ifim ay) 
Erosion and Sedimentation 

in the Kenai River, Alaska

By KEVIN M. SCOTT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1235

Prepared in cooperation with the 

U.S. Fish and Wildlife Service

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Scott/ Kevin M./ 1935-

Erosion and sedimentation in the Kenai River/ Alaska.

(Geological Survey professional paper ; 1235)

Bibliography: p. 33-35

Supt. of Docs, no.: I 19.16:1235

1. Sediments (Geology) Alaska Kenai River watershed. 2. Erosion 

Alaska Kenai River watershed. I. U.S. Fish and Wildlife Service.

II. Title. III. Series: United States. Geological Survey

Professional Paper 1235.

QE571.S412 553.7'8'097983 81-6755

AACR2

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page

Abstract _-_----___---_-______________________________ 1

Introduction __________________________________________ 1

The Kenai River watershed -_-------_---__--_____________ 3

Climate ____________________________________ 3

Vegetation ________________________________________ 3

Hydrology ____________________________________________ 4

Quaternary history of the Kenai River valley --__----_-___--_ 6

Evidence of proglacial lake in Cook Inlet -_---___________ 6

Terraces and river entrenchment ______________________ 9

Topography of the Kenai Lowlands and course of the Kenai

River _--------_-_____________-___-______________ 9

Channel of the Kenai River _--------_____________________ 12

Stream type_______________________________________ 12

Channel pattern ___________________________________ 13

Investigation of underfit condition _--------____________ 13

Flow/depth variation within meanders and with differing

channel pattern __________________________________ 14

Asymmetry of cross sections at bends __________________ 15

Slope ------------____-______-__._________________ 15

Bed material __________________________________________ 15

Page

Bed material Continued

Gravel dunes in channel below SkilakLake ___-----_-___- 17

Armoring of the channel _---____------------_-_-----_ 18

Possible effects of armoring on salmon habitat ___________ 19

Surficial deposits of the modern flood plain _____________ 19

Suspended sediment ________________________________ 20

Bank erosion __________________________________________ 23

Methodology ----_-_-______-----__---_----__-----_- 23

Mechanics of bank erosion low banks and high banks _ _ _ _ 24

Rates of bank erosion ______________-_---_-_-___-_-__ 24

Possible recent increase in bank erosion ________________ 26

Development and the Kenai River channel __________________ 28

Consequences of development _________--_--_--______- 28

Canals ..._________________-----_-------_- 30

Groins and boat ramps ______________________________ 31

Excavated boat slips _----_____-___------_-___-----_- 32

Bank-protection structures _-__-___-__----______--_-_ 32

Gravel mining and commercial developments ____________ 32

Conclusions ____----__---______--___---------___------- 33

References cited ________-______________------__-------- 33

ILLUSTRATIONS

FIGURE

7.

8.

9.

10.

11.

12.

13.

14.

15.

16. 

17-19.

Page 

Location map of the Kenai River downstream from SkilakLake _____--_____-----__-----_----------------------- 2

Hydrographs of monthly discharge at gaging stations at Cooper Landing and Soldotna ______-_--__---------__------ 4

Successive downstream hydrographs for the flood of September 1974 originating from an unnamed glacially dammed

lake in the headwaters of the Snow River ______-______-_________--_---------__--------------------------- 5

Aerial photograph showing the Kenai River at the Soldotna Bridge ________--_-__-_-___-----_--------------_---- 8

Profiles of the Soldotna terrace and the Kenai River (water surface at intermediate flow level) measured along the valley

axis ---_____________________________________________________---_--_-___--------------------_------ 9

Graphs showing channel width at bankfull stage, slope of water surface, meander wavelength, number of channels, and

sinuosity index -_______________________________-______________--_____---__------------------_------- 10

Aerial photograph of the Moose River channel between 1.3 and 2.6 miles upstream from its junction with the Kenai River 11 

Drainage network in the Kenai River watershed near the front of the Kenai mountains ________________-_-____----_- 12

Plot of meander wavelength against bankfull discharge ----_-_------__-----------_--------------------------- 13

Plot of meander wavelength against channel width at bankfull stage _________--__-_____--_-_-----_------__------ 14

Plot of bed- material size against river miles ________-_---___---__--------_--------_--------------_--------- 16

Aerial photograph of the Kenai River between approximate river miles 47.5 and 46.9 ___________-_-_-----___------- 17

Plot of water discharge against suspended-sediment concentration, Kenai River at Soldotna ____________--_____----- 20

Plot of water discharge against suspended- sediment concentration, Kenai River at Soldotna, August 23 to December 5,

1979 --------_-__________-___ ____ ______________________ _ _________________ ___ ________________ 22

Map of reach in upper section of Kenai River, showing bank-erosion rates _-_-_-_--____-_--____--------__------_- 25

Map of reach in lower section of Kenai River, showing bank-erosion rates _ --__________-______----------_----__-- 25

Aerial photographs showing:

17. Kenai River between approximate river miles 16.7 and 15.3 ______________________________________________ 27

18. Kenai River between approximate river mDes 38.2 and 37.0 ______________________________-__----__------_ 28

19. Kenai River between approximate river miles 44.8 and 42.9 _______________________________-----_------_-- 31

m
IV CONTENTS

TABLES

Page

TABLE 1. Late Quaternary history (Wisconsinan to present) of the Cook Inlet area and correlation with the geomorphology of the

KenaiRiver ____________________________________________ _________________________________ 7

2. Statistical analysis of maximum flow depths at cross sections measured August 23-24,1974 ________________________ 15

3. Aerial photography of the Kenai River downstream from SkilakLake ________----_--_----------------_------_--- 23

4. King salmon taken by sport fishing in the Kenai River, 1974-79 _-______________-________-----_---------_------ 26

5. Summary of channel characteristics pertinent to determining sensitivity of the Kenai River to development ____________ 29

CONVERSION FACTORS

Multiply inch-pound unit

°F (degree Fahrenheit)

in.( inch)

ft (foot)

mi (mile)

mi2 (square mile)

ft3 /s (cubic foot per second)

By

6/9 (F-32)

2.540x10

3.048x10

1.609

2.590

2.832X10,-2

To obtain metric unit

°C (degree Celsius)

mm (millimeter)

m (meter)

km (kilometer)

km2 (square kilometer)

m3 /s (cubic meter per second)

National Geodetic Vertical Datum of 1929 (NGVD of 1929), the reference surface to which relief features and altitude data are related, and 

formerly called "mean sea level," is herein called "sea level."

Any use of trade names or trademarks in this publication is for 

descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
EROSION AND SEDIMENTATION IN THE KENAI RIVER,

ALASKA

By KEVIN M. SCOTT

ABSTRACT

The Kenai River system is the most important freshwater fishery in 

Alaska. The flow regime is characterized by high summer flow of glacial melt water and periodic flooding caused by sudden releases of 

glacier-dammed lakes in the headwaters. Throughout most of its 

50-mi course across the Kenai Peninsula Lowlands to Cook Inlet, the 

river meanders within coarse bed material with a median diameter 

typically in the range 40-60 mm. Every nontidal section of the stream 

is a known or potential salmon-spawning site.

The stream is underfit, a condition attributed to regional glacial recession and hypothesized drainage changes, and locally is entrenched 

in response to geologically recent changes in base level. The coarseness of the bed material is explained by these characteristics, combined with the reservoirlike effects of two large morainally impounded lakes, Kenai and Skilak Lakes, that formed as lowland 

glaciers receded. Throughout the central section of the river the 

channel is effectively armored, a condition that may have important 

long-term implications for the ability of this section of channel to 

support the spawning and rearing of salmon.

The 3.8-river-mile channel below Skilak Lake contains submersed, 

crescentic gravel dunes with lengths of more than 500 ft and heights 

of more than 15 ft. Such bed forms are highly unusual in streams 

with coarse bed material. The dunes were entirely stable from 1950 

to at least 1977, so much so that small details of shape were unmodified by a major glacial-outburst flood in 1974. The features are the 

product of a flood greatly in excess of any recorded discharge.

The entrenched section of the channel has been stable since 

1950-51 or earlier; only negligible amounts of bank erosion are indicated by sequential aerial photographs. Bank erosion is active both 

upstream and downstream from the entrenched channel, however, 

and erosion rates in those reaches are locally comparable to rates in 

other streams of similar size. Although erosion rates have been generally constant since 1950-51, evidence suggests a possible recent 

decrease in bank stability and an increase in erosion that could be 

related to changes in river use.

The high sustained flow of summer encourages a variety of 

recreation-related modification to the bank and flood plain canals, 

groins, boat ramps, slips, embankments, as well as commercial developments. As population and recreational use increase, development can pose a hazard to the productivity of the stream through increased suspended-sediment concentration resulting directly from 

construction and, with greater potential for long-term impact, indirectly from bank erosion. A short-term hazard to both stream and 

developments is the cutoff of meander loops, the risk of which is increased by canals and boat slips cut in the surface layer of cohesive, 

erosion-resistant sediment on the flood plain within nonentrenched 

meander loops. A significant long-term hazard is an increase in bank 

erosion rates resulting from the loss of stabilizing vegetation on the 

high (as high as 70 ft) cutbanks of entrenched and partly entrenched 

sections of channel. Potential causes of erosion and consequent vegetation loss are river-use practices, meander cutoffs, and groin construction.

INTRODUCTION

The Kenai River is a large proglacial stream draining 

the inland side of the Kenai Mountains and crossing 

the lowlands of the Kenai Peninsula to Cook Inlet. The 

most obvious feature of the river in the lowlands is the 

presence of coarse bed material in association with a 

meandering pattern; in the spectrum of bed-material 

sizes of meandering streams, the Kenai River is near 

the coarse end. Both the bed material and the channel 

pattern reflect previous geologic intervals when discharge was greater and glaciers were more widespread. 

Glaciers continue to influence the hydrology of the 

river, extending today within the watershed to altitudes 

below 500 ft. The major flood discharges have originated historically from outbursts of a glacier-dammed 

lake every 2 to 4 years.

The Kenai River system is the most heavily used 

freshwater fishery in Alaska (U.S. Army Corps of Engineers, 1978, p. 126). Salmon fishing attracts increasing numbers of visitors from the Anchorage area during the summer, particularly for the runs of king salmon. The development associated with this recreational 

use, though small in scale, is expanding rapidly along 

the downstream 45.5 river miles that lies mainly in a 

corridor of State-owned and private lands outside the 

Kenai National Moose Range (fig. l). The potential for 

further development, evidenced by the demand for recreational property and the population increases in 

communities within the corridor, is large. For details 

on the environment of the river and the associated 

197-mi2 corridor, readers can refer to the comprehensive survey by the U.S. Army Corps of Engineers 

(1978).

The section of the river described in this report is the 

50.3 mi of channel below Skilak Lake, a large 

moraine-impounded lake with influence on the flow regime of the river (fig. l). The purpose of the study is to 

describe the recent history, geomorphic characteristics, and sedimentation system of the stream 

downstream from Skilak Lake and to indicate the 

types, locations, and timing of development that could 

prove harmful to the fluvial habitat in its ability to 

support the spawning and rearing of salmon. This report is concerned mainly with developments in the

l
151°00' 

COOK INLET

60°30'

KENAI NATIONAL MOOSE

KENAI NATIONAL MOOSE

EXPLANATION

16'

River mile

Bend number

01234! 4 5 6 7 KILOMETERS 

1 M 1

012345 MILES

Lake 

EX

0 200 400 600 KILOMETERS

FIGURE 1. Configuration of the Kenai River downstream from Skilak Lake. Glacial lakes capable of yielding potentially hazardous floo

discharges are numbered in accordance with Post and Mayo (1971).
THE KENAI RIVER WATERSHED

category of alterations to the navigable channel for 

which a permit from the Department of the Army is required. Upland development and land-use changes are 

not considered.

In a stream the size and type of the Kenai River, increased suspended-sediment transport will be the first 

general effect of development with the potential to be 

deleterious to the physical stream system, chiefly 

through deposition of fine sediment in the pores of the 

streambed gravel. Consequently, the present levels of 

suspended-sediment concentration and the possible 

causes of future increases are emphasized. Other 

changes in the sediment system are, of course, possible.

The most important feature of the environment to the 

economy of the area is the ability of the Kenai River to 

act as the freshwater habitat for salmon taken directly 

by sport fishing and indirectly by commercial fishing in 

Cook Inlet. Four species (king, sockeye, silver, and 

pink) use the river for spawning in runs from early 

spring to as late as December. The presence of chum 

salmon has also been reported. The young of three valuable species (king, sockeye, and silver) are found in 

the stream year round. Every nontidal part of the river 

is a known or potential spawning site for at least one 

species (U.S. Army Corps of Engineers, 1978, fig. 27).

Salmon-producing habitats are sensitive to many factors, but most importantly to sedimentation and water 

temperature (Meehan, 1974, p. 4). The deposition of 

fine sediment, with the consequent loss of permeability 

in streambed gravel during the time of egg and fry development, has been described by many studies as the 

most detrimental sedimentation effect (for example, 

Meehan and Swanston, 1977, p. l). The deposited sediment reduces the flow of oxygen-bearing water within 

the gravel where eggs and alevins (preemergent fry) 

are incubating. It may also act as a physical barrier to 

the emergence of fry and may cause changes in the 

population of aquatic insects on which the young salmon depend for food.

Erosion and sedimentation have been described as 

the most insidious of civilization's effects on aquatic 

life, in that the processes may go unnoticed and the 

damage can be widespread, cumulative, and permanent (Cordone and Kelley, 1961, p. 189). Unlike most 

causes of degradation in water quality, erosion and the 

resulting increase in sediment transport may be 

triggered by a set of conditions and then may continue 

to increase or even accelerate after the triggering circumstances have ceased. The possible causes of such a 

response and why this form of response could occur 

along the sections of the Kenai River with high, presently stable cut banks are one focus of this report.

