–10.

Measured Points

Original (27 May '92)

Summer 1 (22 Sep '92)

Winter ( 2 Jun '93)

Summer 2 (22 Sep '93)

Figure 30. Examples of spatial variation in erosion rates measured at gully headwall sites. Only changes are

shown by symbols.

lower accretion rates than the ponds (Table 7),

while within gullies, rates could far exceed these

values. Lower sedimentation rates in marshes

may reflect problems in sampling within the

thick sedges, including an unknown amount

trapped in the vegetation, rather than the actual

rate of accretion.

The relationship of rate to elevation is in part

a response to the number of times sites are inun

Gully

Thalweg

Scarp 

Profile

Stakes

Hub

Original (27 M

Summer 1 (22

Winter ( 2 Jun

Summer 2 (22

Measured P

Slump

Block

a. Site 1.

0 1 m

Original (27 May '92)

Summer 1 (22 Sep '92)

Winter ( 2 Jun '93)

Summer 2 (22 Sep '93)

Measured Points

Scarp

Profile

Hub

Stakes

Parachute

Pool

0 5 m

Parachute

Pool

SEDIMENTATION RATES

Sedimentation rates vary with morphological

unit and, in a general sense, with elevation. The

overall trend is an increase eastward from the

Eagle River levees, across the mudflats and into

the ponds. Marshes, however, apparently had
33

Table 7. Gross sedimentation rates in 1992 and 1993.

Range (mm)

Morphological Summer Winter Summer

unit 92 92–93 93

Levee 1– 2 1–5 1– 5

Vegetated upper mudflat 3– 6 3–10 1– 8

Unvegetated lower mudflat 5– 12 7–12 3– 10*

Pond—plate 18 – 35 8–28 2– 12

Pond—cup 5–38 6– 15

Marsh 2– 10 (est) ND† ND

Gully 100– 300 28 –75** 60**

* Larger values from craters.

† ND = no data.

** Based on small sample size.

events covering each landform. During the entire period, 110 tides exceeded the 4.6-m elevation necessary to enter the ponds (Table 8). In

contrast, levee elevation data suggest that during the same period, only 15 tides were sufficiently high to inundate them completely. The

average sediment accumulation in the ponds

from May 1992 to September 1993 was 46.5 mm,

yielding a gross rate per tidal cycle of 0.4 mm.

The average rate per tidal flooding on levees is

0.3 mm.

The higher rates of sedimentation in ponds

than on river levees clearly show the more important role of tidal inundation than Eagle River

floods as a sediment source and thus, within the

study area, sedimentation is tidally dominated.

The large amount of suspended sediment in

Knik Arm increases its importance relative to

the Eagle River, and contrasts with other tidally

dominated salt marshes and mudflats where

glaciers are not important sources of marine suspended sediments. The role of the river is lessened by its apparently lower suspended sediment transport rate, including the lack of sediment influx during winter.

The amount of material that originated by resuspension of pond bottom sediments ranged

from 8 to 16 mm for the summer of 1992, or approximately equivalent to the thickness of new

sediments deposited in the ponds during the

dated, but other factors appear important as

well. Assuming a general elevation of 4.6 m for

ponds to flood (a 9.48-m Anchorage datum tide),

Table 8 shows a minimum of 21 sediment-bearing tidal floodings would have filled ponds,

mainly during the late July to early August and

late August to early September tidal cycles.

Through September 1992 to May 1993, the

ponds were inundated by a minimum of 66 sediment-bearing tides monthly (except December),

with the highest tides in March and April. Although there may have been additional flooding

during summer because of higher discharges,

the range in sedimentation rates for ponds during the winter with more flooding tides was

similar to that of the previous summer (1 to 38

mm). This similarity may reflect several other

factors, including lower sediment concentrations

in winter flood waters, or ice cover effects, such

as the freeze-on and removal of pond bottom

sediments during freezeup or breakup. From

May to September 1993, 23 sediment-bearing

floods occurred, mainly from July through September. These rates were generally lower than in

either the summer or winter of 1992, ranging

from 1 to 15 mm. The reasons for these reduced

rates are not yet known.

