Physical, Chemical, and 

Biological Aspects of the 

Duwamish River Estuary 

JKing County, Washington 

1963-67

(GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-c

{Prepared in cooperation with the 

\Municipality of Metropolitan Seattle
Physical, Chemical, and 

Biological Aspects of the 

Duwamish River Estuary 

King County, Washington 

1963-67

By J. F. SANTOS and J. D. STONER

ENVIRONMENTAL QUALITY

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-C

Prepared in cooperation with the 

Municipality of Metropolitan Seattle

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director

Library of Congress catalog-card No. 78-182395

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

Washington, D.C. 20402 - Price 40 cents (paper cover)

Stock Number 2401-1207
CONTENTS

	Page

Abstract _____________________________________ Cl

Introduction __________ ___ ___ _ _________ C2

Purpose and scope of the investigation _________________ C5

Methods and equipment _ _ ____ _ _ C6

Sampling and field measurements _________________ C6

Special equipment ___ _ _ C7

Analysis _______ __ _ ___ _____ _ ___ C7

Acknowledgments ____________________________ C7

Previous investigations _________________________ C8

Hydrography ______________________________ _ C9

The Duwamish River basin C9

Precipitation __________ _ _____ _ C13

Tides ____________________________________ C14

Volume of the estuary _________________________ C15

Estuary mechanics _______________________________ C17

Distribution of salinity _____________ _ _ _ * C19

Circulation and mixing _________________________ C23

Salt-wedge advection ____________________ C23

Dispersion _____________________ _ C25

Flushing time ____________________________ C27

Dissolved oxygen ____________________________ _ C32

Previous data ____________________________ C34

Annual variation _____________ ___________ _ C39

Correlation studies ______________ __ _ _ _ C39

Renton Junction _____ _ _ C39

East Marginal Way __ _ ____ _ C41

Sixteenth Avenue South __________ _ _ _ C41

Spokane Street ____________________________ C47

Area of critical dissolved oxygen _ C47

Phytoplankton __________________________________ C52

Biochemical oxygen demand _ __ _ ___ C54

Experiments ________ ___ _____ _ C54

Longitudinal profiles C56

Chemical character of the fresh water ___ __ _ C59

Major constituents C59

Trace elements _____ _ _ C63

Fluvial sediments ________ _ _ __ C67

Continuing and future studies __________ _ __ _ C69

Summary and conclusions ____________________ C70

References _______________________ _ _ C72

m
IV CONTENTS

ILLUSTRATIONS

Page

FIGURE 1. Map showing Metro's network of facilities for waterpollution control ______________________ C3

2. Map showing study area and location of sampling

stations ____________________________ C4

3. Map of the Duwamish River basin ____________ CIO 

4-12. Graphs showing:

4. Annual mean discharges of the Green River near

Auburn, 1937-65 _________________ Cll

5. Comparison of monthly mean discharges of Green

River for the driest and wettest years 1937-65 C12

6. Mean monthly and minimum monthly discharges

of Green River near Auburn, 1937-65 _____ C12

7. Low-flow frequency of Green River near Auburn,

1946-63 _______________________ C13

8. Average monthly precipitation at Seattle-Tacoma

International Airport, 1945-66 _________ C14

9. Average monthly precipitation during 1966 for

two stations at different altitudes ________ C15

10. Tide curve for the Duwamish River at station 6.9 

illustrating various terms pertaining to tidalfluctuations _____________________ C16

11. Comparison between tide stage predicted for station 6.9 using data from Elliott Bay and tide 

stages measured at station 1.9 _________ C17

12. Cumulative-volume curves for mean higher high

water and mean lower low water _____ CIS 

13. Schematic diagram of an idealized stratified estuary _ C20 

14-41. Graphs showing:

14. Vertical salinity profiles at selected stations for

various rates of fresh-water inflow _______ C21

15. Longitudinal salinity profiles for selected rates of

fresh-water inflow ______________ C22

16. Surface- and bottom-velocity plots at station 7.7,

June 29-30, 1965 _________________ C24

17. Longitudinal stratification profiles for selected

fresh-water-inflow rates ___ ___ _ C26

18. Flushing time of Duwamish River estuary from

station 21.0 to mouth _____________ C29

19. Generalized predicted tide stage at station 6.9,

August 29-September 1, 1966 ________ C31

20. Longitudinal configuration of dye clouds in the

Duwamish River estuary, August 29, 1966 C31

21. Longitudinal configuration of dye clouds at successive times of lower low water _______ C32

22. Plot of peak concentrations of rhodamine B found 

during successive times of lower low water, 

August 29-September 1,1966 __________ C33

23. Fresh-water inflow and DO content at station 7.7,

May-October 1949 _________________ C35
CONTENTS V

Page

FIGURES 14-41. Graphs showing Continued

24. Fresh-water inflow and DO content at Station 7.7.

May-October 1955 _________________ C38

25. Fresh-water inflow and DO content at station 7.7,

May-October 1956 _________________ C40

26. Fresh-water inflow for the period 1948-66, and 

monthly mean DO content at selected stations 

for periods of data collection _________ C43

27. Daily mean DO content and daily standard deviation at station 21.0 _______ ________ C44

28. Daily mean DO content and daily standard deviation at station 12.6 ________________ C45

29. Daily mean DO content and daily standard deviation at surface at station 7.7 __________ C46

30. Daily mean DO content and daily standard deviation at bottom at station 7.7 __________ C48

31. Daily mean DO content and daily standard deviation at surface at station 1.9 __________ C49

32. Daily mean DO content and daily standard deviation at bottom at station 1.9 ___________ C50

33. Mean daily fresh-water inflow, tidal-prism thickness, and minimum bottom DO at station 7.7, 

June-September 1966 ______________ C51

34. Relation of phytoplankton bloom-time, as indicated by chlorophyll a concentration to freshwater inflow and tidal-prism thickness, during 

1965 and 1966 ___________________ C53

35. Variation of minimum, mean, and maximum BOD 

to distance above river mouth, at a mean daily 

inflow rate of 710 cfs _______________ C56

36. Longitudinal distribution of BOD at higher high 

water or lower low water at various rates of 

fresh-water inflow, January-October 1966 ___ C57

37. The relation of DO and BOD to depth at station 

9.5 during a phytoplankton bloom, August 9, 

1966 _________________________ C58

38. Relation of chloride concentration to specific

conductance _________ _ C62

39. Increase in discharge-weigh ted average nitrate 

concentrations in the Green River near Auburn 

and in the Duwamish River at station 16.2 _ _ C63

40. Relation of molybdenum concentration to specific

conductance _____ ___ ____ C65

41. Comparison of channel cross-section areas at station 12.6 during two rates of fresh-water inflow C68
VI CONTENTS

TABLES

Page 

TABLE 1. Estuary classification ______________________ C25

2. Lengths of hypothetical estuary segments and percentages of

contained fresh water for various rates of river inflow __ C28

3. Monthly mean DO in near-surface and near-bottom samples at

selected stations on the Duwamish River estuary _________ C36

4. Chemical analyses of water at selected stations in the

Duwamish Eiver basin ____________________ C60

5. Range in concentration of trace elements in the GreenDuwamish River prior to discharge of effluent from the 

Renton Treatment Plant _____________________ C64

6. Analyses of special trace-element samples collected at selected

stations, showing theoretical versus actual concentrations __ C66
ENVIRONMENTAL QUALITY

PHYSICAL, CHEMICAL, AND

BIOLOGICAL ASPECTS OF

THE DUWAMISH RIVER ESTUARY,

KING COUNTY, WASHINGTON,

1963-67

By J. F. SANTOS and J. D. STONER

ABSTRACT

This report describes the significant results to 1967 of a comprehensive study 

that began in 1963 to evaluate what changes take place in an estuary as the 

loads .of raw and partially treated industrial and municipal wastes are replaced by effluent from a secondary treatment plant. The study area is the 

Duwamish River estuary, about 18.3 river kilometers long. At mean sea level 

the estuary has a water-surface area of about 1 square mile and a mean width 

of 440 feet. At the lowest and highest recorded tides, the volume of the estuary 

is about 205 and 592 million cubic feet, respectively. The estuary is well 

stratified (salt-wedge type) at fresh-water inflows greater than 1,000 cfs 

(cubic feet per second), but when inflow rates are less than 1,000 cfs the 

lower 5.6 kilometers of the estuary grades into the partly mixed type. The crosschannel salinity distribution is uniform for a given location and depth. Salinity 

migration is controlled by tides and fresh-water inflow. At fresh-water inflow 

rates greater than 1,000 cfs, water in the upper 8.4 kilometers of the estuary is 

always fresh regardless of tide. At inflow rates less than 600 cfs and tide 

heights greater than 10 feet, some salinity has been detected 16.1 kilometers 

above the mouth of the estuary. Studies using a fluorescent dye show that 

virtually no downward mixing into the salt wedge occurs; soluble pollutants 

introduced at the upper end of the estuary stay in the surface layer (5-15 ft 

thick). On the basis of dye studies when fresh-water inflow is less than 400 cfs, 

it is estimated that less than 10 percent of a pollutant will remain in the 

estuary a minimum of 7 days. Longitudinal dispersion coefficients for the surface layer have been determined to be on the order of 100-400 square feet 

per second.