Acknowledgments This study was completed in cooperation with the U.S. Fish and Wildlife Service, to the 

personnel of which the writer is indebted for much 

helpful discussion and the supply of aerial photographs. Many local residents shared their knowledge of 

the past behavior of the Kenai River and helped form 

the writer's historical perspective on the stream.

THE KENAI RIVER WATERSHED

The Kenai River drains 2,200 mi2 of the Kenai Peninsula, encompassing a watershed that extends from the 

icefields of the Kenai Mountains westward to Cook Inlet. Summer flow originating as melt water from iceand snow-covered terrain dominates the hydrologic 

system of the river. Approximately 210 mi2 of the 

drainage basin consists of glaciers or permanent snowfields, of which 130 mi2 is part of the Harding Icefield 

and attached valley glaciers (fig. l).

CLIMATE

The climate of the watershed is transitional between 

the wet and. relatively mild marine climate of coastal 

areas and the colder and dryer continental environment 

of interior Alaska. The high sustained flow in the Kenai 

River in middle and late summer reflects the combination of melt water and superimposed storm runoff. 

More than half the annual precipitation falls in the 

4-month period from July through October, with an 

average of almost 4 in. occurring in September, the 

wettest month.

Annual rainfall totals vary greatly within the drainage basin because of the orographic effect of the Kenai 

Mountains on storm systems moving northward from 

the Gulf of Alaska. In the lowlands downstream from 

Skilak Lake the annual precipitation is less than 20 in. 

Southeastward in the progressively higher parts of the 

basin, precipitation totals increase markedly and probably exceed 80 in. at the crest of the range. The regional 

distribution of precipitation is reflected in the altitudes 

to which glaciers descend many outlet glaciers extend 

to the tidewater of the Gulf of Alaska; within the Kenai 

River drainage basin, however, valley glaciers reach no 

lower than 500 ft.

VEGETATION

The flood plain of the Kenai River and the surrounding terrain are covered by Alaskan taiga association of 

w lite spruce and hardwoods, locally with black spruce 

on north-facing slopes and poorly drained areas (Helmers and Cushwa, 1973, figs. 1, 2; U.S. Army Corps of 

Engineers, 1978, fig. 3l). Evidence of stream behavior
EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

can be obtained from vegetation bordering streambanks and on flood plains. Areas of active bank erosion 

may be characterized by spruce trees leaning at angles 

into the river as their root support is progressively 

eroded. When nearly horizontal, the trees are known as 

"sweepers," named with good reason by early-day 

raftsmen and hazardous to modern river runners as 

well. Ice damage in spruce trees on flood plains is evidence of ice-jam flooding and, if datable by dendrochronology, can serve as evidence of flood frequency 

(Levashov, 1966). Several episodes of ice damage are 

detectable on trees of the flood plain within meander 

loop 3-H.

The interior meander loops of the Kenai River do not 

show the vegetational age succession that would be expected under conditions of rapid channel change. Some 

meanders do, however, show a variation in vegetation 

type within the point-bar deposits that corresponds to 

differences in sediment texture. As documented by Gill 

(1972) in the Mackenzie River delta, coarse-textured 

deposits with a lower water content and higher soil 

temperature encourage the growth of such hardwoods 

as balsam poplar. The finer textured deposits commonly support mature stands of spruce. The differences in texture mark the episodic accretion by which 

the meander loops develop the coarser deposits correspond to the more rapid periods of accretion.

HYDROLOGY

The most obvious characteristic of flow in the Kenai 

River is the continuous rise in discharge that begins in 

May, followed by flow at sustained high levels throughout the summer and then by recession during the 

period from October to January (fig. 2). It is this unusual pattern of relatively uniform high flow during the

summer months, reflecting the melting of glaciers and 

lake storage of melt water, that makes feasible the 

riverbank development in which bed and bank material 

is simply bulldozed to form canals, boat slips, and 

groins. The stage variation of a typical subarctic stream 

would make this kind of development nearly useless.

The mean annual flow of the Kenai River at Soldotna 

is 5,617 ftYs or 37.95 in.( 1965-78). Annual peak flows 

generally occur in August or September at discharges 

in the range 20,000-30,000 ftYs. From freezeup in late 

November or December to breakup, occurring ordinarily in April but as early as February, flow levels base 

within the range 800-1,700 ftYs.

The Kenai River begins at the outlet of Kenai Lake, a 

glacially sculpted lake extending fiordlike for 22 mi inland from the front of the Kenai Mountains to within 

15 mi of Seward on the Gulf of Alaska. Downstream 

from the outlet of Kenai Lake at Cooper Landing, the 

river flows for 17 mi before entering Skilak Lake. The 

50-mi course of the stream between Skilak Lake 

and Cook Inlet is the subject of this report; the 17-mi 

segment between the major lakes is excluded.

Major headwater tributaries of the Kenai River are 

the Trail and Snow Rivers, which enter Kenai Lake 

from the north and south, respectively. The major 

tributary entering the Kenai River between the lakes is 

the Russian River, famous for a run of sockeye salmon 

during which they can be taken on artificial lures. 

Other large tributaries include the Skilak River, which 

drains the Harding Icefield and flows directly into 

Skilak Lake, and the Killey River, which joins the Kenai 

River 6 mi below the outlet of Skilak Lake. All these 

streams have significant areas of their headwaters covered by permanent ice and snow, and as a group they 

supply the high summer melt-water flow of the Kenai 

River.

10,000 

5000 

0

1 ' ' ' ' ' ' . ' ' ' J ' 

1 1 

 J . .

 %

 ». . . * <0IUI

OC/5

Q £ 25,000

>-

ILU 20,000 K it!

°o 15,000 

^3 10,000

5000

1 1

..... 

. n *

....()" ' ...» y..* * , . .* v%.» **"»* '....* *v...* 'y^.* .y.^* ^

1965 1966 1967 1968 1969 1970 1971 1972 

YEAR

1973 1974 1975 1976 1977 1978

FIGURE 2. Mean monthly discharges at gaging stations. A, Kenai River at Cooper Landing. B, Kenai River at Soldotna. Black arrows show 

times of release of glacial lakes in Snow River; white arrow shows times of release of glacial lakes in Skilak River.
HYDROLOGY

Downstream from the Killey River, all tributaries to 

the Kenai River drain only the Kenai Lowlands. Runoff 

from these streams is dominated by snowmelt runoff, 

with annual peaks generally in April or May. Poorly integrated drainage and numerous lakes and marsh 

areas, as well as lower rates of precipitation, result in 

comparatively low annual runoff. The largest of these 

streams is the Moose River, which joins the Kenai River 

at river mile 36.2. The Funny River, and Beaver, Soldotna, and Slikok Creeks, are other lowland 

tributaries, of which Beaver Creek is the only stream 

with more than sporadic flow records (1967-78).

Anderson and Jones (1972, pi. 2) presented a summary of all discharge information for the Kenai River 

downstream from Skilak Lake as of 1972. These data 

and subsequent information can be obtained from the 

series of annual reports entitled " Water Resources 

Data for Alaska," published by the U.S. Geological 

Survey. Gaging-station records on the Kenai River have 

been obtained since 1947 at Cooper Landing, and since 

1965 at the Soldotna bridge at river mile 21.1.

The Kenai River is noteworthy for a low variation in 

annual peak flows during the period of measurement. 

There are, however, three potential sources of major 

flooding on the stream in addition to the normal 

sources of flow melt water and storm runoff: (l) sudden discharges from glacially dammed lakes, (2) outburst floods of water stored in or under glaciers, and 

(3) ice jams. Each is discussed in the following paragraphs.

The annual peak discharges from melt water and 

storm runoff have been generally less than the annual 

peaks that resulted from the sudden release of glacially 

dammed lakes. The historical peak discharge at Soldotna occurred September 9, 1977 instantaneous 

peak discharge was 33,700 ftVs in response to the release of a glacially dammed lake in the Snow River 

drainage basin (fig. l). The lake is one of two potentially hazardous such lakes in the watershed for which 

Post and Mayo (1971, sheet l) recommended monitoring. The lake at the headwaters of the Snow River has 

caused outburst flooding periodically since 1911 or 

earlier. Typical of the floods is that occurring in 1974 

(fig. 3) and yielding the peak discharge of record on 

the Kenai River at Cooper Landing. This lake has 

yielded floods at intervals, most commonly from 2 to 4 

years in length and at levels apparently related to systematic changes in glacier size. Post and Mayo (1971, 

p. 4) cited reports that flooding historically has occurred most commonly in November, December, or 

January. In recent years (1964, 1967, 1970, 1974, 

1977), however, flooding has occurred in August and 

September at times of high base flow derived from 

melt water. If this trend continues, the flood hazard 

from lake releases will increase.

The second potentially hazardous glacial lake occurs 

in the headwaters of the Skilak River (fig. l) and discharges directly to Skilak Lake. This glacial lake 

yielded a comparatively small volume of flow in 

January 1969 (fig. 2), but the flood wave fractured 

large volumes of ice on the Kenai River, thereby causing locally serious flooding from the resulting ice jams 

(Post and Mayo, 1971, p. 4). Aerial observations by the 

U.S. Weather Service on October 18, 1979, revealed 

that the lake has refilled (S. H. Jones, oral commun., 

1980), apparently setting the stage for another outburst flood.

A phenomenon similar to glacial lake discharges is 

the outburst of water impounded beneath glaciers. 

Though potentially originating from any glacier of at 

least moderate size, floods entirely from subglacial 

outbursts have not been specifically recorded on the 

Kenai River. They may not, however, have been observed if originating in uninhabited areas like the 

Skilak or Killey River drainage basins in the period before to flow measurement at Soldotna (before 1965). 

Part of the glacial lake in the Skilak River headwaters 

is formed beneath the Skilak Glacier, and that lake discharges subglacially into the Skilak River (S. H. Jones, 

written commun., 1980).

30,000

10 15 20 

September 

1974

30 5 

October

FIGURE 3. Successive downstream hydrographs for flood of September 1974 originating from an unnamed glacially dammed lake 

(No. 26 of Post and Mayo, 1971) in headwaters of the Snow River. 

Concurrent discharge record of nearby Trail River is .illustrated for 

comparison. Instantaneous peak discharges were 26,400 ft3 /s in 

the Snow River, 23,100 ft3 /s in the Kenai River at Cooper Landing, and 26,900 ft3/s in the Kenai River at Soldotna.
6 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

The final cause of flooding is ice jams, from which an 

additional hazard is the channel-erosion effects with 

which they are associated on other northern rivers 

(MacKay and others, 1974). Jams on the Kenai River 

are most common near Big Eddy, a point of constriction in a tight meander at river mile 14.3 (fig. l). The 

probability of ice jamming at Big Eddy led the U.S. 

Army Corps of Engineers (1967, exhibit 4) to calculate 

upstream flood-hazard levels that are as much as 10 ft 

above the stage of a flood, with a recurrence interval of 

50 years. Potential levels of flooding from ice jams at 

Big Eddy exceed levels of the 50-year flood as far upstream as Soldotna.

QUATERNARY HISTORY 

OF THE KENAI RIVER VALLEY

The flood plain, terraces, and valley of the Kenai 

River reflect the influence of glacial events to a high 

degree. The modern landscape of the river, extending 

even to variations in channel pattern and size of 

channel-bed material, is partly a function of glacial action, including sculpture by glacial ice, deposition from 

receding ice sheets, and changing base levels related to 

the effects of glaciation or tectonics. The final major 

Quaternary glaciation of the Kenai Lowlands did not 

end until about 5,000 years ago, and today an outlet 

glacier from the Harding Icefield reaches to within 7 mi 

of the head of Skilak Lake.

An understanding of the recent glacial history of the 

Kenai Lowlands is prerequisite to interpreting the 

modern Kenai River. The sequence of events, their 

ages, and an interpretation of their effects on the river 

are presented in table 1. The glacial history of the 

Kenai River area and the surrounding region was 

studied in detail by Karlstrom (1964), and most of the 

general aspects and terminology in the following discussion are based on his work.

The Cook Inlet region has undergone five major Pleistocene glaciations and two major subsequent advances. 

In stratigraphic order (youngest to oldest), the major 

glaciations are:

Naptowne 

Knik 

Eklutna 

Caribou Hills 

Mount Susitna

Much of the Kenai Lowlands was covered by ice during the first three major glaciations. During Knik time, 

however, glaciers from the Kenai Mountains reached 

only as far as river mile 26.7. According to Karlstrom 

(1964), farther southwest in Cook Inlet, Knik glaciers 

from the Kenai Mountains coalesced with those from

the Alaska Range and dammed the regional drainage 

into a proglacial lake that existed periodically and at 

successively lower levels until near the end of the Naptowne Glaciation. However, the periodic existence of 

this lake a major premise of Karlstrom's 

interpretations has not been verified by subsequent 

investigations in the Cook Inlet region.

Deposits of the three youngest major glaciations are 

present along the Kenai River in the study area, but it 

is the events of the youngest episode, the Naptowne 

Glaciation, that dominate the geomorphic history of the 

stream. The spatulate Naptowne end moraines are the 

most prominent topographic feature of the Kenai Lowlands, extending as far as river mile 38.9. The type 

localities of the Naptowne Glaciation and several of its 

subsidiary advances are located along the river within 

the study area (the town of Naptowne is now known as 

Sterling). The sequence of advances within the Naptowne Glaciation is, stratigraphically:

Tanya

Skilak

Killey

Moosehorn

The age of the Naptowne Glaciation has been revised 

downward to less than 14,000 B.P. (Pewe, 1975, p. 