Gross sedimentation rates were higher within

ponds than on river levees (Table 7); this appears related to the total number of flooding

Table 8. Number of flooding events reaching

critical heights to cover landforms in 1992 and

1993.

Number of flooding events

Critical height Summer 92 Winter 92–93 Summer 93

Ponds (4.6 m) 21 66 23

Mudflats (4.87 m) 13 36 15

Levees (5.21 m) 2 9 4
same period (Table 7). During the late August to

early September flooding cycle, 3–6 mm of new

material was deposited, while 2–6 mm of material had been resuspended and redeposited. During the winter 1992–93 period, resuspension sedimentation ranged from 2 to 15 mm, or similar to

that of the summer 1993 amounts.

Within gullies, deposition occurs mainly along

the lateral slopes away from the channel thalweg.

Depending on the depositional process, rates

could range as high as 10 to 30 cm for the summer period, with several or more centimeters

being deposited during a single flood and ebb

cycle. These rates primarily reflect adjustments in

the gully cross section, rather than deposition

from tidal flood waters. Mass movements of

slope material account for the majority of the

sediment deposited on lateral slopes. Sedimentation rates are therefore highly variable from

place to place. Note that the 1992–93 winter and

1993 summer gully measurements are not

directly comparable to the 1992 summer measurements because most of the 1992 stakes within

the center of the gully were lost to erosion or

burial.

SYSTEM DYNAMICS AND REMEDIATION

Eagle River Flats is a dynamic estuarine salt

marsh that is actively undergoing progressive

and significant changes in the physical environment. These changes result from the interaction

of multiple physical processes with various factors that include external forces such as a high

tidal range, a glacial river, two relatively large

sediment sources and a cold climate. The location

of ERF within an active earthquake zone adds

further to the complexity and the potential for future rapid, but probably unpredictable, physical

changes. If the area subsides, or conversely uplifts, because of an earthquake, major changes in

the hydrology and terrain can be expected. While

pond areas may increase because of subsidence,

as happened in 1964, the drainage system and

river channel may respond with increases in

rates of erosion or sedimentation because the

drainageways will be out of equilibrium with the

altered physical environment. In addition,

ground explosions and cratering during the use

of ERF as a military firing range since the early

1940s have caused physical changes to the terrain, hydrology and surface drainage. Craters,

for example, have locally increased erosion in

drainages and enhanced gully expansion.

The inherent complexity of this dynamic environment makes it extremely difficult to predict what effects potential remedial measures

for WP contamination will have on the physical

system and conversely what short- and longterm effects the physical system will have on the

effectiveness and success of proposed remedies.

Understanding both the system’s response and

the effects of remedial measures are critical to

developing cleanup strategies. Further analysis

of the processes and factors determining tidal

flat hydrology and sedimentology are required

to predict the system’s response to a particular

remedial technology. The remediation process

may be enhanced or hindered by the physical

system. Questions regarding remediation will

be addressed in our continuing investigations of

the physical system dynamics of Eagle River

Flats.

LITERATURE CITED

Andrew, J. and G. Cooper (1993) Sedimentation

in a river dominated estuary. Sedimentology, 40:

979–1017.

Allen, J.R.L. (1982) Sedimentary structures:

Their character and physical basis. In Developments in Sedimentology, vol. 30A, B. New York:

Elsevier.

Allen, J.R.L. (1990a) Constraints on measurements of sea-level movements from salt marsh

accretion rates. Journal of the Geological Society,

London, 147: 5–7.

Allen, J.R.L. (1990b) Salt marsh growth and

stratification: A numerical model with special

reference to Severn Estuary, southwest Britain.

Marine Geology, 95: 77–96.

Allen, J.R.L. (1992) Large-scale textural patterns

and sedimentary processes on tidal salt marshes

in Severn Estuary, southwest Britain. Sedimentary Geology, 81: 299–318.

Allen, J.R.L. and J.E. Rae (1988) Vertical salt

marsh accretion since the Roman period in the

Severn Estuary, southwest Britain. Marine Geology, 83: 225–235.