Four water-quality stations automatically monitor DO (dissolved oxygen), 

water temperature, pH, and specific conductance; at one station solar radiation 

also is measured. DO concentration in the surface layer decreases almost

Cl
C2 ENVIRONMENTAL QUALITY

linearly in a downstream direction. Minimum DO concentration in the surface 

layer is usually greater than 4 mg/1 (milligrams per liter). The smallest DO 

values are consistently recorded in the bottom layer at the station 7.7 kilometers 

above the mouth; monthly means of less than 3 mg/1 of DO have occurred at 

this point. Manual sampling shows that the DO sag in the bottom layer oscillates between 7.7 and 10.4 kilometers above the mouth of the estuary. Multipleregression analysis shows that the surface DO content can be estimated from 

the fresh-water inflow and water temperature. Tidal exchange and fresh-water 

inflow indirectly control the bottom DO content. Information available from 

previous studies failed to indicate a progressive decrease in DO content during 

the period 1949-56, but data from the present study suggest a slight general 

decrease in the annual minimum DO concentrations in both the upper and 

lower layers. Average nitrate concentration in fresh water at station 16.2 has 

increased progressively since 1964, by amounts greater than those which can 

be attributed to the Renton Treatment Plant, 4.3 kilometers upstream from 

station 16.2.

The BOD (biochemical oxygen demand) in both surface and bottom layers is 

generally less than 4 mg/1 of oxygen, but values greater than 6 mg/1 have 

been measured during a period of phytoplankton bloom. Phytoplankton blooms 

can occur during periods of minimum tidal exchange and fresh-water inflows of 

less than 300 cfs if solar radiation and water temperature are optimum. Nutrients (nitrogen and phosphorus compounds) do not control the occurrence of a 

bloom, because sufficient quantities of these nutrients are always present. Nutrients in the treated effluent may increase the biomass of the bloom. Traceelement studies have not defined any role that these elements may play in 

algal growth.

The inflowing fresh water contains principally calcium and bicarbonate and 

has a dissolved-solids content ranging from 33 to 71 mg/1. During the study 

period, concentrations of suspended sediment ranged from 20 to 1,000 mg/1, and 

the maximum sediment load was 31,000 tons per day. Because bottom deposits 

contain large amounts of coal from upstream sources, it was not possible to 

quantitatively determine other organic material by carbon analysis.

INTRODUCTION

The Duwamish River estuary, the important industrial waterway 

in South Seattle, Wash., has been receiving wastes since the early 

1900s, but the effects of such waste disposal were not considered 

serious until the 1940's. Since then an increase in wastes resulting 

from population and industrial expansion have degraded the quality 

of the estuarine water to the extent that fisheries-resources agencies and commercial interests have become concerned about the 

aquatic life in the estuary.

In 1958 the people in the greater Seattle area voted to form the 

Municipality of Metropolitan Seattle, generally known, and herein 

referred to, as Metro. Metro is a federation of 16 cities (1967) and 

rural areas in the Seattle-Lake Washington drainage area that 

united to deal with the growing problems of waste-water disposal 

in the area. Metro's comprehensive plan for water-pollution control
DUWAMISH RIVER ESTUARY, WASHINGTON C3

includes an extensive network of sewer trunklines and several 

sewage-treatment plants (fig. 1).

Types of wastes presently (1967) entering the estuary includes 

raw and partially treated sewage, as well as wastes from manufacturing and food-processing plants (Peterson and others, 1955, 

p. 7). Since June 1965 the Renton Treatment Plant has been discharging its effluent into the Duwamish River, and the increasing 

amount of treated sewage effluent has been increasing as new sewer 

trunklines are constructed. As a result, the direct discharge of 

raw or partially treated wastes to the river is decreasing. Inflow 

to the Duwamish River of such wastes will probably be mostly 

eliminated within Metro's boundaries by 1970, when all proposed

Scheduled for completion by 1970

D TP

FIGURE 1. Metro's network of facilities for water-pollution control. After 

Municipality of Metropolitan Seattle.
C4 ENVIRONMENTAL QUALITY

sewer trunklines are complete. Then, only several minor sources 

outside Metro's jurisdiction probably will continue to use the estuary for direct waste disposal, and the principal discharge will be 

the treated sewage from the Renton Treatment Plant. In 1963 Metro 

and the U.S. Geological Survey began the present cooperative study 

of water quality in the lower Duwamish River (fig. 2).

ELLIOTT BAY 

(PUGET SOUND)

SEATTLE

"East Waterway

West Waterway

Station 1.9 

Spokane Street

Diagonal Treatment Plant 

Station 3.4

Station 4.5 

South Fidalgo Street

Station 5.6 

1st Avenue South

Station 6.9 

8th Avenue South

Station 7.7 

.venue South

Station 9.5 

Turning basin Station 10.4 

Boeing Bridge

Station 12.6 

East Marginal Way

Station 12.2 

Pacific Highway Station 13.2 

119th Street foot bridge

Station 16.2 

Cherry Street

Station 17.9 

Golf course foot bridge

FIGURE 2. Duwamish River estuary and location of sampling stations, in 

kilometers upstream from river mouth.
DUWAMISH RIVER ESTUARY, WASHINGTON C5

PURPOSE AND SCOPE OF THE INVESTIGATION

The objective of the study is to determine the chemical, physical, 

and ecological changes that take place in the estuary as raw or 

partially treated wastes are replaced by treated effluent from the 

Renton Treatment Plant. The investigation has been divided into 

several parts which, though related, lend themselves to separate 

study. The various parts of the investigation are grouped as follows:

1. Hydraulics of the estuary, including determination of the movements of the salt-water and fresh-water bodies; time of travel 

of the fresh water and residence time of the salt water in the 

estuary; and mixing and dispersion of the two waters during 

various conditions of river flow and tide stage.

2. Physical and chemical characteristics of the water, including 

temperature and density, sediment load DO (dissolved oxygen) content, BOD (biochemical oxygen demand), and reaeration rates.

3. Chemical quality of the fresh and salt waters in the estuary and 

of the inflowing fresh water upstream from major waste discharge, including determination of variations in concentrations 

of chloride, nitrate, phosphorus, alkalinity, and selected trace 

constituents.

4. Certain aspects of the ecology of the estuary, including determinations of the primary production of oxygen by phytoplankton 

(tiny free-floating plants), number and species in the plankton 

community, and variations related to physical and chemical 

conditions.

The results of these individual studies will be used to identify 

the changes in the estuary resulting from the shift in waste-disposal 

practices and hopefully also will allow prediction of environmental 

conditions in the estuary for various rates of effluent discharge 

from the Renton Treatment Plant.

Studies being conducted concurrently in the Duwamish River 

estuary by Metro personnel include: (1) Sanitary quality of the 

water; (2) inventory of migrant and resident fishes using the 

estuary; (3) enumeration and identification of benthic organisms; 

and (4) studies of the BOD of bottom deposits.