14), considerably later than reported by Karlstrom 

(1964). Dating by Karlstrom of the post-Moosehorn 

events appears reasonable in light of this revised age 

and is shown in table 1.

The initial phase of the Naptowne Glaciation, the 

Moosehorn advance, was named for the Moosehorn 

Rapids in the Kenai River at river mile 39.4 near the 

margin of the Naptowne end moraine. Moraines of the 

Killey advance, named for exposures along the Killey 

River, a major tributary to the Kenai River, extend to 

river mile 40.5; and those of the subsequent Skilak advance, named for exposures around the edge of Skilak 

Lake, occur as far downstream as river mile 48.4.

EVIDENCE OF PROGLACIAL LAKE IN COOK INLET

The existence of a proglacial lake in Cook Inlet, or at 

least its chronology as interpreted by Karlstrom, has 

been thrown open to question by a revised origin and 

radiometric age of a unit previously thought to represent a middle Wisconsinan interstadial event. In the 

Anchorage area a distinctive deposit of silty clay, the 

Bootlegger Cove Clay (Miller and Dobrovolny, 1959), 

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8 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

Clay caused disastrous slides during the 1964 Alaska 

earthquake, it has been the subject of additional study 

that has established an entirely marine origin (Hansen, 1965, p. 20) and an age of about 14,000 B.P. 

(Schmoll and others, 1972, p. 1109). Pewe (1975, p. 

74) concluded that the interpretation of a glacial lake 

occupying the upper part of Cook Inlet during Knik and 

Naptowne time is refuted by this later evidence.

At least some of the features attributed by Karlstrom 

to a freshwater lake have other explanations. The Soldotna terrace, a well-developed surface bordering the 

Kenai River over much of its lower course, was interpreted as a lake terrace in mapping by Karlstrom 

(1964, pi. 4) but is described here as a former floodplain surface, an origin in common with other alluvial 

terraces. The Soldotna terrace grades to one of two 

well-developed terrace levels bordering Cook Inlet.

These levels occur 50 and between 100 and 125 ft 

above present sea level and were interpreted by 

Karlstrom as lake terraces (table l). They are, however, more likely marine in origin, on the basis of the 

extent of their development. It is difficult to envision 

an ice-floored lake spillway being sufficiently stable for 

the interval necessary for cutting of the terraces. The 

changes in base level consequently are more likely due 

to isostatic rebound or tectonic uplift than to changes 

in level of the hypothesized lake. Favoring the lake 

hypothesis is Karlstrom's mapping of other higher 

strandlines indicating lake levels at altitudes too high 

(table l) for reasonable explanation by sea-level 

change due to isostatic rebound or tectonics. The existence of these higher strandlines could not, however, 

be confirmed during field investigations in the Kenai 

River watershed.

FIGURE 4. Kenai River at the Soldotna bridge. The river is entrenched 30 to 40 ft below the Soldotna terrace, upon which part of the town of 

Soldotna is visible here. Wakes in the river are caused by large boulders, the presence of which is characteristic of the entrenched section 

of the stream. Direction of flow is toward upper left. Reach visible in photograph extends from approximately river miles 21.5 to 20.7. 

Scale, 1:4,800, or 1 in.= 400 ft. Photograph credit: U.S. Army Corps of Engineers.
QUATERNARY HISTORY OF THE KENAI RIVER VALLEY 9

TERRACES AND RIVER ENTRENCHMENT

The Soldotna terrace, here named informally for the 

town constructed upon it (fig. 4), is the most prominent topographic feature in the Kenai River valley between river miles 13 and 31. The terrace averages about 

a mile in width, is covered with mature taiga vegetation, and occurs at altitudes generally from 25 to 50 ft 

above the present Kenai River flood plain. It dominates 

the valley upstream from river mile 17.6, above which 

point the river is entrenched in the terrace surface and 

little modern flood plain exists. The entrenchment, 

which extends beyond the upstream end of the terrace 

as far as river mile 39.4, is a result of a lowering in 

base level, from the level to which the terrace was 

graded, to present sea level.

Karlstrom (1964, pi. 4) interpreted the section of the 

Soldotna terrace between river miles 31 and 27 as a 

river terrace and the remaining part as a hanging deltaic complex associated with a pro glacial lake of Naptowne age. The entire terrace upstream from Soldotna 

(river mile 22) is here interpreted as a former flood 

plain of the Kenai River. Profiles of the terrace and 

river channel measured along the valley axis (fig. 5) 

show that the terrace is graded to a height above present sea level.

The extension of the Soldotna terrace below the town 

probably correlates with the 100- to 125-ft-high marine 

terrace. A well-developed 50-ft marine terrace is also 

present, and figure 5 portrays possibility that the alluvial part of the Soldotna terrace grades to this lower 

level. The town of Kenai is mainly on this lower terrace, 

which is not represented by obviously correlative alluvial equivalents along the Kenai River.

TOPOGRAPHY OF THE KENAI LOWLANDS 

AND COURSE OF THE KENAI RIVER

The poorly drained, lake-dotted Kenai Lowlands contain many abandoned channels that are visible on aerial photographs yet do not form a drainage system 

which is obviously integrated with the present network. 

The channels, though well developed at some localities, 

are discontinuous and not easily traceable. Karlstrom 

(1964, p. 15) believed that the pattern locally suggests 

scabland topography formed under torrential-flood 

conditions.

Changes in the drainage of the Kenai River system 

have occurred within a geologic time span that is apparently too short for any but partial adjustment of the 

channel in pattern and bed material size, for example. 

A change in pattern (an increase in wavelength 

downstream from river mile 36; fig. 6) below the point 

of inflow of the tributary Moose River suggests that 

discharges proportionally larger than those now 

supplied by the Moose River have occurred in the past. 

If true, the Kenai River downstream from the Moose 

River is underfit to a greater degree than is the river

200

150 -

100

45678 

DISTANCE ALONG VALLEY AXIS, IN MILES

10 11 12

FIGURE 5. Profiles of the Soldotna terrace and the Kenai River (water surface at intermediate flow level) measured along the valley axis. River 

miles are shown inset Altitudes were derived photogrammetrically, and absolute values are only accurate within the approximate range of 

± 10 ft Relative differences between altitudes of terrace and river at a point are believed accurate to within ±2 ft
10 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

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DISTANCE ABOVE CHANNEL MOUTH, IN MILES

FIGURE 6. Distance above channel mouth (in river miles) against: channel width at bankfull stage (A), slope of water surface (B), meander

wavelength (C), number of channels (D), and sinuosity index (£").
QUATERNARY HISTORY OF THE KENAI RIVER VALLEY 11

upstream from the junction. This effect would be in addition to the probable basinwide underfit condition reflecting the general reduction in precipitation that has 

occurred with glacial recession.

Figure 7 illustrates the channel of the Moose River a 

short distance upstream from its junction with the 

Kenai River. The underfit condition is pronounced. The 

present channel is approximately 80 ft wide where it 

meanders within a paleochannel 600 to 700 ft wide. 

The Moose River paleochannel appears to be a natural 

upstream extension of the lower part of the Kenai River, 

from both the similarity in pattern and the trends of 

the two channels at their junction.

Topography at the front of the Kenai Mountains indicates several past variations in drainage in which the 

Moose River would have yielded much greater flow 

than at present. It is possible to project an extension of 

the Skilak Glacier to the head of Skilak Lake where it 

could divert the Kenai River into the headwaters of the 

East Fork of the Moose River (fig. 8). The course of 

probable diversion is today a chain of lakes, beginning 

with Hidden Lake in the gap between Hideout Hill and 

the hills north of Skilak Lake and continuing with the 

Seven Lakes, each connected by the drainage that becomes the East Fork of the Moose River. This 

hypothesized diversion probably occurred with the 

Skilak advance of the Naptowne Glaciation and could 

also have occured during the Tanya advance. Tanya end 

moraines have not been recognized in the Skilak Lake 

area, although they were mapped by Karlstrom (1964, 

pi. 4) at their type locality near Tustumena Lake. An 

advance of the Skilak Glacier similar in distance and 

gradient to the relation between the Tanya end 

moraines and the Tustumena Glacier, the extension of 

which was the type Tanya advance, could have diverted 

the Kenai River into the Moose River drainage.

The effects of earlier glaciations on the drainage pattern would have been greater. Drainage from the area 

of Kenai Lake, which was glacier filled during much of 

pre-Tanya Naptowne time, may also have entered the 

Moose River drainage north of Hideout Hill (fig. 8). 

During the maxima of Skilak and earlier Naptowne advances, glacial lobes from the Kenai Lake valley entered the Moose River basin and discharged large volumes of melt water.

No matter what scenario of melt-water drainage is 

hypothesized, during each of the Naptowne advances 

the tendency was for greater proportions of the total 

discharge of the Kenai River basin to have entered the 

Kenai Lowlands from the Moose River than from the 

present Kenai River channel above the confluence with 

the Moose River. The Kenai River channel downstream 

from the Moose River thus has had a constant drainage 

area, and the overall decrease in discharge in that 

channel has reflected the general climatic change. The 

channel upstream from the Moose River, however, reFIGURE 7. Moose River channel between 1.3 and 2.6 mi upstream 

from its junction with the Kenai River a striking.example of confined meanders occurring within a large sinuous paleochannel. 

Flow is toward bottom of photograph. Downstream change in pattern of present channel from meandering to straight is in response 

to entrenchment of the Kenai River. Scale, 1:12,000, or 1 in. = 

1,000 ft. Photograph credit: U.S. Army Corps of Engineers.
12 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

fleets partially offsetting changes in climate and drainage area. The effects of change in drainage area have 

been to reduce the high discharges at times of glacial 

maxima. In consequence, the channel of the Kenai 

River below the Moose River reflects a history of adjustment to greater absolute change in discharge than 

does the channel upstream from the tributary, and this 

difference in adjustment is reflected in the channel and 

sediment characteristics described in the following sections.

CHANNEL OF THE KENAI RIVER

Study of the channel pattern, degree of entrenchment, position of riffle bars, symmetry of cross sections, and slope permits a description of the Kenai 

River that, in combination with the subsequent sections 

on bank erosion and development, can be used to assess the relative susceptibility of various sections of the

stream to the actions of man. The information will be 

presented in the following section but will be applied in 

the final section on river development.

STREAM TYPE

The Kenai River can be fitted to an engineering classification of streams (Brice and Blodgett, 1978, p. 94; 

Brice, 1981, fig. 5) that emphasizes lateral stability 

the potential for bank erosion. The classification is 

based on observable channel properties that show an 

association with varying degrees of lateral stability. The 

section of the Kenai River between river miles 39.4 and 

17.6 has characteristics similar to the type described as 

equiwidth point bar. Such streams are relatively stable. 

Upstream and downstream from this section the Kenai 

River more closely fits the category described as 

wide-bend point bar. This type of stream is generally 

less stable than equiwidth point-bar streams.

150°30' 150°00'

Area of 

figure 7

60°30'

01 23456 78 KILOMETERS

I i i i i i i i i

I I I r \~^ I

012345 MILES

1000

FIGURE 8. Drainage network in the Kenai River watershed near the front of the Kenai Mountains. Limits of subsidiary advances within the

Naptown Glaciation are shown (mainly fromKarlstrom, 1964, pi. 4).
CHANNEL OF THE KENAI RIVER 13

CHANNEL PATTERN

Variations in channel pattern can be empirically useful in assessing differences in susceptibility to bank 

erosion. For example, the straighter, less sinuous 

reaches of a stream tend to be significantly more stable 

than the more sinuous reaches, solely on the basis of 

the observation that bank erosion is most intense at 

channel bends. Channel pattern can be in part described by means of a sinuosity index (SJ.), defined as 

the ratio of the thalweg length to the length of the 

meander-belt axis (Brice, 1964, p. 25). Although the 

symmetry of channel bends is not considered in calculating the index, channels can be described by boundary values of the index. In the classification used here, 

reaches with a sinuosity index greater than 1.25 are 

described as meandering, those with an index between 

1.05 and 1.25 are sinuous, and those with an index less 

than 1.05 are straight (Brice and Blodgett, 1978, p. 

70).

The sinuosity indexes of overlapping reaches 4 mi in 

length are shown in figure 6, where values are plotted 

at midpoints 2 mi apart. That the Kenai River varies in 

sinuosity is readily seen. Three intervals of meandering 

channel are present: the first, between Skilak Lake and 

river mile 34.8; the second, between river miles 21.8 

and 13.4; and that farthest downstream, between river 

mile 9.0 and the mouth. This last interval shows the 

downstream increase in channel width and meander 

amplitude associated with tidal augmentation of flow.

The river branches into multiple distinct channels 

(anabranches) in two reaches (fig. 6). The upstream 

anabranched reach, between river miles 42.7 and 39.6, 

is part of the first meandering section. The downstream 

anabranched reach, between river miles 15.8 and 11.4, 

includes part of the middle meandering section. The islands within the upstream anabranched section of 

channel are mainly covered with mature spruce. Vegetation on islands in the downstream anabranched section is less dense, but is generally mature and indicates 

that the islands are only rarely inundated.

INVESTIGATION OF UNDERFTT CONDITION

The possibility of an underfit condition can be investigated by comparison of the channel pattern with discharge and channel width. Paired observations have 

shown that meander wavelength is a function of 

bankfull discharge according to the relation (Inglis, 

1949, p. 147; Leopold and Wolman, 1970, p. 216)

X = 36Q°-5.

Dury (1965, p. 5; 1970, p. 273; 1976, p. 223) analyzed 

several sets of paired observations of wavelength and

discharge. His original set of 105 pairs of data gives 

the relation (Dury, 1970, p. 273)

X = 30Q° 5.