Bartsch-Winkler, S. and A.T. Ovenshine (1984)

Macrotidal subarctic environment of Turnagain

and Knik Arms, Upper Cook Inlet, Alaska: Sedimentology of the intertidal zone. Journal of Sedimentary Petrology, 54: 1221–1238.

Bloom, A.L. (1984) Peat accumulation and compaction in a Connecticut coastal marsh. Journal

of Sedimentary Petrology, 34: 599–603.

Boon, J.D. (1975) Tidal discharge asymmetry in a

34
35

tion rates on tidal salt marshes. Journal of Sedimentary Petrology, 47: 1484–1490.

Hubbard, J.C.E. and R.E. Stebbings (1968) Spartina marshes in southern England III. Stratigraphy

of the Keyworth marsh, Poole Harbour. Journal of

Ecology, 56: 707–722.

Kearney, M.S. and L.G. Ward (1986) Vertical accretion rates in brackish marshes of a Chesapeake

Bay tributary. Geo-Marine Letters, 6: 41–49.

Keene, H.W. (1971) Postglacial submergence and

salt marsh evolution in New Hampshire. Maritime Sediments, 7: 64–68.

Kohsiek, L.H.M., J. Alphen, P. Bloks and P. Hoekstra (1981) A water and sediment balance of a

salt marsh in the Eastern Scheldt Estuary (Research Rept. RWS Nr. DDWT 81.042).

Krone, R.B. (1987) A method for simulating historic marsh elevations. In Coastal Sediments ‘87

(N.D. Krause, Ed.). New York: American Society

of Civil Engineers, p. 316–323.

Lawson, D.E. (1993) Glaciohydrologic and glaciohydraulic effects on runoff and sediment yield in

glacierized basins. USA Cold Regions Research

and Engineering Laboratory, Monograph 93–2.

Lawson, D.E. and B.E. Brockett (1993) Preliminary

assessment of sedimentation and erosion in Eagle

River Flats, southcentral, Alaska. USA Cold Regions Research and Engineering Laboratory, CRREL Report 93–23.

Leopold, L.B., M.G. Wolman and J.P. Miller (1964)

Fluvial Processes in Geomorphology. San Francisco:

W.H. Freeman, Co.

Letzch, W.S. and R.W. Frey (1980) Deposition and

erosion in a Holocene salt marsh. Journal of Sedimentary Petrology, 50: 529–542.

McCann, S.B., J.E. Dale and P.B. Hale (1981) Subarctic tidal flats in areas of large tidal range,

southern Baffin Island, eastern Canada. Geographie Physique et Quaternaire, 35: 183–204.

McLaren, P., M.B. Collins, S. Gao and R.I.L. Powys

(1993) Sediment dynamics of the Seven Estuary

and Inner Bristol Channel. Journal of the Geological

Society of London, 50: 589–603.

Ovenshine, A.T., D.E. Lawson and S.R. BartschWinkler (1976a) The Placer River silt: An intertidal

deposit caused by the 1964 Alaskan earthquake.

Journal of Research, U.S. Geological Survey, 4: 151–

162.

Ovenshine, A.T., S.R. Bartsch-Winkler, N.R.

O’Brien and D.E. Lawson (1976b) Sediment of the

high tidal range environment of upper Turnagain

Arm, Alaska. In Recent and Ancient Sedimentary

Environments in Alaska (T.P. Miller, Ed.). Memoir,

Alaskan Geological Society, p. M–1 to M–26.

salt marsh drainage system. Limnology and Oceanography, 20: 71–80.

Carling, P.A. (1981) Sediment transport by tidal

currents and waves: observations from a sandy

intertidal zone (Burry Inlet, South Wales). In

Holocene Marine Sedimentation in the North Sea Basin (S.D. Nio, R.T.E. Schüttenhelm and Tj.C.E.

van Weering, Ed.). I.A.S. Special Publication 5:

65–80.

Carling, P.A. (1982) Temporal and spatial variation in intertidal sedimentation rates. Sedimentology, 29: 17–23.

Collins, L.M., J.L. Collins and L.B. Leopold (1987)

Geomorphic processes of an estuarine marsh:

preliminary results and hypotheses. International

Geomorphology 1986, p. 1049–1072.