This report summarizes significant results of the study to date 

(1967) and discusses work planned for the future to accomplish 

the long-term objective of the study in the estuary. The report also 

briefly describes water-quality conditions upstream, beyond the 

immediate study area, which relate directly to conditions in the 

estuary. The preliminary results described in this report pertain
C6 ENVIRONMENTAL QUALITY

to (1) the estuary mechanics, or how the estuarine system functions, (2) distribution of DO and BOD in the estuary, (3) factors 

that influence phytoplankton bloom, and (4) chemical and physical 

characteristics of the fresh and salty waters.

Six reports on various phases of this study have been published 

separately: two are on dispersion in the estuary and inflowing fresh 

water (Williams, 1967; Fischer, 1968), three describe phytoplankton and related water-quality conditions in the estuary (Welch, 

1968 and 1969; Welch and Isaac, 1967), and the other deals with 

prediction of the intrusion of salt water into the estuary (Stoner, 

1967).

METHODS AND EQUIPMENT

The ecological and hydraulic environment in the Duwamish 

River Estuary is especially difficult to study because of the variable 

character of the estuary. Seasonally the estuary varies from a wellstratified salt-wedge type to a partially stratified, partially mixed 

type. (See section on "Salt-Wedge Advection" for description of 

estuary types.) This dynamic environment makes duplication of 

experiments virtually impossible. Experiments made on samples 

taken from the estuary produce results that represent one set of 

transitory conditions and are not necessarily representative of specific or "average" conditions; experiments that include living 

organisms are especially nonrepresentative. The changes in inflow 

rate and in water-quality input to the system (waste and sediment 

load) also add to the difficulty of defining specific causative factors 

for observed results.

Most of the methods and equipment used to date (1967) in this 

investigation are well known in the fields of hydrologic and waterquality studies and are not described in detail in this report. Some 

of the methods and equipment, however, were developed during 

this investigation. Insofar as the innovations are significant to the 

results of this investigation and may be applicable to similar studies 

elsewhere, they are described in detail either in the present report 

or in other reports resulting from this investigation.

SAMPLING AND FIELD MEASUREMENTS

Water sampling was done from a boat or a bridge. All samples, 

except those taken for dye studies, were collected in a 4-liter sampler made of polyvinyl chloride. The sampler is equipped with a 

flexible rubber hose and clamp for dividing the 4 liters into containers for specific determinations of phytoplankton, specific conductance and major chemical constituents, trace elements, DO, and 

BOD. Samples for DO and BOD always were drawn off first. Sam-
DUWAMISH RIVER ESTUARY, WASHINGTON C7

pies for dye studies were taken separately in 16-ounce glass bottles. 

Throughout the report reference will be made to surface or bottom 

in discussing the elements of the study. "Surface" samples were 

collected about 3 feet below the water surface, and "bottom" samples 3 feet above the bottom.

Suspended-sediment samples were collected by using a depthintegrating sampler (DH-49) from a fixed facility on the bridge 

at station 21.00. Samples for bedload were taken using a bedmaterial sample (BM-54). Field measurements of specific conductance, salinity, and water temperature were obtained by using an 

electrodeless induction-type salinometer.

SPECIAL EQUIPMENT

Automatic water-quality monitors installed at stations 1.9, 7.7, 

12.6, and 21.0 (fig. 2) sample water from 3 feet below the surface 

and measure specific conductance. DO, temperature, and pH. In 

addition, at stations 1.9 and 7.7, water is sampled automatically 

from 3 feet above the bottom and is analyzed for the same parameters except pH. At station 12.6 solar radiation also is measured. 

Positive-displacement pumps are used to bring the river water to 

the monitor. All plumbing is made of polyvinyl chloride.

ANALYSIS

Chemical constituents were determined using procedures given 

in Rainwater and Thatcher (1960). Trace elements were determined 

spectrographically, employing the procedures of Silvey (1967). 

Analysis of chlorophyll was made by the method of Richards and 

Thompson (1952) as modified by Creitz and Richards (1955). DO 

was determined by using the azide modification of the Winkler 

method (American Public Health Association, 1965).

Both suspended-sediment and bed-material samples were analyzed using the procedures of the U.S. Geological Survey.

ACKNOWLEDGMENTS

Chemical-quality data for the Green River were collected in 

cooperation with the Department of Water Resources and the Water 

Pollution Control Commission of the State of Washington. The 

Seattle Department of Engineering provided the results of their 

1948-49 Duwamish River estuary sanitary survey.

The authors are grateful for the helpful suggestions provided 

by C. V. Gibbs, Executive Director of Metro, and G. D. Farris, Superintendent, Water Quality and Industrial Wastes of Metro; Prof. 

E. B. Welch, University of Washington; and R. W. Paulsen, Hydrologist, U.S. Geological Survey.
C8 ENVIRONMENTAL QUALITY 

PREVIOUS INVESTIGATIONS

Formal water-quality investigations of the Green-Duwamish 

River began in 1948, and during the period 1948-63 several investigations were made. The first formal study was a bacteriological and 

DO survey made during June-September 1949 by the Washington 

State Pollution Control Commission (Sylvester and others, 1949).

An investigation also was made by the Engineering Department 

of the City of Seattle (written commun., 1967) from November 

1948 to April 1950. Samples were taken for measurements of DO, 

BOD, salinity, water temperature, and pH at seven stations in the 

downstream section of the estuary. The samples were collected at 

the top and bottom at approximately twice-weekly intervals. The 

resulting report contains data only, and no interpretations or conclusions were made. The data indicate, however, that there was no 

serious oxygen depletion in the lower Duwamish River estuary 

under the rate of fresh-water inflow recorded.

The Washington State Pollution Control Commission conducted 

another sanitary-quality survey of the Green-Duwamish River 

during the summer of 1955. The commission reported on bacteriological character, DO, BOD, and toxic compounds (Peterson and 

others, 1955). The report states that the amount of dissolved oxygen 

in the river was, in general, satisfactory; however, it also states 

that the samples were collected during a year of high fresh-water 

inflow and of low water temperature and that low amounts of dissolved oxygen certainly could exist during low-water years in the 

estuary.

The present and future effect of pollution in the Green-Duwamish 

River was the subject of a University of Washington post-graduate 

thesis by R. W. Okey (written commun., 1957). Okey's evaluation 

was based on temperature, DO, BOD, and chloride data collected 

during the summer and fall of 1956. He concluded that unless increases in streamflow can be obtained, no additional discharges of 

oxygen-demanding wastes of any magnitude can be tolerated at 

the present level of DO. He also stated that the construction of 

Howard A. Hanson Dam should have no material effect upon DO 

levels in the lower estuary.

In 1956 the City of Seattle authorized a survey of sewerage and 

drainage problems and methods of solving the problem of water 

pollution. The survey (Brown and Caldwell Engineers, 1958) reported on types, amounts, and composition of wastes being discharged to the Duwamish River, the effect of various waste loads 

on DO, estimates of flushing time, and needed waste-treatment 

facilities. The report resulting from the survey was the basis for 

Metro's comprehensive sewerage plan.
DUWAMISH RIVER ESTUARY, WASHINGTON C9

The most recent report (Isaac and others, 1964) was published 

by Metro; it comprehensively covers water-quality conditions during 

the low-flow period of 1963 and provides some comparisons with 

the 1962 low-flow period.

HYDROGRAPHY 

THE DUWAMISH RIVER BASIN

The Green-Duwamish River study area is 11.9 miles (19.1 kilometers) long. At mean sea level the water-surface area is about 1 

square mile and has a mean width of 440 feet. All river distances 

used in this report are reckoned from the mouth, kilometer 0, which 

was arbitrarily established at a line drawn across the mouth of the 

West Waterway (fig. 2).

Prior to 1906 the drainage basin of the Duwamish River exceeded 

1,600 square miles and included the Cedar, Green, Black, and White 

River basins. In 1906 the White River was permanently diverted to 

the Puyallup River to the south. In 1916 the Black River (fig. 2), 

which received the flow from the Cedar River (fig. 1) and Lake 

Washington, had its flow reduced when the level of Lake Washington (fig. 1) was lowered 9 feet after completion of a ship canal to 

Puget Sound and it ceased to be a major tributary to the Duwamish 

River. These changes reduced the Duwamish River to its present 

drainage area of 483 square miles (fig. 3).

The Green River drains more than 90 percent of the Duwamish 

River basin (Richardson, 1962). Fresh-water discharge from the 

remainder of the basin is negligible in comparison.