Also, meander wavelength is related to width of 

bankfull channel according to the relation (Leopold 

and Wolman, 1970, p. 216)

,1.1 X =6.5u;

Dury (1976, fig. 2) summarized 173 pairs of values of 

wavelength and width and calculated the relation

1.019 X =9.76u;

Leopold and Wolman (1970, p. 216-217) showed that 

wavelength was more directly dependent on width than 

on discharge when data were compared for a large 

range of stream sizes. Bankfull discharge is considered 

here as equivalent to channel-forming discharge and is 

calculated as the discharge at a recurrence interval of 

1.58 years in the annual series.

The plot of wavelength with bankfull discharge (fig. 

9) indicates that the channel in each of the three 

meandering sections tends to be underfit; that is, the 

data points are in or above the upper ranges of the 

data of Inglis and Dury. In such cases of apparent underfit, a meander of a given size is associated with an 

uncommonly low channel-forming discharge, leading

100,000

10,000

1000

EXPLANATION

"Upstream meandering section

*Middle meandering section

 Downstream meandering section

1000 10,000 100,000 

BANKFULL DISCHARGE, IN CUBIC FEET PER SECOND

FIGURE 9. Meander wavelength against bankfull discharge. Lines 

representing limits of Inglis (1949) and Dury (1965, 1970, 1976) 

data are approximate.
14 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

to the assumption that discharge has decreased since 

the meanders were formed.

The data points in the plot of wavelength against 

width at bankfull stage (fig. 10) tend to cluster in or 

above the upper ranges of the data plotted by Leopold 

and Wolman (1970, fig. 7.13) and Dury (1976, fig. 2). 

Thus the channel width is smaller for a given 

wavelength than would be expected by comparison with 

other streams, as the likely result of the meander pattern of the Kenai River having been formed during a 

previous period of higher discharge with, of course, the 

width of the channel reflecting the present, lower discharge.

Meanders from the tidal section of channel mainly 

plot below the mean lines of the data in the LeopoldWolman and Dury studies (fig. 10), but the significance of this relation is not known because those authors included no data from tidal reaches. The 

downstream meandering channel reflects tidally augmented flow and shows the consequent characteristic 

increase in channel width, and so it should expectably 

indicate an underfit condition relative to the freshwater 

discharge of the stream, as it does in the plot of figure 

9.

FLOW DEPTH VARIATION WITHIN MEANDERS 

AND WITH DIFFERING CHANNEL PATTERN

The measurement of a series of cross sections by the

100,000

10,000

1000

EXPLANATION '

Upstream meandering section 

Middle meandering section 

A Downstream meandering section

nge of Dury data

Range of Leopold and 

Wolman data

100 1000 10,000 

CHANNEL WIDTH AT BANKFULL DISCHARGE, IN FEET

FIGURE 10. Meander wavelength against channel width at bankfull 

stage. Lines representing ranges of previous data are approximate.

U.S. Army Corps of Engineers (1967, 1973, 1975, 

1978) permits analysis of flow depths according to position in the meander course and the type of channel 

pattern. The discharge at Soldotna during the 2-day 

period of the survey was in the relatively narrow range 

of approximately 11,500 to 11,900 ft3 /s. The cross sections, therefore, represent the bed at a moderate flow 

level, approximately 70 percent of bankfull discharge, 

in the reaches between river miles 47 and 26.

In meandering streams the shallows analogous to riffles occur at the crossovers or points of inflection in the 

meander curve, and the pools are found at the bends, 

with the deepest point near the outside or concave 

bank. If this pattern of pools and riffles is not present 

or if it occurs with a different spacing relative to the 

meanders, some aspect of the fluvial environment is 

preventing the normal adjustment of the bed response 

to flow. For example, the meanders may be relict from 

a period of previous, generally higher discharge, or the 

mobility of the bed may have been reduced by the process of armoring, in which finer sediment is selectively 

removed and the bed is rendered progressively immobile. Dury (1970, p. 268) recognized an underfit 

condition in which the old meanders continue as the 

stream channel, but in which the pools and riffles assume an irregular distribution reflecting the new reduced discharge.

The following statistical analysis was made to investigate the spacing of bars. Maximum depths in selected 

sections were grouped in table 2 according to whether 

the channel was meandering or sinuous to straight. The 

data from meandering channels were subdivided by 

location whether the section was at a bend or at or 

near a crossover and were further grouped according 

to whether the meander was free to migrate laterally or 

was entrenched. Hypothesis testing of the differences 

between the means of the data subgroups for meandering channels yielded unexpected results. There was no 

significant difference between the maximum depths in 

bends and at crossovers, a result suggesting, when 

considered with other evidence, that the channel is underfit. The morphology is similar in some respects to 

that of the Illinois River (Rubey, 1952, p. 123-136), a 

stream with a stable and deep uniform channel that occupies a valley formed by large proglacial discharges.

There was also no significant difference between the 

maximum depths in the channels of meanders that are 

free to migrate and those that appear to be entrenched. 

The only significant difference was found between the 

depths in all meandering channels and the depths of 

sinuous or straight reaches. Maximum depths in the 

channel where it is sinuous or straight are less than 

those where the channel meanders, with a probability 

in excess of 0.99.
BED MATERIAL 15

ASYMMETRY OF CROSS SECTIONS AT BENDS

Cross sections of the channel of a meandering stream 

are characteristically asymmetric at bends, with the 

point of maximum depth close to the outside of the 

bend. Of the nine sections located at meander bends, 

only four show the expected asymmetry, each in meanders free to migrate. Although none of the sections 

from entrenched meanders shows asymmetry, the 

sample size is too small for significance tests. Absence 

of or abnormal asymmetry can be regarded as evidence 

of underfitness, as exemplified by the Illinois River 

(Rubey, 1952, p. 129), where the normal asymmetry is 

reversed and the deepest part of the channel is close 

against the inside of meander bends.

SLOPE

The slope of the water surface of the Kenai River has 

a variation of at least an order of magnitude in the values determined from 5-ft contour increments and plotted at the midpoint of each increment (fig. 6). These 

data should be viewed as approximations to the actual 

slope because of their photogrammetric derivation. The 

accuracy of photogrammetric altitudes is such that, 

over short longitudinal increments of channel, expectable inaccuracies in altitude can yield significant differences in slope. Field observations likewise fail to 

confirm some of the slope data in figure 6 as more than 

approximations, useful for comparison only.

At the largest scale there is a difference in slope between the meandering reaches and the long middle section of the river with only a slightly sinuous configuration. The meandering reaches generally have the lesser 

slope, in accord with the general inverse correlation between sinuosity and slope.

At a smaller scale within the meandering reaches, the 

tendency toward anabranching, which is commonly associated with an increased gradient (see Mollard, 

1973, fig. l), does not fit this tendency, according to 

the data of figure 6. The anabranched and meandering 

sections of the river apparently have a lesser slope than 

some sections of single meandering channel. The 

reason for this anomaly is that parts of the upstream 

and middle meandering sections are entrenched. The 

entrenched meanders have the greatest slope, shown in 

figure 6 and verified by field observations, of any part 

of the river except the Moosehorn Rapids.

BED MATERIAL

The bed material of the Kenai River is among the 

coarsest recorded for a meandering channel of similar 

size (compare data inKellerhals and others, 1972). The 

reasons are both geologic and hydrologic. The coarse 

material reflects initial transport by glaciers, which 

throughout the Pleistocene covered at first all, and 

then successively lesser, parts of the drainage basin. 

Coarse bed material was supplied directly from melting 

ice and outwash discharges and subsequently was derived throughout the length of the stream from erosion 

of previous glacial deposits. Numerous boulders too 

large for transport by even the highest discharges remain in the channel throughout the entrenched sections of the river (fig. 4).

TABLE 2 Statistical analysis of maximum flow depths at cross sections measured August 23-24,1974

[Location of sections, between river miles 26 and 47. Probable variation in discharge, less than 5 percent. Depths are in feet. SJ., sinuosity index]

Data grouped as indicated

Channel Position Channel "free" x x x

pattern in meander or entrenched n (range) ox (range) ox (range) ox

meandering

Sinuous

or straight

Bend

Crossover

Not determined

Nonentrenched 

meanders

Entrenched

meanders

Nonentrenched 

meanders

Entrenched

meanders

Entrenched

to varying 

degrees

6

o

4

o

8

11.1

(8.7-13.5)

10.7

(7.7-13.7)

10.3 

(8.3-12.0)

11.4

(9.2-15.1)

(5.8-10.2)

1.9

o.u

1.5

0*0

1.7

10.9

(7.7-13.7)

10.7 

(8.3-15.1)

2.1

2.2

10.8 

(7.7-15.1) 2.1
16 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

As glaciers receded within the Kenai Mountains, 

transition from a braided to a meandering channel occurred as the flow regime changed to one of lesser discharge and greatly decreased sediment supply. Similar 

changes in pattern have been widely noted throughout 

areas peripheral to receding glaciers. In the Kenai 

River, formation of the large lakes left by the receding 

glaciers first Skilak Lake and then Kenai Lake acted 

much as the construction of reservoirs. Downstream 

degradation and partial armoring of the channel occurred in response to the sediment-entrapment effects of 

the lakes. The pronounced entrenchment of the channel below the Soldotna terrace, however, is attributed 

mainly to degradation consequent to change in base 

level rather than to the downstream effects of the 

lakes.

The size of bed material in the active channel is 

shown in figure 11 as the median diameter (Dso). These 

data were obtained from large emersed bars by 

pebble-counting techniques that are statistically valid 

for coarse sediment (Wolman, 1954). Several estimates of the median grain size were made during a 

boat traverse of the river, and these points are so designated. The estimates were made only for the submersed gravel dunes found in the reaches downstream 

from Skilak Lake (fig. 12).

The distribution of median sizes of bed material (fig. 

11) reflects the entrenchment and partial armoring of 

parts of the river. The comparatively finer grained bed 

material upstream from river mile 39.4, site of the 

Naptowne end moraine and the Moosehorn Rapids, 

coincides with the reaches in which higher erosion rates 

were documented (see section on bank erosion). The 

extremely coarse bed material (D5o = 122 mm) in the 

channel at the end moraine functions as the base level 

for the river upstream to Skilak Lake and has prevented upstream extension of the entrenchment. The 

bed material below river mile 39.4 is coarser than that 

upstream, remaining in the range 40-60 mm throughout the entrenched part of the channel downstream 

from the Moose River. Below river mile 20, bed material becomes gradually finer, and, correspondingly, 

bank-erosion rates locally increase to rates comparable 

to those in the reaches upstream from river mile 39.4.

The roundness (Meehan and Swanston, 1977) and 

size (McNeil and Ahnell, 1964) of bed material have 

been related to success rates of salmon spawning in 

southeastern Alaska. Survival of salmon eggs was 

slightly higher in angular than in round gravel. The 

roundness of Kenai River bed material fell within 

categories defined as subrounded or rounded, and no 

significant longitudinal variation was detected. Little 

variation in productivity consequently can be ascribed 

to this factor. Gravel permeability, which correlated 

strongly with salmon survival rates, was found to be 

negatively related to the percentage by volume of sediment passing a 0.833-mm sieve. In measuring the size 

distribution of Kenai River bed material, the percentage of material in size fractions finer than sand was not 

determined because, for statistical validity of the results, large volumes of material would have to be excavated and separated before the fine components could 

be sieved. However, the percentage of sediment of 

sand size or finer (<2 mm) was determined during the 

pebble-counting process. Using those percentages for 

comparison a conservative approach because only part 

of the sediment finer than 2 mm would pass the 

0.833-mm sieve the bed material of the Kenai River is 

highly permeable and contains a relatively small proportion of fine sediment. The streams studied by 

McNeil and Ahnell (1964, fig. 7) contained bed material of which 5 and 20 percent was finer than 0.833

10 12 14 16 18 20 22 24 26 28 30 32 34 

DISTANCE ABOVE CHANNEL MOUTH, IN MILES

36 38 40 42 44 46 48 50

FIGURE 11. Bed-material size against river miles. E, estimated.
BED MATERIAL 17

mm. At all measurement sites the surficial bed material 

of the Kenai River contained less than 5 percent sediment finer than 2 mm. No great significance should be 

placed on this comparison because of the greater 

coarseness of the Kenai River bed material and the differences in sampling techniques. If it had been possible 

to measure samples of the Kenai River bed near the 

thalweg, the percentage of fine sediment would have 

been greater.

GRAVEL DUNES IN CHANNEL BELOW SKILAK LAKE

The reach containing crescentic gravel dunes that are 

visible on aerial photographs between the outlet of 

Skilak Lake and river mile 46.5 is among the most productive on the river in its ability to support heavy 

spawning of several types of salmon (see data summarized by U.S. Army Corps of Engineers, 1978, fig. 

27). Whether the productivity relates to the bedforms 

or to the effects of suspended-sediment retention in 

Skilak Lake, leading to minimal deposition of fine sediment in this reach, is not known. Crescentic dunes are 

a highly unusual mode of transport in gravel-bed 

streams. Both the coarseness of the bed material composing the dunes in the Kenai River and the scale of 

the dune forms (fig. 12) are exceptional.

Active dunes of comparable and larger sizes occur in 

much larger rivers, such as the Mississippi and Missouri Rivers, but are associated with finer, generally 

sand size bed material. The dunes in the Mississippi 

River are as much as 22 ft in height and range in 

length from 100 to 3,000 ft (Lane and Eden, 1940). 