Dalrymple, R.W., B.A. Zaitlin and R. Boyd (1992)

Estuarine facies models: conceptual basis and

stratigraphic implications. Journal of Sedimentary

Petrology, 62: 1130–1146.

Dionne, J.C. (1988) Characteristic features of

modern tidal flats in cold regions. In Tide-Influenced Sedimentary Environments and Facies (P.L.

deBoer, A. van Gelder and S.D. Nio, Ed.). Boston: D. Reidel Publishing Company, p. 301–332.

Evans, C.D., E. Buch, R. Buffler, G. Fish, R.

Forbes and W. Parker (1972) The Cook Inlet environment: A background study of available knowledge. University of Alaska Sea Grant Program.

French, J.R. (1991) Eustatic and neotectonic controls on saltmarsh sedimentation. In Coastal Sediments ‘91 (N.C. Kraus, K.J. Gingerich and D.L.

Kriebel, Ed.). New York: American Society of

Civil Engineers, p. 1223–1236.

French, J.R. (1993) Numerical simulation of vertical marsh growth and adjustment to accelerated

sea-level rise, north Norfolk, U.K. Earth Surface

Processes and Landforms, 18: 68–81.

French, J.R. and T. Spencer (1993) Dynamics of

sedimentation in a tide-dominated backbarrier

salt marsh, Norfolk, U.K. Marine Geology, 110:

315–331.

Frostick, M.E. and I.N. McCave (1979) Seasonal

shifts of sediments in an estuary mediated by algal growth. Estuarine, Coastal and Shelf Science, 9:

569–576.

Gurnell, A.M. and J.C. Clark, Ed. (1987) GlacioFluvial Sediment Transfer. New York: John Wiley

and Sons.

Hanson, H.C. (1951) Characteristics of some

grassland, marsh and other plant communities

in western Alaska. Ecological Monographs, 21:

327–378.

Harrison, E.Z. and A.L. Bloom (1977) Sedimenta-
36

Racine, C.H., M.E. Walsh, C.M. Collins, D.J. Calkins, B.D. Roebuck and L. Reitsma (1992a) Waterfowl mortality in Eagle River Flats, Alaska. USA

Cold Regions Research and Engineering Laboratory, CRREL Report 92–5.

Racine, C.H., M.E. Walsh, B.D. Roebuck, C.M.

Collins, D. Calkins, L. Reitsma, P. Buchli and G.

Goldfarb (1992b) White phosphorus poisoning

of waterfowl in an Alaskan salt marsh. Journal of

Wildlife Diseases, 28(4): 669–673.

Racine, C.H., M.E. Walsh, C.M. Collins, D. Lawson, K. Henry, L. Reitsma, B. Steele, R. Harris and

S.T. Bird (1993) White phosphorus contamination of salt marsh sediments at Eagle River Flats,

Alaska. USA Cold Regions Research and Engineering Laboratory, CRREL Report 93–17.

Reed, D.J. (1990) The impact of sea-level rise on

coastal salt marshes. Progress in Physical Geography, 14: 465–481.

Richard, G.A. (1978) Seasonal and environmental variations in sediment accretion in a Long Island salt marsh. Estuaries, 1: 29–35.

Rosenberg, D.H. (1986) Wetland types and bird

use of Kenai Lowlands. Anchorage, Alaska. U.S.

Fish and Wildlife Service, Special Studies.

Röthlisberger, J. and H. Lang (1987) Glacial hydrology. In Glacio-Fluvial Sediment Transfer (A.M.

Gurnell and J.C. Clark, Ed.). New York: John

Wiley and Sons, p. 207–284.

Stoddart, D.R., D.J. Reed and J.R. French (1989)

Understanding salt marshes. Estuaries, 12: 228–

236.

Small, J.B. and L.C. Wharton (1969) Vertical displacements determined by surveys after the

Alaska earthquake of March 1964. The Prince

William Sound, Alaska, earthquake of 1964 and

aftershocks. In Research Studies and Interpretive

Results, Geodesy and Photogrammetry (L.E. Leipold and F.J. Wood, Ed.). Washington, D.C.: U.S.