The Green River heads on the west slope of the Cascade Range 

and flows north and east about 60 miles (96.5 km) to its confluence 

with the Black River below which it becomes known as the Duwamish River (fig. 2). The upper reaches of the Green River and its 

tributaries flow in steep-sided and heavily forested mountain valleys.

Howard A. Hanson Dam (fig. 3), primarily a flood-control structure, was completed in 1962. This dam has altered the natural 

stream regimen by tending to reduce peak discharges and increase 

summer flows. Below the dam, the Green River flows through a 15- 

mile (24.1 km) gorge where many springs contribute to the discharge. The only sizable tributaries downstream from the gorge are 

Newaukum and Big Soos Creeks. Downstream from Auburn, inflow 

is negligible except during periods of heavy local rains.

The City of Tacoma diverts 113 cfs (cubic feet per second) daily 

from the Green River for municipal and industrial use. The diversion point is 21/2 miles (4.0 km) southeast of Palmer.
CIO ENVIRONMENTAL QUALITY

** ^V.

B

a

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t

00

1

fa
DUWAMISH RIVER ESTUARY, WASHINGTON Cll

Streamflow data for the Duwamish-lower Green River are available from two gaging stations, one near Auburn and one at Tukwila 

(fig. 3). Streamflow records for these stations have been published 

by the U.S. Geological Survey (1937-60; 1961-66).

For the gaging station near Auburn, a long-term (1937-66) discharge record is available. This long-term record at Auburn has 

been correlated with a short-term record, 1960-66, for the gaging 

station at Renton Junction1 (sta. 21.0) about 20 miles downstream, 

but the correlation is not applicable for a rising river stage. Thus, 

the record at Renton Junction cannot be extended back in time on 

a day-to-day basis, although the correlation has been found to be 

valid for monthly and yearly means. In addition, the river level at 

the gaging station at Renton Junction (sta. 21.0) is influenced by 

the tide at low discharges, and records collected at this station are 

considered fair that is, accurate to within 8 percent.

1800 p=

Z 1600 

O S 1400

1200

1000

< C 800

g 400

200

=1 3000

2500 -

2000

1500 co 

z tr

1000

500

z m 

5o

WATER YEAR

FIGURE 4. Annual mean discharges of the Green River near Auburn, 1937 

65. Heavy dashed line is mean of 1,349 cfs.

Figure 4 shows the annual discharges of the Green River near 

Auburn. Maximum annual discharge occurred in 1959 and was 132 

percent of the 29-year mean; the minimum, in 1941, was 49 percent 

of the 29-year mean.

Figure 5 shows the monthly mean discharges for 1941 and 1959, 

respectively the driest and wettest years during the period 1937-65.

1 The gaging station is listed as "12-1133.5 Green River at Tukwila Wash." in the streamflow 

records published by the U.S. Geological Survey (1937-60).
C12 ENVIRONMENTAL QUALITY

LJ O

O Z

2?°

< o

X LJ 

O V)

Eft

z a

< iLJ LJ 

5^

X ffi

13

4000

3200

2400

1600

800

NDJ FMAMJ J AS

FIGURE 5. Comparison of monthly mean discharge of the 

Green River near Auburn for the driest year (1941, shaded 

bars) and wettest year (1959, shaded plus unshaded bars) 

during the period 1937-65.

Normally, rains cause high runoff during the late fall and winter 

months, and snowmelt maintains this high-runoff period into June. 

During July the runoff decreases rapidly, and the minimum occurs 

at most places in August. The normal seasonal pattern of flow is 

shown by the long bars in figure 6. This figure also shows the minimum monthly discharges for the period 1937-65.

2500

2000

1500

1000

500

A S

FIGURE 6. Mean monthly (unshaded plus shaded bars) 

and minimum monthly (shaded bars) discharges of 

Green River near Auburn, 1937-65.
DUWAMISH RIVER ESTUARY, WASHINGTON C13

Low fresh-water flows are of particular importance to waterquality conditions in the lower estuary. Welch (1969, p. 53) has 

demonstrated that when fresh-water inflow decreases to about 300 

cfs a phytoplankton bloom can occur if the other related factors 

(principally sunlight, tidal prism thickness, and water temperature) 

are favorable. This bloom usually is followed by a decrease of DO 

in bottom water (Welch, 1969, fig. 16) to a value less than many 

authorities (McKee and Wolf, 1963) consider adequate for the 

maintenance of fish life. Figure 7 shows the low-flow frequency 

for the Green River at Auburn; however, because the frequency 

distribution is based on natural flows prior to flow regulation by 

Howard A. Hanson Dam, caution should be used in its application.

Howard A. Hanson Dam, with an active storage capacity of 

105,160 acre-feet, is primarily a flood-control structure. Water is 

stored in the dam during periods of floodflow but is released as soon 

as practicable afterwards to prepare for any subsequent flood. Usuall in late May the dam starts retaining water to augment low 

summer flows. The low-flow augmentation is designed to maintain 

100 cfs in the lower reaches of the Green River.

PRECIPITATION

The rate and character of the fresh-water inflow to the estuary 

are determined largely .by the amount, distribution, and type of 

precipitation in the Duwamish River basin. The annual precipitad 2000

z 

o 

o

$ $ 1000

o

(T -J Eju-

"-u. 500 (-0

UJ

Eg 

^ 200

Z Q 100

- uj

UJ(-

£2 50 

$Q

20

10

r \

183-Day 

90-Day _

2000

1000

«Z

500

200 £ u

S3 

100 < I

50 5 £

Z IU"

20

1.01 1.1 1.5 2 345 10 20 30 50 

RECURRENCE INTERVAL, IN YEARS

FIGURE 7. Low-flow frequency of Green River near Auburn, 1946-63, 

prior to regulation by Howard A. Hanson Dam.
C14 ENVIRONMENTAL QUALITY

tion varies widely throughout the basin, depending mostly on altitude and proximity to the crest of the Cascade Range. SeattleTacoma International Airport (fig. 3) receives an average of 38.9 

inches per year, whereas the upper slopes in the eastern part of the 

basin receive more than 100 inches per year. Data from the Environmental Science Services Administration (U.S. Weather Bureau, 

1967) show that most of the precipitation occurs during the cooler 

months of October through March; in the lowlands it is in the form 

of rain, but at the higher altitudes it falls mostly as snow. Snow 

falls occasionally on the lowlands but does not remain long.

Figure 8 shows the average monthly precipitation recorded at 

the Seattle-Tacoma International Airport for the period 1945-66, 

and figure 9 compares the 1966 precipitation at the airport station 

with that at Palmer (fig. 3), which is about 520 feet higher than 

the airport station and about 20 miles closer to the crest of the 

Cascades.

F M A M O N D

FIGURE 8. Average monthly precipitation at SeattleTacoma International Airport, 1945-66. Data from Environmental Science Services Administration.

TIDES

The U.S. Coast and Geodetic Survey (1963-66) has a tide-recording station in Elliott Bay, data from which have been projected to 

station 6.9 in the Duwamish River. Stages in the estuary have been 

measured only when very precise tide data were needed for special 

studies. The values from station 6.9 for various tidal stages are 

shown in figure 10. Throughout this investigation the term "tide 

stage" is referenced to mean lower low water (MLLW), which is
DUWAMISH RIVER ESTUARY, WASHINGTON C15

6.6 feet below mean sea level. Mean range of tide in the Duwamish 

River is 7.5 feet, and mean diurnal range is 11.1 feet. Recorded tides 

have ranged from minus 4.6 to plus 14.7 feet.

During the investigation, tide stage has been measured at several 

stations over a tidal cycle (25 hrs), and close agreement between 

the measured and predicted stages has been found. Figure 11 compares the tide stage measured at station 1.9 with the stage predicted for station 6.9. For all practical purposes the stage predicted 

for station 6.9 is applicable to the reach between stations 1.9 and 7.7.

15

z 

o

I .0

o

UJ

tr V)

Q. UJ

il

£zO ~

 36

32

28

20 >

16 0

8 <

M J S O N D

a 0

FIGURE 9. Average monthly precipitation during 1966, 

for two stations at different altitudes. Shaded bars represent precipitation at Seattle-Tacoma International 

Airport, altitude 375 feet; shaded plus unshaded bars 

are precipitation at Palmer, altitude 895 feet. Data 

from Environmental Science Services Administration.