The dunes of the Kenai River likewise vary in size, as 

indicated by their submersed images on aerial photography. The largest dunes are at least 500 to 600 ft in 

length, approximately equivalent to the mean channel 

width in this reach. Smaller dunes are developed on the 

larger forms and are common in lengths of more than 

50 ft. The maximum height of the dunes, estimated 

from water depths in the intervals between the shallow 

riffles that mark the crests of the forms, is at least 15 

ft. The ratio of height to length for the Kenai River 

gravel dunes appears to be greater than that for the 

sand dunes measured in larger rivers.

Features of comparable coarseness and scale have 

been reported to result from an exceptional flood discharge, such as a surge from a dam failure (Scott and 

Gravlee, 1968, fig. 18), but in these unusual instances 

the features are not subsequently active. The gravel 

dunes of the Kenai River were examined with aerial 

photography taken in 1950, 1972, and 1977 to determine the degree of their activity. Where best developed, between river miles 48.5 and 46.5, the dunes

show a surprising and remarkable similarity in position. Resolution is relatively poor on the 1950 photographs, but the positions of the major forms are clearly 

the same as those in 1972 and 1977. Striking comparisons of the 1972 and 1977 photographs show that even 

the small irregularities of dune morphology did not 

change in that interval, one that included a major flood

FIGURE 12. Kenai River between approximate river miles 47.5 and 

46.9. Crests of large crescentic gravel dunes appear just below 

water surface as darker areas. Flow is from bottom of photograph 

toward top. Scale 1:4,800 or 1 in. = 400 ft. Date: July 11, 1977. 

Photograph credit: U.S. Army Corps of Engineers.
18 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

discharge in 1974 (fig. 3). The dune forms, like the 

traveling bars of an incipiently meandering channel 

and nearly all other types of submersed dune forms, 

are the type of bedform that migrates progressively 

and changes position at least seasonally. The rate of 

movement of the Mississippi River dunes described 

above ranged from a few ft per day to as much as 81 ft 

per day (Lane and Eden, 1940). The historical stability 

of the Kenai River dunes indicates that, like the forms 

described by Scott and Gravlee (1968), they are the 

product of an exceptional flood event, one probably 

greatly in excess of any flood during the period of flow 

records.

The question of why the dune forms are confined to 

the 3.8 river miles downstream from Skilak Lake is not 

so easily answered. The presence of the dunes coincides 

almost exactly with the reach that appears "drowned"; 

that is, the channel shows evidence of having been 

formed at lower water-surf ace elevations. This part of 

the river presently functions in part as an extension of 

the lake channel width is large and irregular; banks 

show little evidence of erosion. The most likely reason 

for the "drowned" channel is the presence of gravel in 

the form of the dunes, which have effectively plugged 

the reach. The cause of the flood that introduced the 

gravel and molded it into dunes is unknown, but, as 

noted, the event was of exceptional recurrence interval. 

The effect of wave action in introducing suspended sediment into the river at the outlet of Skilak Lake is described in the discussion of suspended sediment. Similarly, it is possible that a flood surge traversing the 

lake mobilized sufficient coarse sediment at the lake 

outlet to form the dunes and aggrade the channel to its 

present configuration.

ARMORING OF THE CHANNEL

Armoring is the process whereby finer sediment is 

progessively removed, leaving the coarsest material to 

armor the bed surface. It occurs when the high flows 

that transport the coarse material no longer occur, as 

happens when a reservoir is built upstream. In places 

where the change in flow regime is engineered, the armoring commonly involves only the surface of the bed, 

is one particle diameter in thickness, and is easily observable (Vanoni, 1975, p. 181-182). As the term is 

applied here, to the sedimentologic response to a longterm natural reduction in flow, the results are less obvious and do not appear as a pronounced size difference immediately below the bed surface.

The bed material within the entrenched channel (between river miles 39.4 and 17.6) has a size distribution 

in which a significant proportion of the particles is not 

credible under the present flow regime, and the evidence of this condition includes sediment size and 

channel stability. The causes are threefold: the longterm decline in flow accompanying glacial recession, 

the reservoirlike effects of Skilak Lake, and, to an unknown extent, the presence of coarser underlying 

gravel than is present outside the entrenched reaches.

The size data in figure 11 are mainly from emersed 

bar surfaces; the average bed material in a cross section is likely to be coarser. The most visible feature of 

the armored reaches is the presence of large boulders, 

which protrude above the water surface at normal 

levels of summer flow (see fig. 4) and may exceed 13 ft 

in intermediate diameter. In other streams the size of 

the Kenai River, the bed material normally will be 

moved by discharges not greatly in excess of bankfull 

discharge. Field calculations of tractive force compared 

with known critical values (for example, Baker and Ritter, 1975, fig. l) indicate that only discharges greatly 

in excess of bankfull or channel-forming discharge will 

transport the coarse fractions of the size distributions 

in the entrenched reaches. These calculations are not 

presented here because of the confidence limits applicable to the slope data and therefore to the values of 

tractive force. The general conclusion is believed to be 

valid.

It should not be concluded that no movement of 

coarse bed material occurs in the entrenched channel. 

Competence is sufficient to transport coarse sediment 

supplied from reaches upstream and from tributaries 

to the entrenched reaches. Both sources have lower 

flow competence, in the case of the upstream river because of a lesser slope. The basic gravel framework of 

the entrenched channel is, however, stable at bankfull 

flow.

As will be documented in the discussion of bank erosion, the entrenched channel has been generally stable 

since 1950. Over much of the entrenched channel no 

detectable erosion has occurred, within the limits of 

accuracy of the measurement techniques. This situation 

contrasts with that both upstream and downstream, 

where extensive amounts of bank erosion have occurred.

Excavation of the submersed bed material to determine the size gradation within the bed was not practical because of flow levels during the fieldwork in summer and early fall. The size gradation is probably slight 

compared with the armoring resulting from such engineered changes in flow as that seen in the channel 

downstream from a dam. The size difference may exist 

chiefly with respect to comparison of the size of bed 

material with that of the underlying outwash gravel. 

The important observation, however, concerns the 

competency of flood flows of a frequency that in 

nonarmored channels would readily move most sizes of
BED MATERIAL 19

particles present in the bed. In the Kenai River, only 

the most extreme floods would mobilize the bed material in the entrenched section of channel.

POSSIBLE EFFECTS OF ARMORING ON SALMON HABITAT

From spawning to the time the young salmon leave 

the interstices of the gravel, the oxygen supply is critical (see Phillips, 1974, p. 65-68). The initial shaping 

and sorting of the redd by the adult fish serves the dual 

purpose of increasing the flow rate within the gravel, 

through the irregularity of bed surface thus produced, 

and removing deposits of fine sediment from within the 

pores of the gravel. For the several months during 

which the young remain in the gravel, they are vulnerable to any renewed deposition of fine sediment. Even 

where dissolved-oxygen concentration is high, newly 

deposited sediment can act as a physical barrier to fry 

emergence.

Einstein (1968) studied the progressive clogging of 

spawning gravel in flume experiments and observed 

that silt particles filter slowly down through the pores 

without any systematic horizontal motion, settling on 

top of individual clasts and filling the pores from the 

bottom up. These observations show that the armoring 

of the channel has important implications for the productivity of the Kenai River in terms of its ability to 

support the spawning and rearing of salmon. If bed 

material is too coarse to be moved by a normal range 

of flow, as is the gravel in the entrenched channel, fine 

sediment will gradually accumulate within the pores of 

the gravel and reduce the permeability. Because the infiltrating fine sediment was observed to move only in a 

general vertical direction, lateral redistribution in the 

bed apparently will not occur. Thus, in an armored bed 

the clogging of the gravel pores is an irreversible process. Only the movement of the gravel framework, by 

either the spawning fish or an exceptional flood, will 

flush out the accumulating fine material.

Observations by personnel of the U.S. Fish and 

Wildlife Service (Wayne Pichon, oral commun, 1979) 

show that salmon, particularly king salmon, can construct redds in bed material as coarse as that in the 

armored channel. Study of spawning locations verifies 

that the armored reaches are the sites of active spawning (U.S. Army Corps of Engineers, 1978, fig. 27). Although salmon are capable of building redds in the 

material and thus cleansing it at a point, it seems likely 

that the productivity of a progressively silting reach 

would decline.

The historical rate of fine-sediment deposition in the 

gravel of the armored reach has not detectably reduced 

the permeability of the bed at the depth necessary for 

spawning and rearing. Before concluding that this will

continue to be true, two factors should be considered. 

First, the rate of interstitial deposition will increase 

with any increase in suspended-sediment transport 

that may result from development or other maninduced change. Second, an exceptional flood competent to mobilize and cleanse the armored bed will not 

necessarily occur. The flood that emplaced the gravel 

dunes in the reach below Skilak Lake may have been 

competent to mobilize the bed material in the armored 

reach, but its magnitude and cause, as well as its age 

(other than pre-1950), are unknown. Similar floods are 

likely to be the result of geologic events, such as the 

breaching of landslide and glacial dams, and thus their 

probabilities are not predictable from a short series of 

annual flows.

The stability of the reach containing the gravel dunes 

indicates that the above conclusions apply to it as well. 

This at first seems unlikely because of the relatively 

finer bed material of which the dunes are composed. 

The dunes themselves, however, have dammed the 

channel and reduced the slope and thus the competence of a given discharge.

SURFICIAL DEPOSITS OF THE MODERN FLOOD PLAIN

A flood plain exists lateral to the nonentrenched sections of the Kenai River, but only small segments are 

found along the entrenched channel. Like the flood 

plains of the group of streams described by Wolman 

and Leopold (1957), the underlying material consists 

mainly of channel deposits. Only at the surface is there 

a distinct segregation of cohesive material within the 

size range of silt (0.004-0.625 mm) and clay (<0.004 

mm). This layer of sediment deposited during overbank 

flow is as thick as 6 ft and is laced with roots that act as 

a strong binding agent. It is well developed in the interior of nonentrenched meander loops.

A "mat" of root-bound fine-grained sediment is a 

characteristic of northern rivers and, because of either 

the absence of permafrost or the presence of a thick 

active layer (depth of summer thaw in permafrost), is 

particularly well developed in subarctic streams. This 

layer serves the important function of stabilizing riverbanks by retarding the slumping that occurs in response to erosion of the underlying noncohesive channel deposits (Scott, 1978, p. ll). As the channel deposits are eroded, the cohesive layer may fold down to 

protect the bank from further erosion for a period as 

long as years. In such cases it has been likened by Russian observers of northern streams to a cloth draped 

over the edge of a table. The layer also acts to protect 

meander loops from cutoffs. Observations of arctic and 

subarctic streams by the writer indicate that cutoff is 

preceded by stripping of the surface cohesive layer.
20 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

This process may extend over several successive high 

flows in smaller streams, or it may occur entirely at the 

time of the flow causing the cutoff in larger streams.

Any cutting or removal of the surface layer where it 

occurs along the banks on the active flood plain of the 

Kenai River will create an increased potential for bank 

erosion. A boat slip without riprap, for example, and 

cut transverse to the flow direction creates a point of 

attack from which the cohesive layer can be stripped. 

Once the cohesive layer is lost, the underlying channel 

deposits are subject to rapid erosion that could lead to 

a meander cutoff.

SUSPENDED SEDIMENT

Sediment sufficiently fine grained to be transported 

in suspension affects the salmon habitat in a variety of 

direct and indirect ways (see Meehan, 1974, p. 5-7). As 

described previously, the main detrimental effect of 

fine sediment occurs consequent to deposition, through 

the reduction of gravel permeability during egg and fry 

development. Suspended sediment can be directly 

harmful to fish if concentrations are both high and persistant, but the requisite levels are not well defined. 

After a literature survey, Gibbons and Salo (1973, p. 6) 

concluded that prolonged exposure to sediment concentrations of 200-300 mg/L is lethal to fish, although 

other studies report higher levels. High concentrations may also detract from the esthetic and recreational values of a fishery. Because salmon are sight 

feeders, angling success is reduced and competition 

with species more tolerant of turbidity is increased 

with a significant rise in suspended-sediment concentration (Phillips, 1971, p. 65).

Subarctic alpine streams are characterized by a limited and specialized macro invertebrate fauna that is 

adapted to the glacial melt-water environment (Hynes, 

1970). It is logical to assume that even minor changes 

in habitat could affect the macro invertebrate population and thus the fish fauna dependent on it for food 

(U.S. Army Corps of Engineers, 1978, p. 102).

Unfortunately, the effects on the salmon habitat of 

specific values of suspended-sediment concentration 

have not been established. The preferred environments 

and times for salmon spawning are clearly those with 

the least suspended sediment. Concentrations were observed to be "minor" (less than about 30-50 mg/L) 

during the spawning and incubation periods in the 

most stable producing areas for sockeye and pink salmon (Cooper, 1965, p. 6). Also, experiments comparing deposition rates from flows with 20 and 200 mg/L 

of suspended sediment indicate the "necessity for 

maintaining very low suspended sediment concentration in waters flowing over salmon spawning grounds" 

(Cooper, 1965, p. 61).

Values of suspended-sediment concentration in the 

Kenai River at Soldotna ranged as high as 151 mg/L in 

24 samples collected from 1967 to 1977. The typical 

concentration during summer flow fell within the range 

10-100 mg/L. A sample taken on September 9, 1977 

the date of the peak discharge of record, 33,700 

ftYs yielded a concentration of 104 mg/L. The only 

comparable nearby stream, the Kasilof River, has a 

similar melt-water flow regime and likewise drains a 

large moraine-impounded lake, Tustumena Lake. The 

stream is, like the Kenai River, the site of important 

salmon runs. Suspended-sediment concentration in 

that stream, from 19 samples collected between 1953 

and 1968, fell within the uncommonly narrow range 

15-45 mg/L. This lower, narrower range can be ascribed mainly to the greater sediment-retention effect 

of Tustumena Lake, but it could be due in some part to 

lesser river use and bank development relative to the 

Kenai River.