Coast and Geodetic Survey, Volume III, p. 21–

33.

USGS (1981) Water resources data for Alaska,

water year 1980. U.S. Geological Survey Water

Resources Division, Annual Report for Alaska.

USGS/WRD/HD-81/004 and USGS/WRD/AK80-1.

Vince, S.W. and A.A. Snow (1984) Plant zonation

of an Alaskan salt marsh. II. An experimental

study of the role of edaphic conditions. Journal of

Ecology, 72: 669–684.
37

APPENDIX A: RECESSION RATE SUMMARY

(sites located on Figure 8)

Erosion rate (m)

Hub Stake Summer ‘92 Winter Summer ‘93

Drainage B

Site 1 (established 27 May 92)

B 28 1.3 2.1 3.9

96 1.3 3.2 2.1

26 2.1 0.6 0.6

98 1.3 0.5 0.9

29 0.6 0.9 0.9

97 0.0 0.6 0.1

27 0.0 0.3 0.3

Min 0.0 0.3 0.1

Max 2.1 3.2 3.9

Site 2 (established 27 May 92)

A 92 0.0 0.1 0.0

38 0.0 0.4 0.0

87 0.1 0.3 0.6

48 4.9 2.0 0.9

86 0.4 0.0 0.0

43 0.0 0.0 0.0

91 0.1 0.3 0.3

47 0.2 0.6 0.0

93 0.0 0.4 0.0

45 2.0 1.3 0.2

100 — — 1.2

101 — — 1.2

102 — — 0.8

Min 0.0 0.0 0.0

Max 4.9 2.0 1.2

Parachute gully (drainage C)

Site 3 (west side, established 27 May 92)

31 32 0.1 0.0 0.0

81 0.0 0.4 0.0

41 0.0 0.3 0.0

88 0.6 0.0 0.0

33 0.0 0.9 0.0

Min 0.0 0.0 0.0

Max 0.6 0.9 0.0

Site 4 (south side, established 27 May 92)

85 34 1.0 — —

95 0.8 0.5 0.0

44 0.7 — —

80 3.1 1.5 0.0

49 0.2 — —

78 0.1 — —

42 0.3 0.8 3.8

Min 0.1 0.5 0.0

Max 3.1 1.5 3.8

Site 5 (east side, established 27 May 92)

94 84 0.1 0.8 0.0

36 0.1 0.2 0.2

76 0.2 0.6 0.1

40 0.0 0.1 0.0

89 0.7 0.0 0.0

37 1.3 0.2 0.0

Min 0.0 0.0 0.0

Max 1.3 0.8 0.2

“In between” drainage–southern fork (drainage D)

Site 6 (west side, established 28 May 92)

64 57 2.4 0.0 0.0

4 0.8 0.3 0.0

68 0.1 0.0 0.0

3 0.1 0.1 0.3

71 0.4 0.1 1.1

5 0.1 0.0 0.0

51 0.9 0.0 0.0

2 1.8 0.6 0.0

56 1.0 0.3 0.3

Min 0.1 0.0 0.0

Max 2.4 0.6 1.1

Site 7 (southeast upstream side, established 28 May 92)

66 6 1.1 0.0 0.5

61 0.1 0.2 0.4

1 0.1 0.3 0.0

53 0.0 0.0 0.0

7 0.0 0.4 0.0

52 0.1 0.2 0.8

Min 0.0 0.0 0.0

Max 1.1 0.4 0.8

“In between” drainage–central fork (drainage D)

Site 8 (east side, established 28 May 92)

13 65 0.1 0.0 0.0

8 0.1 1.0 0.0

58 0.1 0.1 0.4

17 0.0 0.0 0.0

60 0.0 0.4 0.0

12 0.1 0.0 0.0

54 0.1 0.5 0.1

9 0.0 0.1 0.0

62 0.0 0.0 0.2

Min 0.0 0.0 0.0

Max 0.1 1.0 0.4

Site 9 (south side, established 28 May 92)