VOLUME OF THE ESTUARY

The areas of 21 cross sections were determined for computing 

estuary volume at various tide stages. In the lower 11.3 river km 

of the estuary, the fathometer soundings on charts of the U.S. Army 

Corps of Engineers (1965) were used to determine the channel 

depths of the cross sections and were supplemented by hand soundings where necessary. To determine volumes in the upper 9.7 km, 

hand soundings were made across cross sectional areas. The 21
C16 ENVIRONMENTAL QUALITY

133J Nl '33V1S 3QI1

SU313W Nl '39V1S 3QI1
DUWAMISH RIVER ESTUARY, WASHINGTON C17

cross sections were chosen so that the volume interval between them 

was as uniform as possible. The geometry of each particular channel 

segment provided the basis for determining whether the volume 

was computed as a rectangular parallelepiped, half a cylinder, or 

half a frustum of a pyramid. Figure 12 shows the relation of 

cumulative volume to distance, for mean higher high water and 

mean lower low water. Volumes were calculated for nine tide levels 

from the lowest recorded tide to the highest recorded tide, 205 and 

592 million cubic feet, respectively. The graph shows that the 

logarithm of cumulative volume is almost a linear function of distance. These data have been used to test some of the flushing-rate 

theories reported in the literature. (See section on "Flushing Time.")

12

10

±* 6

w z

1 1 1 1 1

r\ = ^ v =

 TT^ / \ / / 

\ ; \ /

V

\ I

\ "' \ y ^ * \ /

i i i ^X i I

3 rot-' oj 

STAGE, ABOV TIDEINMETERS 

MEAN LOWER WAT LOW 

0600 1200 1800 

AUGUST 22, 1966

2400 0600 1200 

AUGUST 23, 1966

1800

FIGURE 11. Comparison between tide stage predicted for station 6.9, using 

data from Elliott Bay (dashed line) and tide stages measured at station 1.9 (dots).

ESTUARY MECHANICS

The effect of the Renton Treatment Plant effluent upon the estuary is dependent primarily upon two things distribution and residence time of the effluent. The following discussions of salinity 

distribution, mixing and circulation, flushing, and dye studies are 

presented to provide a better understanding of the processes involved in the distribution of the treatment plant effluent throughout 

the estuary.
CUMULATIVE VOLUME, IN MILLIONS OF 

CUMULATIVE VOLUME, IN MILLIONS OF CU
DUWAMISH RIVER ESTUARY, WASHINGTON C19 

DISTRIBUTION OF SALINITY

The distribution of salinity throughout the estuary varies vertically, longitudinally, and laterally with time. The vertical and longitudinal variation of the salinity distribution at a given station is 

dependent upon the combined effects of the fresh-water inflow and 

the tidal conditions. The maximum and the minimum salinity concentrations are governed primarily by the fresh-water inflow, which 

varies seasonally. Highest inflow rates generally occur between 

October and April, and the lowest occur during the period July to 

September. Salinity concentrations are greatest throughout the 

estuary during the period of minimum fresh-water inflow.

To define the lateral salinity variation throughout the estuary 

the distribution of salinity was measured through several different 

cross sections at two different rates of fresh-water inflow. At a 

fresh-water inflow of approximately 1,800 cf s, the salinity at a given 

depth for a specific station was uniform from one side of the channel 

to the other. At a fresh-water flow of 300 cfs, the salinity in the 

estuary was laterally uniform also, except where the effluent from 

the Diagonal Treatment Plant (fig. 2) discharges into the estuary. 

There, because the effluent is of a lower salinity than the estuary 

water into which it discharges, it produces a lower salinity pattern 

at depth along the east bank for a short distance downstream, 

beyond which the pattern returns to lateral uniformity. The estuary 

is not wide enough for the lateral-salinity distribution to be significantly affected by the Coriolis force (effect of the earth's rotation) . In general, for a given depth and upstream distance, salinity 

was laterally constant at all rates of fresh-water inflow which 

occurred during the study.

The vertical salinity distribution, however, is strikingly nonuniform. Vertical salinity profiles taken at various stages of tide 

and rates of fresh-water inflow show the presence of a saline wedge 

typical of a stratified estuary (fig. 13). The wedge is distinct in 

the profiles upstream from river km 5.6 at all measured rates of 

fresh-water inflow. In the lower estuary, downstream from station 

5.6, at flows less than 2,000 cfs, the boundary between the surface 

layer and the saline wedge becomes less distinct. Salinity stratification becomes well developed at fresh-water inflows greater than 

1,500 cfs (fig. 14). As the fresh-water inflow decreases, vertical 

stratification becomes less sharply defined; at fresh-water flows of 

less than 1,000 cfs, stratification is no longer clearly defined in 

the lower section of the estuary (fig. 14). Even at the lower flows, 

however, the vertical salinity gradient of the estuary is still strong, 

and the ratio of top-to-bottom salinity rarely exceeds 0.6. For any 

given river flow, vertical stratification increases in an upstream
C20 ENVIRONMENTAL QUALITY

Net water velocity is downstream and 

surface layer becomes more saline

Salt water being mixed into surface layer

Fresh-water 

inflow

:x:x::: Salt-water wedge j::X:Xv::X 

Net water velocity is upstream 

X-:and salinity is constant Xf.;X;

Increasing salinity

Vertical salinity profile 

at section B-B '

Increasing salinity

Vertical salinity profile 

at section A-A'

FIGURE 13. Schematic diagram of an idealized stratified estuary showing net 

circulation pattern and salinity profiles.

direction to the end of the wedge. The depth of the surface layer, 

when stratification is evident, varies directly with the fresh-water 

inflow; this layer ranges from 5 to 15 feet in thickness.

Longitudinal distribution of salinity is dependent upon the tide 

conditions and the rate of fresh-water inflow. Salinity in the surface 

layer increases in a downstream direction, whereas salinity in the 

bottom layer is relatively constant except near the upstream end of 

the salt-water wedge. The salinity of the surface layer is controlled 

primarily by the rate of fresh-water inflow (fig. 15). The surface 

salinity at station 1.9 (Spokane Street Bridge) varies from less than 

4 ppt (parts per thousand), at fresh-water inflows of 5,900 cfs or 

greater, to more than 15 ppt at inflows of 500 cfs or less.

The salinity in the bottom layer, although relatively constant, 

does decrease slightly in an upstream direction, probably owing 

mostly to slow diffusion. The upstream end of the wedge terminates
DUWAMISH RIVER ESTUARY, WASHINGTON C21

10

25

- i

Station 7.7

SALINITY, IN PARTS PER THOUSAND

- 7

35

FIGURE 14. Vertical salinity profiles at selected stations for various 

rates of fresh-water inflow.

over a relatively short distance. The average salinity throughout 

the main body of the salt wedge decreases very slightly with increase 

in the fresh-water inflow. This decrease in salinity is due to the 

slightly larger proportion of fresh water in the mixture that enters 

the wedge at its downstream end (fig. 13).

The upstream extent of the wedge is dependent upon fresh-water 

inflow and tide height, except that inflows greater than 1,000 cfs 

will prevent intrusion of the wedge farther upstream than station
C22 ENVIRONMENTAL QUALITY

DISTANCE ABOVE MOUTH, IN KILOMETERS 

1 2 34 567

30

25

Bottom

1 -L. 1

I^H3^~~^-~-~

1

 - -^ "^ 

1R 15

 5900 cfs

 1700 cfs

 500 cfs

(£.

CL 10

\

\

\

Surface

X

012345 

DISTANCE ABOVE MOUTH, IN MILES

FIGURE 15. Longitudinal salinity profiles for selected rates of 

fresh-water inflow.

12.6 (East Marginal Way), regardless of tide height (Stoner, 1967). 

At low fresh-water inflow (generally less than 600 cfs) and at tide 

heights greater than 10 feet above mean lower low water, the saltwater wedge has extended upstream approximately 16 km above 

the mouth. Conversely, the wedge has been pushed downstream as 

far as 6.4 km above the mouth at low tide and high rates of inflow.