Limited sampling from the Kenai River at Cooper 

Landing, at the outlet of Kenai Lake, suggests the presence of generally low concentrations of suspended sediment at that point. The concentrations in 24 samples 

taken between 1956 and 1974 at discharges from 420 

to 19,100 ftYs ranged from 2 to 26 mg/L, except for 

one measurement of 72 mg/L. Concentration at the 

discharge of 19,100 ft3 /s was only 2 mg/L, sampled 

September 20, 1974 the day before the peak discharge of record that resulted from release of the glacial lake in the Snow River drainage (hydrograph in 

fig. 3).

All pre-1979 measurements from the Kenai River at 

Soldotna are plotted in figure 13. A sharp distinction in 

the relation between water discharge and sediment 

concentration is evident in the data representing discharges of January through May and those for the 

period June-September. A similar difference is evident 

in the sediment-transport curve for the station (not 

shown), in which water discharge is compared with sediment discharge rather than concentration. The groupings of data seen in figure 13 represent the sustained 

low-flow period of winter and spring and the prolonged 

period of high melt-water flow throughout the summer. 

They illustrate the important conclusion that concentrations can vary widely within each range of flow. The 

biota of the Kenai River consequently will be at 

greatest risk to increases in concentration due to construction activity during the low-flow period. An influx 

of sediment that caused little change in concentration 

levels during the summer could result in significant 

adverse impact during winter and spring.
SUSPENDED SEDIMENT 21

Neither the base concentration levels nor the shortterm variations in concentration are evident from the 

scattered historical samples shown in figure 13. To illustrate these aspects of the sediment system of the 

river and to provide a basis for future comparison, 

daily sampling at Soldotna was begun on August 23, 

1979, and continued until December 5, 1979 (fig. 14). 

During this period, suspended-sediment concentration 

ranged from 1 to 52 mg/L, at mean daily discharges of 

5,260 to 21,600 ft3 /s. In comparison with previous flow 

records (fig. 2), the mean discharge of 11,800 ftfVs in 

September 1979 was typical. Unfortunately for purposes of comparison, flow later in the fall of 1979 was 

abnormally high. The mean discharge for October of 

14,000 ftVs was more than 50 percent above the previous high mean discharge for the month, and the 

mean flow in November of 7,330 ft3 /s exceeded the 

previous high by a similar proportion.

Throughout the period of daily sampling, concentration levels based near or below 10 mg/L and generally 

increased above that level in the early stages of a rise 

in flow (fig. 14). An unexpected pattern of variations in 

concentration with flow is the seeming gradual rise in 

base concentration as discharge underwent its seasonal 

decline in late October and November. From base values of approximately 5 mg/L in early September and 

mid-October, the typical base concentration increased 

to about 10 mg/L in the period from late October to 

the end of data collection on December 5. Although the

reason for this anomalous increase as flow declined is 

not known, one possible cause is wind-generated wave 

action on Skilak Lake.

Each daily rise in concentration of more than 5 mg/L 

accompanied a significant increase in discharge in 

comparison with the preceding day (fig. 14). The sharpest daily changes in concentration and discharge occurred early in both major rises in discharge during the 

measurement period. On days following the peak in 

concentration, discharge continued to increase, most 

notably during the rise in discharge that began on September 13. Concentration peaked on September 15 and 

then generally declined for the five subsequent days as 

discharge continued to increase.

Speculations concerning the sources of this suspended sediment are possible. The relation of water 

discharge and sediment concentration described in the 

preceding paragraph is designated as advanced (or 

leading) sediment concentration (Guy, 1970, p. 22); 

that is, the peak concentration precedes the peak of the 

water-discharge hydrograph, in this case markedly so. 

This relation is the most common and is consistent with 

transport of loose sediment by the first direct runoff. 

However, in the Kenai River at Soldotna the concentration is so advanced that the bulk of the sediment is 

clearly of local derivation, originating in the section of 

watershed downstream from Skilak Lake. This conclusion is expected, given the sediment-entrapping function of the lake, and narrows the sources of much of

100,000

10,000 CD D 

O

1000

9-23-74 ( 

8-23-79 . 9-2-67 ,

9-9-77_

fc ^^

'"l 0-16-69

7-14-71

7-31-69

6-24-70

^-8-21-68 

'8-2-68

'6-19-68

9-9-69 10-22-67

10-23-68

5-17-68

-324-71

3-5-70. 

3-19-69

1-28-71

2-27-69

4-2-68

' 1-15-69

0.1 1.0 10 100 1000

CONCENTRATION, IN MILLIGRAMS PER LITER 

FIGURE 13. Water discharge against suspended-sediment concentration, Kenai River at Soldotna. All pre-1979 data are shown.
22 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

the sediment to the Killey River basin and bank erosion 

along the Kenai River.

Scattered sampling of the tributaries entering the 

river downstream from Skilak Lake shows that 

suspended-sediment concentration is generally very 

low, especially in the subordinate storm-runoff peaks 

of middle and late summer. Snowmelt peaks are the 

dominant discharge events in the flow records of these 

lowland streams, and the runoff is greatly retardedtypical of marshy subarctic terrain. The Killey River is 

the exception: it drains a watershed that extends to 

nearly 6,000 ft in altitude (timberline is approximately 

2,000 ft) and includes the Killey Glacier, an extension 

of the Harding Icefield. Runoff from the Killey River 

basin contributes to the early part of any rise in the 

Kenai River that occurs in response to a basinwide 

storm. Traveltime of flood waves from the headwaters 

is unknown, but it would be measured in hours as opposed to days for a flood wave from the Snow River 

drainage (fig. 3). Unfortunately, storm sediment concentrations of the Killey River are unknown. Observations indicate that they are relatively high. Two sets of 

aerial photographs (i960, 1977) of the Kenai-Killey 

confluence show a turbid plume, representing the unmixed contribution of the Killey River, extending several miles downstream in the Kenai River. Sequential 

aerial photography also indicates that the Killey River 

channel is actively eroding; a neck cutoff of a meander 

1.5 mi upstream from the confluence occurred between 

1950 and 1972.

The dispersion in concentration at a given discharge 

is mainly due to variations in natural sedimentproducing processes. There is no increase in concentration over time evident in the limited data of figure 13 

that can be ascribed to development or river use. This 

result may reflect the small number of samples taken 

at low flows. The effect of canal dredging and cleaning, 

which are probably accomplished mainly during lowflow periods, are limited in time and would have been 

sampled only by extreme change. Local residents report that episodes of abnormally high turbidity are 

caused by dredging of canals. This high turbidity prob60

ul 50

40

30

20

10

t_> 

00

o

2 ~Q

uJZ

|§

C/5 in 

O ^~

^~ 'i'

 1 in

Q

2

<

UJ

5

20,000

16,000

12,000

8000

4000

i i i i i i i i i i

 

 

. .. .." .... . ' 

_. 

' . ... t » » m

* » . » " -.

i_ - * ... 

.... 

* .». « > .....(< -

.«.».*

i i i i i i i i i i

31 10 20 30 10 20 31 10 20 30 1

Aug. Sept. Oct. Nov. Dec

1979

FIGURE 14. Water discharge against suspended-sediment concentration, Kenai River at Soldotna, August 23 to December 5, 1979.
BANK EROSION 23

ably correlates with increased suspended-sediment 

concentration.

One cause of dispersion in concentration levels at a 

given discharge is wind action on Skilak Lake. The lake 

is at the foot of large icefields and is periodically swept 

by violent winds that have caused the deaths of more 

than 20 boaters. The association of wind action on the 

lake, high turbidity levels in the lake, and turbid flow 

in the downstream part of the Kenai River has been observed by S. H. Jones of the U.S. Geological Survey 

(written commun., 1979). These observations coincide 

with those of Smith (1978), who described sediment 

movement in a glacier-fed lake in Alberta in response 

to wind-generated currents. In addition to generating 

high turbidity throughout Skilak Lake, wind-induced 

waves may erode lake-bottom and shoreline sediment 

in the vicinity of the outlet. The entrained sediment 

may then be introduced into the river as part of the 

suspended-sediment load.

Size measurements of the suspended sediment from 

the Kenai River at Soldotna indicate that 38 to 71 percent falls within the size ranges of silt and clay. Comparison with data from other Alaskan streams, including those fed by glacial melt water and controlled by 

lakes, shows that size distribution to be typical. Turbidity measurements from the station are too few for comparison or generalization.

BANK EROSION

An unknown but probably significant amount of the 

suspended-sediment load in the Kenai River is presently derived from bank erosion. Future increases in 

suspended sediment thus will be caused by any type of 

development or river use that increases bank erosion. 

The historical rates at which banks have been eroded 

can indicate which sections of the river are likely to be 

the most vulnerable to future man-induced changes.

Bank-erosion rates were determined by comparing 

aerial photographs taken in 1950-51, 1972, and 1977 

(table 3). Additionally, the 1977 photographs were 

compared with ground photographs of the present 

(1979) bank configuration in channel bends. These 

comparisons showed that since 1950 the entrenched 

section of the stream has been exceptionally stable. 

Elsewhere, erosion rates have been comparable with 

those to be expected in a river the size of the Kenai. 

There is an indication that a recent increase in bank 

erosion may be occurring in response to river-use practices.

METHODOLOGY

Amounts of erosion were measured by superimposing

TABLE 3. Aerial photography of the Kenai River downstream 

from Skilak Lake

Date Agency Scale Area covered

1:36,000 Entire river

1:12,000 Downstream

from Soldotna.

June, August 1950 _ U.S. GeoloJune, August 1951 _ gical Survey 

May 1965 __-_ U.S. Army

Corps of Engi

neers 

September 1972 _ U.S. Army 1:12,000 Upstream from

Corps of Engi- Soldotna.

neers

July 1977 _____ U.S. Army 1:4,800 Entire river 

Corps of Engineers

the projected image of one photograph on another of a 

differing date. If the projection is precise, the differences in bank position correspond to erosion and accretion of the channel in the interval between the sets of 

photography. For this study, projections were made 

with a Bausch&Lomb Zoom Transfer Scope. This 

technique permits immediate comparison of photographs of greatly differing scale a distinct advantage 

over previous methods. Because the procedure is not 

described in the literature, it will be discussed here in 

detail.

Use of the Zoom Transfer Scope involves viewing one 

photograph directly through a binocular eyepiece. On 

that photograph is projected the image of a second 

photograph, with the scale of the projection continuously variable with a zoom control to as much as 14x. 

The image of the smaller scale photograph is projected 

on to the larger, and the illumination of either may be 

varied with a rheostat. In matching the images, it is 

useful to vary one of the illumination controls rapidly 

so that the two photographs are seen in alternating 

succession. Then, once the scale and position of the 

photographs have been correctly matched, channel 

changes will stand out with remarkable clarity.

The main obstacle to precise measurement of channel 

change is scale variation in the aerial photographs. On 

each photograph the scale changes with distance from 

the center, reflecting the vertical orientation of the 

camera. Consequently, on each pair of photographs it 

is necessary to match geographic features in the immediate vicinity of each bank segment as it is analyzed. 

Features useful in matching photographs of the Kenai 

River include individual trees, large boulders, roads, 

and houses. The need to match features on or near the 

bank segment being studied cannot be overemphasized. Generally, the scale variation was such that, 

if one bank was matched, the opposite bank of the 

stream would not be matched, even in reaches where 

no bank erosion had occurred.
24 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

MECHANICS OF BANK EROSION LOW BANKS AND HIGH BANKS

Although permafrost is not present in significant 

amounts, the low banks bordering most of the nonentrenched parts of the Kenai River, and its flood plain 

where present, erode in a manner similar to the bank 

erosion of streams in permafrost areas (Scott, 1978, p. 

10). Channel deposits erode, thereby undercutting the 

stabilizing surficial layer of cohesive sediment. All 

areas of relatively rapid bank erosion, with rates comparable to those of small and medium-sized rivers 

elsewhere (Wolman and Leopold, 1957, table 4), involve the low banks.

The low banks downstream from approximately river 

mile 14 are composed of cohesive, clay-rich sediment 

interbedded with less cohesive silt and sand, and locally with coarser sediment. Erosion progresses most 

rapidly in the sand and gravel layers and triggers bank 

failure by slumping. This bank material represents 

tidal and shallow marine deposition during the marine 

transgression near the close of the Naptowne Glaciation 

(table l). Modern tidal deposition is occurring as far 

upstream as river mile 12, but the deposits now subject 

to erosion mainly represent the earlier interval of deposition.

The high banks are those extending well above the 

level of bankfull stage to heights as much as 70 ft. They 

occur along entrenched sections and locally along 

nonentrenched sections of the river. The banks are 

composed mainly of glacial-outwash gravel that is distinctly finer grained and more poorly sorted than the 

modern channel deposits. Most cut banks are covered 

with mature spruce and historically have been stable. 

Where the high banks are eroding, the slope is undercut at the base, and the vegetated surface is progressively unraveling. Trees and mats of vegetation slide 

into the river until the entire slope becomes composed 

of loose gravel at the angle of repose. The slope angle 

is nearly the same as that of the completely vegetated 

banks, showing that the history of the banks is one of 

erosion interrupted by a geologically recent interval of 

low erosion rates that has allowed the mantling and 

stabilizing of the slopes by vegetation. The period of 

high-bank stability may now be ending in response to 

increased river use, a possibility discussed below.

RATES OF BANK EROSION

The position of the high banks of the entrenched 

channel in 1977 was remarkably similar to their position in 1950-51. Rates of erosion less than 1 ft per year 

were the rule. At most sites there was no detectable 

change in bank position, within the limits of accuracy 

of photographic comparisons and with adjustments for

differing flows levels shown on the photographs.