10 72 0.0 0.4 0.0

20 0.1 0.1 0.0

73 0.5 0.1 0.0

23 0.0 0.2 0.7

63 0.0 0.1 9.8

25 0.0 6.3 0.6

Min 0.0 0.1 0.0

Max 0.5 6.3 9.8

Site 10 (west side, established 28 May 92)

16 55 0.1 0.3 0.0

15 0.1 0.0 0.0

70 0.0 0.2 0.0

18 0.1 0.1 0.0

67 0.0 0.1 0.0

24 0.2 0.2 0.0

75 0.0 0.1 0.0

22 0.0 0.1 0.0

59 0.0 0.1 0.0

21 0.1 0.0 0.0

69 0.0 0.5 0.0

Min 0.0 0.0 0.0

Max 0.2 0.5 0.0

Erosion rate (m)

Hub Stake Summer ‘92 Winter Summer ‘93
38

Erosion rate (m)

Hub Stake Summer 92 Winter Summer 93

Site 13 (on southern offshoot to west, established 2 July 93)

109 105 — — 1.8

106 — — 1.7

107 — — 0.1

108 — — —

Min — — 0.1

Max — — 1.8

Site 14 (north of hydrostation, established 2 July 93)

111 112 — — 0.0

113 — — 0.0

114 — — 0.1

115 — — 1.2

116 — — 0.4

117 — — 0.0

Min — — 0.0

Max — — 1.2

Erosion rate (m)

Hub Stake Summer 92 Winter Summer 93

“In-between drainage” - northern fork (drainage D)

Site 11 (east side, established 28 May 92)

79 19 0.0 0.0 0.0

46 0.2 0.0 0.0

39 0.1 0.0 0.0

83 0.1 0.0 0.0

50 0.1 0.0 0.0

77 0.1 0.0 0.0

14 0.1 0.0 0.0

82 0.1 0.0 0.0

35 0.1 0.0 0.1

90 0.0 0.0 1.0

11 0.0 0.0 0.0

30 0.5 0.4 0.1

Min 0.0 0.0 0.0

Max 0.5 0.4 1.0

Bread Truck drainage (drainage F)

Site 12 (on southern offshoot to west, established 2 July 93)

100 104 — — 0.5

103 — — 0.0

102 — — 0.8

101 — — 2.1

110 — — 3.1

Min — — 0.0

Max — — 3.1
1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

6. AUTHORS

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

 REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS 15. NUMBER OF PAGES

16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

 OF REPORT OF THIS PAGE OF ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and

maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,

including suggestion for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington,

VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

46

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

March 1995

Initial Analyses of Eagle River Flats Hydrology and Sedimentology,

Fort Richardson, Alaska

Daniel E. Lawson, Susan R. Bigl, John H. Bodette and Patricia Weyrick

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 95-5

Hanover, New Hampshire 03755-1290

U.S. Army Engineer District, Alaska

Fort Richardson, Alaska

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

The physical environment of Eagle River Flats (ERF), a subarctic tidal flat and salt marsh, is progressively

changing because of the interactions of multiple physical processes, including a high tidal range, two primary

sediment sources, cold climate and location within an active earthquake zone. In addition, ERF has been used by

the U.S. Army as an artillery range, where high explosives or smoke-producing shells have been detonated,

causing cratering and disrupting drainage. The physical environment of ERF needs to be understood to help remediate a problem of unusually high mortality rates in migrating waterfowl. This high mortality of ducks is attributable to ingestion of elemental white phosphorus (P4) particles (from smoke-producing devices), which are

now distributed within near-surface sediments of the ponds and marshes. The complexity of this dynamic environment makes it extremely difficult to predict what physical effects remedial measures for the P4 contamination will have and, conversely, what short- and long-term effects the physical system will have on the effectiveness and success of proposed remedies. Understanding both the system’s response and the effects of remedial

technologies is critical to deciding what measures are used. This report presents the initial analysis of the physical processes of erosion, sedimentation and sediment transport and the factors controlling their activity within a

portion of ERF.

Alaska Hydrology White phosphorus

Artillery Sediments

Eagle River Flats Waterfowl mortality