The salinity distribution in the estuary can be described as a 

two-dimensional system. Any point of salinity data can be described 

in terms of its longitudinal and vertical position only, inasmuch as 

salinity differences in the lateral (cross-channel) direction at each 

point are negligible.

Three cyclic tidal periods in the Duwamish estuary are of importance to the salinity distribution; they are the semidiurnal, diurnal, 

and biweekly periods. (See fig. 10.) Of these three, the semidiurnal
DUWAMISH RIVER ESTUARY, WASHINGTON C23

period (from low to high and high to low tide) has the most pronounced effect upon the salinity distribution. The diurnal period is 

significant because of the inequality of successive high or low tides. 

The biweekly period is significant because a major range of the 

tides occurs during this period.

At station 21.0 (Renton Junction), the sampling point farthest 

upstream, the water is always fresh. The salinity at station 12.6 

(East Marginal Way) is highly dependent on rates of fresh-water 

inflow; it is negligible at inflows greater than 1,000 cfs.

At station 7.7 (16th Avenue South), the variation of salinity 

during the tidal cycle becomes less pronounced than the salinity 

variation at station 12.6, and the average salinity increases with 

decreasing fresh-water inflow. For example, at a fresh-water inflow 

of 3,800 cfs the salinity at the bottom ranged from 27.8 ppt to 0.3 

ppt during a tidal cycle, whereas at an inflow of 297 cfs the salinity 

ranged from 27.8 ppt to 15.4 ppt. The estuary at this station has 

fresh water at all points in the vertical profile only with the combination of very low tide and high rates of fresh-water inflow.

At station 1.9, the farthest downstream primary sampling station, 

the estuary is always saline regardless of tide and the rate of freshwater inflow. The salinity variation during the tidal cycle at this 

station follows the same pattern as that at station 7.7, although 

the variation is smaller even at the higher rates of fresh-water 

inflow.

CIRCULATION AND MIXING

SALT-WEDGE ADVECTION

Vertical salinity profiles indicate that very little of the fresh 

water in the upper layer of the estuary mixes downward into the 

saline-water wedge. Most of the mixing between the saline and 

fresh water occurs at the interface of the two layers, where saline 

water is entrained upward into the overriding fresh-water layer 

(fig. 13). This pattern of circulation in estuaries has been defined 

by Pritchard (1955) as that of the classical salt-wedge estuary. 

Confirmation of the absence of downward mixing was indicated 

by the results of a fluorescent dye study in August 1966. (See 

section on "Flushing Time.")

In this type of estuary, the net movement of water in the saline 

wedge is upstream. The upstream movement of the saline water is 

theoretically necessary to replace the water that has been entrained 

upward into the fresh-water layer along the interface.

The theoretical circulation pattern for the estuary was confirmed by a velocity study made in June 1965 during one complete 

tidal cycle at a fresh-water flow of 425 cfs. The surface and the
C24 ENVIRONMENTAL QUALITY

bottom velocities obtained from this study are plotted in figure 16. 

The net area under the two curves was computed in order to determine the net velocity of each layer. The net velocity of the bottom 

layer was 0.09 fps (feet per second) upstream, and the surfacelayer velocity was 0.13 fps downstream.

2.00

0.75

 0.50

, 0.25

Net surface velocity, 0.13 fps downstream 

Net bottom velocity, 0.09 fps upstream _

-1.00

- -0.25

-0.50

-2.00

0930 1230 1530 1830 2130 0030 0330 0630 0930 1230 

JUNE 29 JUNE 30

FIGURE 16. Surface- and bottom-velocity plots at station 7.7 (16th Avenue 

South), June 29-30, 1965. Solid line, surface velocity; dashed line, bottom 

velocity.

The net circulation of the estuary upstream in the bottom layer 

and downstream in the upper layer and the mixing from the bottom layer to the upper layer places the upstream part of the Duwamish River estuary in the category of Pritchard's salt-wedge estuary 

even at low rates of fresh-water inflow. Vertical salinity profiles in 

the lower estuary, however, are indicative of the partially mixed 

type of estuary at low rates of fresh-water inflow.

A two-parameter system of estuary classification which ma;y be 

useful for water-management purposes has been developed by Hansen and Rattray (1966) and is shown in table 1. This system, which 

categorized estuaries into four general types, is based upon a "stratification number" as well as a "circulation number." The stratification number is given as 8S/S0 and a circulation number Ug /Uf 

where

8S=top-to-bottom salinity difference, in parts per thousand.

<S0=mean cross-section salinity, in parts per thousand.

f/g=surface velocity, in feet per second.

Uf=fYesh water discharge divided by the cross-section area, in 

cubic feet per second per square foot.
DUWAMISH RIVER ESTUARY, WASHINGTON C25 

TABLE 1. Estuary classification

[Modified from Hansen and Rattray, (1966)]

Sub 

Type Circulation type Stratification

1 ___Well-mixed estuary, net flow seaward at all A ___Slight, 

times; upstream salt transfer by 

diffusion.

Do __________________ B ___Appreciable. 

2____Net flow reverse at depth; upstream salt A ___Slight, 

transfer by diffusion and advection.

Do ___________________ B ___Appreciable.

3 ____ Similar to type 2 except advection accounts A ___Slight, 

for 99 percent of upstream salt transfer.

Do __________________ B___Appreciable.

4 _____Salt wedge, with appreciable stratification.

The values used for these four parameters are the mean values 

over the total tidal cycle.

Data from the June 1965 velocity study and from various vertical 

salinity profiles lead to the following conclusions in terms of the 

Hansen and Rattray classification system (table 1):

1. The part of the estuary upstream from station 5.6 for any measured fresh-water inflow is in the type 4 category (salt wedge).

2. At fresh-water inflows greater than 1,000 cfs, the entire estuary 

would be type 4; at lesser inflows the part downstream from 

station 5.6 grades into type 2B. According to the classification 

system, the type 2B estuary, although appreciably stratified, 

has diffusion, as well as advection, as a mode of salt transfer 

and mixing.

Longitudinal stratification profiles of the Duwamish River estuary are shown in figure 17. Three rates of fresh-water inflow are 

plotted against the stratification numbers of Hansen and Rattray 

(1966, p. 322).

DISPERSION

The use of dyes to study dispersion and water movement has 

been reported by many workers. Most of the studies used fluorescein, 

pontacyl pink, or one of the rhodamine compounds. Rhodamine B 

was used in the work reported in this section. The details on instrumentation, standardization, and dye have been reported by Fischer 

(1968), who also provided an excellent discussion of the physical 

and hydraulic factors that produce dispersion.
C26 ENVIRONMENTAL QUALITY

DISTANCE UPSTREAM FROM MOUTH, IN KILOMETERS 

1.6

234 567

1.5

^ 1.4

0| °

in 1.2

D ! 1 z 1.1

g 1.0

iO 0.9

^ 0.8

W 0.7

0.6

0.5

EXPLANATION

- 510 cfs

 - 1070 cfs

_ 1730 cfs

1 234 

DISTANCE UPSTREAM FROM MOUTH, IN MILES

FIGURE 17. Longitudinal stratification profiles for selected fresh-water

inflow rates.

The use of a dye to study time of travel and dispersion in the 

Duwamish River estuary posed three problems. First, the geometry 

of the estuary is such that a very large amount of dye must be 

inserted into the relatively small-volume upstream section so that 

the dye can be detected in the large-volume lower part of the estuary. 

Second, during low flows in the summer, the river contains adult 

salmon migrating upstream to spawn and juvenile salmon migrating 

downstream to Puget Sound. A search of the literature provided 

no indication as to what concentration of rhodamine B might be 

toxic to salmon; therefore, the concentrations of dye that were used 

in the early studies were too weak, and the result was that those 

studies failed. Third, background fluorescence in the Duwamish 

estuary varies with water temperature, salinity, and quantity of 

phytoplankton. In general, the background fluorescence amounted 

to 0.1-0.2 /i.g/1 (microgram per liter) of apparent rhodamine B. 

In most of the studies, therefore, 0.1 /xg/1 was considered to be the 

minimum detectable amount of dye, and concentrations below that 

were considered background fluorescence.