Unfortunately, this generalization does not apply to 

the entire river. Above river mile 39.4 and below river 

mile 17.6 the limits of the entrenched channel are 

areas with low banks eroding at rates as high as 5 ft 

per year. Figures 15 and 16 illustrate the distribution 

of erosion within parts of these two sections of the 

river. Several observations on these figures are pertinent.

First, the eroding areas are local in distribution, and 

even in these less stable reaches, much of the bank has 

not been affected by measurable amounts of erosion. 

The positions of the rapidly eroding banks are not predictable from the configuration of the channel. This effect is not unusual and has been shown in some other 

rivers to be caused by a wandering thalweg. Composition of the banks is a chief control on erosion of the 

Kenai River banks, along with the correlative factor of 

bed-material size. For example, at river mile 40.4 (fig. 

15) the flow impinges at a 90° angle on the right bank, 

yet only negligible erosion of that bank has occurred. 

This section of bank is part of a topographic lineament 

against which the north sides of meanders are deformed upstream from river mile 39.4 (fig. l). Cut 

banks along the lineament reveal glacial till that is resistant to erosion because of its clay-rich matrix.

Second, erosion rates have been relatively constant 

during the period 1950-51 to 1977. This conclusion is 

based on the proportional amounts of erosion in subdivisions of this period. In the downstream area of high 

erosion rates (fig. 16), the amount of erosion between 

1950 and 1965, a 15-year interval, is similar to or 

slightly greater than that between 1965 and 1977, a 

12-year interval. Upstream (fig. 15), most of the erosion occurred between 1950 and 1972, with smaller 

amounts between 1972 and 1977. The intervals reflect 

the dates of the photographs.

Finally, the two sections of the river with the highest 

erosion rates coincide with those sections of the river 

having a tendency to anabranch. In each case the slope 

of the eroding reaches is controlled by a base level a 

short distance downstream. In the upstream reach the 

control is the Naptowne end moraine; in the 

downstream reach the control is sea level.

Tidal action extends upstream approximately as far 

as river mile 12 and affects the reach shown in figure 

16. Jones (1969), in a study of the Kenai River estuary, 

measured tidal velocities at sections as far upstream as 

river mile 11.4, above the illustrated reach. The measurements revealed significant floodtide velocities at 

that point at a time of low streamflow and high tides 

(May, 1969). Bank erosion from upstream tidal flow is 

possible during such periods. The distribution of the re-
BANK EROSION 25

0 100 200 300 METERS

500 1000 FEET

0 100 200 300 METERS

I_____|_____I I I

500 1000 FEET

FIGURE 15. Reach in upper section of the Kenai River, showing bank I^DU-I * t 4.\. v - -D- uo i-ji- -LI -x- ,^r,r, FIGURE 16. Reach in lower section of the Kenai River, showing erosion rates. Solid line is bank position in 1977. , , . , 0 ,.,,. . , , ... inaa bank-erosion rates. Solid line is bank position m 1977.
26 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

corded erosion (fig. 16) indicates, however, that 

downstream flow is the main cause. The erosion of the 

head of the island at the bottom of figure 16 is an 

example, as is the erosion on the inner, upstream side 

of the bend immediately above river mile 10.

POSSIBLE RECENT INCREASE IN BANK EROSION

Although no obvious changes in bank-erosion rates 

could be determined in the period 1950-51 to 1977, 

there is evidence of recent change that possibly forecasts a period of more rapid erosion. The most noticeable change is the number of fresh slide scars on the 

high banks visible in the 1977 photographs. Figure 17 

illustrates these scars on the high banks along the outside of meander 3-H. The features occur where a maturely vegetated bank is undercut and the bank surface 

slides off into the river. The amount of erosion in terms 

of distance of bank retreat has thus far been small. 

Nevertheless, if sliding continues and the entire 

lengths of meander cut banks become active, a serious 

erosion problem will result. Because of the heights of 

some banks (50-70 ft), small amounts of bank retreat 

will add large volumes of sediment to the stream.

To investigate this increase in erosion of the high 

banks, ground photographs were made of the inside of 

all meander bends and then compared with the 1977 

aerial photography. The results suggest that the instability is of recent occurrence and is continuing and 

possibly increasing at the present time (1979). The 

evidence for this conclusion is based on the 1977 

photographs, which are of larger scale (1:4,800) and 

consequently of greater resolution than any preceding 

photographs, as well as on a comparison of that photography with ground photographs. To establish the recent instability of the high banks without qualification, 

it may be necessary to compare the 1977 photography 

with a later set that is equivalent in scale and resolution.

There are several explanations for this apparent increase in slide scars on the high banks. The possibility 

that construction debris was dumped over the banks 

was excluded in most instances. Another possibility is 

that the increased deflection of flow into cut banks as a 

result of construction of groins, boat ramps, and 

bank-protection structures has thus far caused small 

amounts of erosion. The most obvious example is 

meander 1-P near Sterling, where the inside of the 

entrenched-meander bend is studded with 13 groins 

from 15 to 75 ft long (fig. 18). These groins create the 

potential for bank erosion of at least an equivalent distance on the opposite cut bank and the possibility of

larger amounts once the stability of the bank is destroyed. The groins were constructed before 1972, and 

the opposite high bank is beginning to fail by slumping 

near the point opposite the largest groins.

Another explanation is a recent change in river use. 

Beginning approximately in 1974, it was discovered 

that the most efficient sport-fishing technique for king 

salmon consisted of "drifting" the practice of trolling 

from a boat while floating downstream without power 

through a promising reach, and than using power to return to the head of the reach and repeat the maneuver. 

Fishing for most other species, such as silver salmon, 

has continued in large part from anchored boats. The 

practice of " drifting" for king salmon has resulted in a 

substantial increase in the use of high-horsepower 

sport boats and more intensive usage of the boats per 

man-day on the river. These effects are additive to the 

general increase in sport-fishing popularity (table 4). 

An assessment of this problem is beyond the scope of 

this report and should await conclusive study of the 

possible recent increase in erosion rates mentioned 

above. The potential for river-use practices as contributors to increased bank erosion is a significant one, 

however, and should be considered by planners 

whether an increase in erosion can be documented or 

not. Once the stabilizing vegetation on the high banks 

is lost, erosion can potentially accelerate, even if river 

use is subsequently controlled.

The effect of boat wakes on the banks is sufficient to 

initiate and cause continued erosion of the high banks 

without other significant changes. Observations along 

the cut bank of meander 3-H reveal that each wake 

runs up the loose gravel bank as much as 3 or 4 ft, 

eroding and entraining sediment and creating a zone of 

visibly turbid water at the edge of the stream. The bank 

is progressively undercut, and the slope profile is 

maintained by sediment from the upper sections of the 

bank. Where the bank is vegetated or formed of cohesive sediment, the resistance to boat-wake erosion is 

greater.

TABLE 4. King salmon taken by sport fishing in the KenaiRiver, 

1974-79

[Data from Alaska Department of Fish and Game. Annual catch is limited by State 

regulations]

Year Early run

(June)

Late run 

(July) Total

1974.

1975.

1976.

1977.

1978.

1979.

1,685

615

1,555

2,173

1,542

3,661

3,225

2,355

4,477

5,148

5,578

4,634

4,910

2,970

6,032

7,321

7,120

8,295
BANK EROSION 27

FIGURE 17. Kenai River between approximate river miles 16.7 and 15.3. Note concave high bank with slide scars, and canal development and 

forest clearing on flood plain within meander loop. Wakes are caused by boats. Flow is from bottom of photograph to top. Scale, 1:4,800, 

or 1 in.= 400 ft. Date: July 9, 1977. Photograph credit: U.S. Army Corps of Engineers.
28 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

DEVELOPMENT AND THE KENAI RIVER CHANNEL

This part of the report discusses which sections of the 

river are most vulnerable to development and the types 

of development and impacts associated with each. 

Table 5 summarizes the channel characteristics and the 

sensitivity of each section of the stream to development. It will serve as background information on the 

channel for the discussion of development types that 

follows. For use by planners, this section is intended to 

be used in conjunction with the flood-hazard maps prepared by the U.S. Army Corps of Engineers (1967, 

1973, 1975). The existing criteria for development 

permits are presented in the comprehensive report by 

the U.S. Army Corps of Engineers (1978, p. 16-52).

CONSEQUENCES OF DEVELOPMENT

Because the risks of development cannot be quantified, the definition of the hazards to the Kenai River

salmon fishery must be subjective. The exact erosional 

response of the river's banks to certain types of development is unknown, although a significant response 

can be expected on the basis of our knowledge of river 

behavior. Nor can the increase in suspended-sediment 

transport that will result from increased bank erosion 

be stated with any degree of certainty. We know that 

suspended sediment will increase as bank erosion increases, and the studies cited in the section on suspended sediment indicate the potential for decline in 

the salmon fishery with increases in concentration only 

moderately above present levels. Conclusions regarding 

the range of concentration levels that may prove harmful will not, however, meet with agreement among 

those studying salmon habitats.

Additions to suspended sediment that will occur directly from construction activities should be distinguished from the more significant increases in concentration that can occur with the increased bank eroFIGURE 18. Kenai River between approximate river miles 38.2 and 37.0. Flow is from right to left. Scale, 1:4,800, or 1 in.= 400 ft. Date: July

11, 1977. Photograph credit: U.S. Army Corps of Engineers.
DEVELOPMENT AND THE KENAI RIVER CHANNEL 29

sion triggered by some types of development. (This 

section deals with the latter type of hazard unless 

stated otherwise.) An additional potential cause of increased suspended-sediment transport is such upland 

land-use changes as logging, but these effects are 

excluded from the analysis. And possibly more significant than any effect of development is the potential 

adverse impact from river-use practices described in 

the previous section.

In determining what types of development to allow, 

planners are faced with two problems. The first problem involves the fact that, although a type of development may now be insignificant in its effects on the 

river, the cumulative effect of many such developments, combined with other actions in the future, may 

have an important negative effect. An example of such 

a situation, discussed below, is the excavation of boat 

slips in the entrenched section of the river. An approach to this general problem is to continue to 

monitor the productivity and sediment content of the 

stream as development progresses.

The second problem involves the fact that, because 

none of the risks associated with any of the development types can be quantified, cost-benefit analysis

cannot be used directly. This, however, should not 

serve as a rationale for lack of decisions concerning development. This report defines the impacts of each 

common type of development, ranks them in order of 

risk, and indicates (table 5) how the impact will vary 

along the river.

Each development type can be assessed for its potential to cause channel change. The most dramatic 

change, and one that poses a short-term hazard to the 

stream by increasing erosion and suspended sediment, 

is the cutoff of a meander loop. A cutoff is a sudden 

diversion of the main channel that may set up a disequilibrium which causes substantial channel change 

extending beyond the vicinity of the diversion. Cutoffs 

consist of two types: loop or neck cutoffs, in which a 

meander loop tightens until flow cuts across the narrow neck; and chute cutoffs, in which flow cuts across a 

meander loop, generally one less tightly developed and 

one which may have incipient channels between ridges 

of point-bar deposits.

The first effect of a loop cutoff will be seen in the 

change of shape of adjacent meanders in response to 

the local change in slope. The extent of this change has 

been variously reported to be slight or to consist of

TABLE 5. Summary of channel characteristics pertinent to determining sensitivity of the Kenai River to development

Segment of channel 

( river miles)

50.3 to 45.7 

45.7 to 39.4

39.4 to 34.8

34.8 to 2 1.8

21.8 to 17.6

17.6 to 13.4

13.4 to 9.0

9.0 to mouth

Pattern and 

degree of 

entrenchment

Meandering; 

slightly entrenched.

Meandering;

free to 

migrate.

entrenched.

straight; 

entrenched 

within Soldotna 

terrace. 

Meandering;

entrenched 

within Soldotna 

terrace. 

Meandering;

Partially 

entrenched, 

but meanders 

are migrating. 

Sinuous and

anabranching.

tidal regime; 

channel is free 

to migrate.

Underfit conditions

Channel appears 

"drowned" formed at 

lower streambed 

elevations.

present flow regime.

below junction with 

Moose River.

of entire river. 

Underfit- _-------__-_-

present flow regime.

product of present 

flow regime.

Degree of armoring

Partly armored ( stable 

crescentric dunes).

do _____________ _ __

... do ____________ . _..

armored. 

None

do _____.___.___.._._

. *ateof. Relative bank erosion ... . 

, sensitivity under present .

(ff/yrl development

1.0 Low 

5.0 High

< 1.0 Low

<1.0 Do.

<1.0 Do.

2.0 Hfgh

5.0 Do.

2.0 Moderate
30 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

channel realinement extending for miles beyond the 

site of the cutoff. Case histories of cutoffs in streams 

similar to the Kenai River are not useful in forecasting 

the likely effects. A loop cutoff of an entrenched subarctic stream the Pembina River in Alberta was described by Crickmay (i960), but little bank erosion 

outside the point of cutoff apparently occurred because 

the stream, unlike the Kenai River, was entrenched in 

resistant bedrock. Loop cutoffs on the White River in 

Indiana resulted in rapid growth of adjoining meanders, but the effect did not extend very far upstream or 

downstream (Brice, 1973, p. 191). In a new meander 

formed after a chute cutoff on the Des Moines River, 

erosion rates were initially high and then decreased as 

the equilibrium position approximated by the meander 

belt was approached (Handy, 1972). A contrasting result was described by Konditerova and Ivanov (1969), 

who documented a pattern of change in the Irtysh 

River, a tributary of the Ob River in Siberia, in which 

changes in a single "key" meander controlled the deformation of a long sequence of meanders. Perhaps the 

most comprehensive study of the effects of cutoffs is 

that by Brice (1980), who has compiled case histories 

on approximately 60 sites where artificial cutoffs have 

been made. In most places the results were slight, but 

in a few there were drastic effects. The reasons for this 

differential response are not yet known.

Probably the greatest long-term hazard to the stream 

is the loss of stability of the high banks. Once the vegetative cover of the banks is lost, erosion rates and sediment loads could increase rapidly to levels endangering the productivity of the river. After the process begins, the only means of restoring the stability of these 

banks could be a costly engineering solution. The possible effects of river use on the high banks were discussed in the section on bank erosion. A type of development that could have a similar effect is the building of 

groins and boat ramps on the convex banks of meanders. Some loss of high-bank stability could also result 

from a meander cutoff on a nonentrenched part of the 

stream.

CANALS

Where the channel is not entrenched, the interior of 

several meander loops has been developed by means of 

canals bulldozed within the active flood plain for the 

purpose of providing waterfront access to trailer sites 

and homesites. This unusual form of development is 

possible only because of the sustained high flow that 

keeps the water level in the canals within a restricted 

range throughout most of middle and late summer. The 

most extensive canal developments occur within meanders 3-H and 1-H (figs. 17 and 19, respectively).

The unriprapped canals in the interior of meander 

loops are of concern to the stability of the river. The 

canals create a point of attack for flood flows to cut 

through and peel away the surficial layer and erode the 

underlying channel deposits. Once a channel is formed 

in the underlying gravel, the potential is for a cutoff 

and a diversion of the entire channel through that point 

in the neck of the meander.

Meander cutoffs have occurred on the Kenai River, 

probably within historical time, although none has occurred within the post-1950 period documented by aerial photography. The bend labeled meander "1-J" may 

have been a fully developed meander, now cut off, the 

previous course of which is in part marked by a small 

residual channel. Meander loop 1-L (fig. 15) is a 

meander probably in the process of a gradual chute 

cutoff.

The areas at risk from a meander cutoff are those 

where the river channel is not entrenched and the level 

of the interior surface of the meander loop is below the 

level of the Intermediate Regional Flood that which 

will recur once in 100 years on the average but which 

could occur in any given year. The risk of a cutoff is 

associated with lesser floods, but the frequency of 

flows or the depth of flow on the flood plain with which 

the risk is associated cannot be accurately stated.

In the upstream part of the river the areas at greatest 

risk of cutoff potentially triggered by unriprapped 

canal development include the meander loops in the 

reach that extends from river mile 45.7 to river mile 

39.4. Below this section of channel the river is fully entrenched, and upstream to the mouth of Skilak Lake 

the meanders are stable, and the normal pattern of 

pools and riffles are replaced by gravel dunes.

In the downstream part of the river the area at risk 

from channel changes initiated by canals extends from 

river mile 17.6 to river mile 9.0. The channel upstream 

from river mile 17.6 is entrenched, and that 

downstream from approximately river mile 9 is relatively stable within the tidal regime. This section of the 

river includes the area of single greatest risk, meander 

3-H. Here the stream is partly entrenched the interior 

of the meander loop is active flood plain; the outside 

high bank is 40 to 45 ft in height. This bend is the 

tightest of any meander on the river, and the interior 

of the loop has been subject to canal development and 

forest clearing (fig. 17). The consequences of a loop 

cutoff of meander 3-H could be significant. Much of 

the area within downstream loop 3-1 would potentially 

be subject to erosion as the channel adjusted to the 

postcutoff configuration. There is little impediment to a 

major realinement of the stream at this point. The high 

bank on the downstream side of meander 3-H is actively eroding; vegetative cover has been lost, and the
DEVELOPMENT AND THE KENAI RIVER CHANNEL 31

bank is composed of relatively fine grained glaciofluvial 

sediment.

The area upstream from meander 3-1, the apex of 

which is the tight bend known as Big Eddy, is subject 

to periodic ice-jam flooding. The potential for channel 

cutting through the neck of meander 3-H is consequently increased. Ice scars in spruce trees growing 

on the interior-meander flood plain extend to heights of 

approximately 20 ft. Flooding and erosion risks associated with ice jams are present on the entire river, 

of course, but they are pronounced in this place.

GROINS AND BOAT RAMPS

Groins are structures placed at approximately a right 

angle to the bank, commonly for the purpose of preventing bank erosion. Along the Kenai River the structures are emplaced most commonly to provide docking 

facilities and a protected area for boat mooring. The

coarseness of the bed material allows it to be formed 

into groins that are sufficiently stable to remain for 

years with the addition of riprap on the point and upstream side. The riprap may consist of rock- or 

concrete-filled drums, iron bars and cable, tires linked 

with chain, or dumped scrap metal. Without minimally 

maintained riprap, the groins and boat ramps are observed on the sequential aerial photographs to become 

blunted over a period of years as the material is slowly 

eroded.

The greatest development of groins is found on 

meander 1-P (fig. 13), as described in the section on 

bank erosion. They are mainly confined to the entrenched section of the channel, where they are the alternative to canals and boat slips because of the impracticality of excavation in the high banks.

Characteristic of a groin is the formation of an eddy 

downstream from its tip and a resulting deflection of 

flow that can erode the bank. The problem can be

FIGURE 19. Kenai River between approximate river miles 44.8 and 42.9. Interior of meander loop has been developed with canals. Note 

natural channels across neck of meander; one channel has been partly excavated to form a canal. The Killey River enters from bottom of 

photograph. Flow direction is from right to left. Scale, 1:12,000, or 1 in.= 1,000 ft. Date: September 24, 1972. Photograph credit: U.S. 

Army Corps of Engineers.
32 EROSION AND SEDIMENTATION, KENAI RIVER, ALASKA

minimized by emplacing the groin at a slight upstream 

angle. This type of bank scour associated with groins 

and boat ramps on the Kenai River is not normally a 

problem because of the coarse bed material.

The most obvious deleterious result of groin and 

ramp construction is the potential for displacement of 

the channel toward the opposite bank a distance equivalent to the length of the structure. This result has yet 

to occur at meander 1-P because the bank on the outside of the meander bend was stabilized by vegetation 

at the time of construction. At present (1979) the bank 

is beginning to fail by undercutting and slumping, a 

process that can be expected to increase in future years 

if the groins are maintained with the addition of riprap.

If the distance of channel displacement was confined 

to the length of the structures, a cost-benefit analysis 

of their construction would be possible. Unfortunately, 

once the stabilizing vegetation on the bank is lost, the 

erosion potential is much greater, and it is possible for 

a cycle of increased erosion over a period of years to 

begin.

EXCAVATED BOAT SLIPS

Boat slips excavated in the channel bank are probably the most common type of development along the 

Kenai River. In the past the excavated material has 

been dumped to form a small protective groin on the 

upstream side of the slip or'just pushed into the channel. Both methods of disposal, however, are presently 

contrary to the conditions attached to a construction 

permit (U.S. Army Corps of Engineers, 1978, p. 43). 

The slips and the canal systems are excavated and 

cleaned, most commonly during the low-flow period.

The potential for harmful effects of unriprapped boat 

slips varies with location. .Where excavated on the upstream side of a meander loop in the nonentrenched 

part of the stream, a single boat slip can pose a hazard 

by creating a point of attack for flood flows. Meander 

1-H is a bend that would become more vulnerable to 

cutoff through the construction of slips on the upstream side, especially at the locations of natural channels visible in figure 19. Where slips are excavated at 

most locations on the entrenched part of the stream 

(table 5), the individual hazard will be slight, but each 

may form part of a cumulative effect. The need for riprap will also vary greatly with location. Where excavation is in the coarse channel deposits characteristic of 

the entrenched and partly armored sections of the 

river, the need for lining by even coarser material will 

be slight at most locations. Riprap will be advisable at 

most sites outside the entrenched channel.

There are other considerations illustrating the complexity of the impact of boat slips. Excavated boat slips

are a type of development that is not necessary for recreational use of the river. For most owners of riverfront property, a slip can be viewed as a matter of convenience; small boats can be drawn up on the bank at 

any place where the bank height is low enough to make 

a slip feasible. Excavated slips, however, may encourage the use of large, high-horsepower boats of the sizes 

that may be contributing, disproportionate to their 

numbers, to the possible increase in bank erosion discussed previously. With unlimited river use, the granting of permits for boat slips could logically, therefore, 

be assessed for the potential additional effect of encouraging larger boats.

BANK- PROTECTION STRUCTURES

A variety of measures have been employed to support 

and protect homes constructed on the banks of the 

Kenai River. They include concrete walls, gravel berms, 

earthen embankments, piles driven into the bank, and 

chained tires. The purpose is commonly multifold: to 

provide docks, to provide foundations for structures, 

porch and patio areas, or to expand usable lot size, as 

well as acting as revetments to provide protection from 

bank erosion.

The effects on the stream channel of most such bank 

modifications will be slight as long as the original bank 

profile is not greatly changed. Loss of channel capacity 

and concentration of flow toward the opposite bank, 

leading to erosion of that bank, are possible if the 

structures are sufficiently extensive and of sufficient 

height to function locally as flood levees. Indirect effects, related to excavation of gravel and removal of the 

cohesive surface to supply fill for berms and levees, are 

also possible.

GRAVEL MINING AND COMMERCIAL DEVELOPMENTS

At several locations visible on the 1977 aerial photographs it appears that the banks have been mined for 

aggregate. The largest of these sites is on the north 

bank of the Kenai River, approximately 0.2 miles upstream from the junction of the Moose River. The impacts of gravel mining on stream channels have been 

described previously (for example, Scott, 1973; Bull 

and Scott, 1974) and need not be elaborated here. The 

hazards are clear, and, because of abundant sand and 

gravel deposits throughout the area, little rationale 

presently exists for permitting mining of the Kenai 

River banks. In addition to channel diversion and bank 

erosion, there is risk of dumping of the unmarketable 

fine-grained sediment fractions into the river.

Operators of many small fishing resorts have modified the banks to provide ramp access to the stream as
REFERENCES CUED 33

well as convenient parking. At a few sites large volumes 

of gravel have been displaced, most of which has been 

used for fill. At a few resorts developed on higher 

banks, large volumes of gravel apparently have been 

pushed into the channel and subsequently transported 

by the stream. In some cases the gravel ramps extending into the stream are periodically maintained with 

newly excavated gravel. The impacts of these commercial developments, whether they involve extending or 

cutting the natural bank, will correspond to those previously discussed for groins, boat ramps, and slips.

CONCLUSIONS

Suspended-sediment concentrations in the Kenai 

River are naturally low because of sediment retention 

in upstream lakes; levels known from other streams to 

be harmful to salmon habitat are reached only rarely. 

More frequent elevated concentrations may result 

from increase in development of the types now present 

along the navigable channel of the river. These types of 

development are listed in the preceding section in the 

order of their magnitude of impact on the sediment 

system of the stream.

Rates of bank erosion since 1950-51 show that sections of the river differ greatly in their sensitivity to 

development, as indicated in table 5. Throughout the 

central section of the river (between river miles 39.4 

and 17.6) the channel is entrenched, partly armored, 

and has undergone rates of bank erosion that are very 

low to undetectable. Upstream and downstream from 

this section the bank erosion rates are more typical of 

proglacial streams as high as 5 ft per year. Two additional sections of channel are exceptions to this pattern: the initial 3.8 river miles of channel below Skilak 

Lake are highly stable because of the presence of large 

gravel dunes emplaced by a pre-1950 flood surge; also, 

the downstream 9.0 river miles of channel are moderately stable because of the dominance of the tidal regime.

Development along the navigable channel will affect 

the sediment system of the stream in several ways. 

Construction may increase suspended-sediment 

concentration temporarily, with the greatest potential 

for harmful impact between January and May, as indicated by the relation between discharge and concentration for that period. Development can increase bank 

erosion, and thus the suspended-sediment concentration, over the longer term by causing cutoff of meander 

loops, loss of stabilizing vegetation on banks, and loss 

of the cohesive surface layer of flood-plain sediment.

Throughout this report, emphasis has been placed on 

the potential for increased suspended-sediment transport because that is the first general effect of development which is likely to be harmful to the physical 

stream system. The effect on salmon habitat occurs 

mainly through deposition of fine sediment in the pores 

of the streambed gravel in reaches used for spawning 

and rearing. There is additional concern for habitat 

conditions throughout the entrenched and partly armored section of channel. Without the cleansing action 

of flood flows competent to mobilize the coarser bed 

material of those reaches, increased transport of fine 

sediment will result in deleterious rates of deposition 

within the bed. In contrast with normal reaches, flow 

magnitudes competent to move the bed material of the 

armored reaches are greatly in excess of bankfull discharge and may not recur at the frequencies necessary 

to maintain a viable fishery if suspended-sediment 

transport increases.

Bank-erosion rates have been generally constant 

since 1950-51. The high cut banks present in entrenched and partially entrenched sections of channel 

have been mainly vegetated and stable through the 

same period. Loss of stability of the high banks is of 

special concern because of the potential for large, 

long-term contributions to the sediment load of the 

river. Ground photography in 1979 suggests that the 

high banks have locally begun to erode more rapidly, 

although verification of this possibility must await future study. A likely contributing cause of such erosion 

is increased intensity of river use and a recent change 

in sport-fishing technique.

The Kenai River salmon fishery is a major component 

of the economic base of the Kenai Peninsula. It justifies 

continued concern for changes in the sediment system 

of the stream, in response to channel and flood-plain 

development as well as trends in land use and other 

changes within the watershed. This can be best accomplished by monitoring the suspended-sediment 

concentration and the stability of the high banks.

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