The rate of dispersion in a stream is described quantitatively 

by means of a dispersion coefficient. In an early dye study in this 

investigation. Williams (1967) calculated dispersion coefficients of 

100 and 200 to 400 square feet per second for stations 12.6 and 7.7,
DUWAMISH RIVER ESTUARY, WASHINGTON C27

respectively. River flow during the study was about 700 cf s. Williams 

used two different methods for calculating the coefficient at station 

12.6. The first method used the statistical Pearson type-Ill distribution (Godfrey and Fredrick, 1963), which gave a coefficient of 

113 square feet per second. The second method used the formula 

of Taylor (1954), which gave a coefficient of 104 square feet per 

second. Williams rounded the coefficients to 100 square feet per 

second.

In reporting on the more intensive studies made in the summer 

of 1965, Fischer (1968) developed a method for predicting longitudinal dispersion from lateral dye distribution and velocity profiles at a single station. Dispersion coefficients for two experiments 

gave measured values of 70 and 90 square feet per second, whereas 

dispersion coefficients predicted by Fischer's method gave values of 

84 and 91 square feet per second. The dispersion coefficients reported by Fischer were computed from data collected on an outgoing tide, when the estuary behaves essentially like a fresh-water 

river, and the coefficients apply only to the upper 9.7 km of the 

study area.

FLUSHING TIME

Flushing time is the average length of time required for the river 

water and its contained pollutants to move through the estuary. 

Because it determines the residence time of oxygen-depleting pollutants, the flushing time is one of the more important factors 

(along with the quality and rate of inflowing fresh water, tidal 

action, and concentration of the pollutants) controlling the DO 

levels in the estuary. The method proposed by Ketchum (1951b) 

was used to compute the flushing time of the estuary.

Ketchum's method requires a division of the estuary into segments whose lengths correspond to "the average excursion of a 

particle of water on the flooding tide" (Ketchum, 1951b, p. 200); 

the method includes the assumption that the intertidal volume of 

water (and any contained pollutants) in each segment moves downstream to the next segment during each successive tidal cycle. The 

method also assumes vertical homogeneity. The Duwamish River 

estuary, however, is not homogeneous vertically, and some of the 

data had to be modified to fit Ketchum's method. Because the saline 

wedge does not enter into the mixing process except through entrainment at the interface, a theoretical "bottom" was used in 

the flushing-time calculations. The theoretical bottom was placed 

at that depth of the water below which salinity is virtually constant. 

The cumulative-volume curve used in the computation varied because of the differences in depth to the theoretical bottom with
C28 ENVIRONMENTAL QUALITY

different rates of streamflow. Volumes of estuary segments were 

computed at mean high and mean low water. The volume of the 

estuary used in the computation ranged from 242.1x10° cu ft 

(cubic feet) at mean high water for a discharge of 500 cfs to 

323.0x10° cu ft at mean high water for 7,000 cfs.

The total volume escaping from each segment can be calculated 

by dividing the river flow per tidal cycle by the fraction of fresh 

water in that segment of the estuary (Ketchum, 1951b, p. 206). 

The percentages of fresh water for three segments of the estuary, 

for virtually the entire range of fresh-water inflow, are shown in 

table 2. Fresh-water percentages calculated by Ketchum's method 

are shown along with actual percentages from field salinity measurements. The lengths of the hypothetical segments, which are 

based on tidal-excursion distances, vary with the rate of freshwater inflow.

As shown in table 2, at inflows greater than 1,000 cfs the calculations by Ketchum's method were in good to fair agreement with 

the actual data. At lesser inflow rates, however, some of the calculated percentages disagreed substantially from the actual percentages, especially in segment 2, which is farthest downstream. 

This departure between calculated and actual percentages in the 

lower part of the estuary also was described by Ketchum (1951b, 

p. 206) for Raritan Bay in New Jersey.

TABLE 2. Lengths of hypothetical estuary segments and percentages of 

contained fresh water for various rates of river inflow

[Method of Ketchum, (1951a)]

Segment 0

River 

discharge 

(cfs)

510. .....

540. ......

745......

570.......

750.......

800......

800.......

8.90.......

900......

930......

950......

1,070.......

1,150.......

i ocft

1,900......

3,000.......

3,000.......

0 Qftft

5,900.......

Fresh water 

(percent)

Calculated

100

100

100

100

100

100

100

100

100

100

100

100

100 

100

100

100

100

100

100

Actual

98 

95+ 

99+ 

95+ 

95+ 

100 

100 

100 

99+ 

100 

99+ 

98+ 

98+ 

97+ 

100 

97+ 

94+ 

97+ 

97 

94

Segment 1

Fresh water 

(percent)

(ft) Calculated Actual

28,800 

29,600 

34,600 

34,500 

37,700 

35,400 

35,400 

36,400 

36,600 

37,000 

37,000 

38,600 

39,700 

41,000 

48,200 

49,400 

58,600 

58,600 

63,700 

71.600

44 

45 

50 

51 

42 

52 

52 

55 

52 

52 

54 

58 

60 

59 

67 

77

63 

47 

53 

49 

45 

61 

68 

61 

72 

80 

59 

59 

75 

69 

69 

76

(ft)

18,000 

19,400 

18,000 

19,200 

18,000 

18,300 

18,300 

18,800 

19,300 

19,700 

19,800 

20,300 

20,500 

20,200 

18,600 

17,200

Segment 2

Fresh water 

(percent)

Calculated Actual

22 

24.2 

28 

28 

26 

29 

27 

31 

31 

31 

32 

33

46 

32 

32 

28 

31 

47 

46 

44 

44 

60 

36 

43

Length 

(ft)

18,800 

16,400 

16,700 

15,600 

17,900 

16,600 

16,600 

16,500 

16,600 

16,600 

16,300 

16,100
DUWAMISH RIVER ESTUARY, WASHINGTON C29

5000

 2

3J °

o a

ill W

z o: onnn

 HJ ^uuu

o: °-

^ $ £ 1000

^ "

25

o: D

500

onn

- 1 1 1 1 1 1 -

_ . _

^^

 ^v

_ N. _

\^

\

\

\.

\ -

\

\ 

__ \ __

1 1 1 1 1 | N

15,000

10,000

o

CO

D

5000 ° LJ

55

'S* 

o ^

2000 ? ^

LU W

1- LU

1000 $ LU

wLU

cr

LL.

500

10 20 30 40 50 

FLUSHING TIME, IN HOURS

60 70

FIGURE 18. Flushing time of Duwamish River estuary, from station

21.0 to mouth.

The curve in figure 18, derived from the data in table 2, represents at this stage of the investigation the best available estimate 

of flushing time for the Duwamish estuary. A comparison is available from a previous investigation made by Brown and Caldwell 

Engineers (1958). Their calculations of a flushing time of 3.0 days 

at 200 cfs, using an estimated mean mixed depth of 16 feet at mean 

high water, corresponds well with the 65 hours, or 2.7 days, at 

200 cfs shown on the curve in figure 18.

The change in the lower estuary at fresh-water discharge less 

than 1,000 cfs from a salt-wedge type-to a partially mixed type 

probably accounts for the failure of Ketchum's method in the lowinflow range. At the lower discharges the interface between the 

wedge and mixed layer becomes so diffuse that it is nearly impossible to establish accurately a theoretical bottom for the mixed layer. 

Therefore, because the critical DO levels occur during the lowdischarge period, the use of this method to compute flushing time 

has been abandoned.

Several authors have proposed other methods of mathematical 

models intended to evaluate the salinity distribution and (or) flushing time in an estuary. The method of Ippen and Harleman (1961) 

was found to be unsatisfactory because calculated results using this 

method did not compare well with the Duwamish field data. A 

possible reason is that the diffusion coefficient of Ippen and Harleman, determined at the ocean entrance, is assumed to be constant,
C30 ENVIRONMENTAL QUALITY

whereas this value increased in an upstream direction when computed from data for the Duwamish River estuary.

Arons and Stommel (1951) have proposed a "mixing-length theory of tidal flushing" for computing the salinity distribution in an 

idealized estuary where mixing is predominantly due to tides. Their 

equation takes the form 

where VSa=,F(l-L/X),

£=mean salinity, in parts per thousand; 

So^ocean salinity, in parts per thousand; 

L=length of the estuary, in feet, from a point where

salinity=0 to a point where salinity=S0 ; 

.X=distance along the estuary length, in feet; and 

F=a "flushing number" presumed to be characteristic of a 

given estuary, dimensionless.

Longitudinal salinity profiles at selected discharges as predicted 

by the Arons and Stommel method were compared with actual profiles measured in the estuary. That the predicted salinity did not 

compare well with the measured salinity was not surprising because 

S in the equation is intended to be the mean salinity in the vertical 

over the tidal cycle, whereas the measured values of salinity were 

not tidal-cycle means. This method, however, seems to hold enough 

promise to warrant further evaluation after data for a sufficient 

number of tidal cycles have been collected.

In August 1966 another dye study was made to determine how 

long the relatively fresh surface-water layer remains in the lower 

estuary. The dye was released at 0500 hours on August 29, 1966, 

at station 13.2. Figure 19 shows the predicted tide curves for the 

study period. Fresh-water inflow for the preceding 5 days had 

increased gradually; the mean daily discharge at station 21.0 had 

increased from 291 cfs on August 24,1966, to 307 cfs on August 28, 

1966. During the study, inflow decreased from 301 cfs on August 29, 

1966, to 289 cfs on September 1, 1966.

Samples of river water were collected for determination of dye 

concentrations at stations 1.9, 3.7, 4.5, 5.6, 6.9, 7.7, 9.5, and 10.4. 

Samples were taken 3 feet below the surface and 3 feet above the 

bottom. Station 10.4 was not sampled during lower low water, for 

shallow water prevents access by boat. The dye entered the study 

reach in about 2 hours and at the first lower low water, about 6 

hours after the dye release, almost the entire dye cloud was in the 

study area (fig. 20). During successive tides the dye cloud moved 

out of the estuary (fig. 21). In four successive cycles between lower 

low waters, the maximum dye concentration decreased from 18 p.g/1 

to 0.4 pg/l. A plot of peak dye concentration found during subse-
DUWAMISH RIVER ESTUARY, WASHINGTON C31

2400 1200 2400 1200 -2400 1200 2400 1200 

8-29-66 8-30-66 """" 8-31-66 9-1-66

2400

FIGURE 19. Generalized predicted tide stage for the Duwamish River 

estuary at station 6.9, for period August 29-September 1, 1966. 

Point A indicates dye release; points B through F refer to tide 

stages for figures 20 and 21.

DISTANCE UPSTREAM FROM MOUTH, IN KILOMETERS 

123456789 10 11

1 234567 

DISTANCE UPSTREAM FROM MOUTH, IN RIVER MILES

FIGURE 20. Longitudinal configuration of dye clouds in the Duwamish 

River estuary, August 29,1966. Circles, dye cloud just entering study 

area about 2 hours after dye release; squares, dye cloud at lower low 

water about 6 hours after dye release.
C32 ENVIRONMENTAL QUALITY

quent lower low water yielded a curve (fig. 22) that upon extrapolation suggests the dye would be nearly gone within 5 days. Rhodamine B was not detected in the bottom samples collected at stations 1.9, 3.7, and 5.6; this fact suggests that no mixing took place 

between the surface and bottom waters during this study.

In both the 1965 and 1966 dye studies, data were collected during 

nearly identical conditions of tide stage and fresh-water inflow. 

Comparison of the 1965 upper river data with the 1966 lower river 

data indicates that some of a pollutant introduced at station 21.0 

would remain in the estuary a minimum of 7 days.

DISTANCE UPSTREAM FROM MOUTH, IN KILOMETERS 

1 23456789

1 23456 

DISTANCE UPSTREAM FROM MOUTH, IN RIVER MILES

FIGURE 21. Longitudinal configuration of dye clouds at successive times of lower low water in the Duwamish River 

estuary: circles, August 30, 1966, about 26 hours after dye 

release; squares, August 31, 1966, about 51 hours after dye 

release; triangles, September 1, 1966, about 77 hours after 

dye release.

DISSOLVED OXYGEN

Dissolved oxygen has been a key parameter in this and previous 

studies of the Duwamish River estuary. Inasmuch as the degradation of the more common wastes requires their oxidation, the 

variation in content of DO in stream or estuarine water is a sig-
DUWAMISH RIVER ESTUARY, WASHINGTON C33

nificant indicator of the general amount and persistence of pollutants. Furthermore, the DO is especially significant in this and 

similar streams that have commercially and recreationally important fish populations. Changes in DO content not only directly affect 

the respiration of the fish and shellfish, but also influence them 

indirectly by changing the ecological system of which they are 

a part.

50

10

0.5

0.1

10 20 30 40 50 60 70 80 90 100 110 120 130 

TIME SINCE DYE RELEASE, IN HOURS

FIGURE 22. Plot of peak concentrations of rhodamine B found during 

successive times of lower low water, August 29-September 1,1966.

The DO of stream or estuary water is depleted by the oxidation 

of natural materials as well as manmade wastes and is added naturally by aeration at the air-water surface, by mixing with water 

having more DO and (or) by photosynthesis of aquatic plants. (See 

sections on "Phytoplankton" and "Biochemical Oxygen Demand.") 

Commonly the DO concentration in a stream water will decrease 

progressively for a certain distance downstream from a point of 

waste entry because the rate of oxygen depletion by oxidation of 

the wastes is greater than the rate of oxygen enrichment by reaeration or photosynthesis. The concentration of oxygen usually
C34 ENVIRONMENTAL QUALITY

begins to increase again at a sufficient distance downstream as the 

oxygen-depletion rate becomes less than the oxygen-enrichment 

rate. This longitudinal decrease and subsequent increase in DO is 

usually referred to as a "DO sag."

PREVIOUS DATA

Previous investigations by other agencies and individuals provide data for comparison with the DO data from the present study 

and provide a possible indication of changes over the years. Sample 

collection by these earlier investigators, however, was random with 

respect to tides and time. Significant DO data from the survey made 

by the Engineering Department of the City of Seattle during the 

period November 1948-April 1950 (table 3) show monthly mean 

values of DO in milligrams per liter and of percentage saturation 

for the four stations that most nearly correspond with those for 

stations of the present investigation.

During the 1948-50 period the data show that a DO sag near 

the bottom was centered about the 16th Avenue South station 7.7, 

and the minimum DO concentration of 3.7 mg/1 recorded during 

that study was found near the bottom at that location during August 

1949. Although the surface samples show no sag, there is a steady 

decline in percentage saturation of DO in the surface layer in the 

downstream direction. The relationship between DO content and 

fresh-water inflow during the critical summer period (MayOctober) at 16th Avenue South for 1949 is shown in figure 23. 

Mean-daily fresh-water inflow during August and September 1949 

was 262 cfs. Under the conditions of fresh-water flow during that 

survey, no serious oxygen depletion in the lower Duwamish River 

estuary was evident.

In the sanitary survey of the Green-Duwamish River made by 

the Washington State Pollution Control Commission during the 

summer of 1955 (Peterson and others, 1955), the river was sampled 

at 13 stations from Auburn (river kilometer 52.6) to the Spokane 

Street station (river kilometer 1.9). The samples were taken from 

the surface only, at approximately weekly intervals. The freshwater inflow during the sampling period June-October (fig. 24) was 

greater than that during the previous (1948-50) investigation by 

the City of Seattle. Minimum DO concentrations observed on the 

surface also were higher than those of the previous investigation. 

The minimum DO concentration observed at station 7.7 during the 

study by the Pollution Control Commission was 6.0 mg/1.

During the investigation in the summer and fall of 1956 by R. W. 

Okey (written commun., 1957), samples were taken from the top
MAY JUNE JULY AUGUST SEPTEMB

FIGURE 23. Fresh-water inflow and DO content at station 7.7, May-October 194

at the bottom; solid line, fresh-water inflo
C36 ENVIRONMENTAL QUALITY

TABLE 3. Monthly mean DO in near-surface and near-bottom

[Reported from previous studies as follows: November 1948-April 1950 by City of Seattle;

by R. W. Okey

Year Month

Station 12.6 Station 9.5

Surface Bottom Surface Bottom

mg/i

Percent 

saturation at 

field 

temperature

mg/1

Percent 

saturation at

field 

temperature

mg/1

Percent 

saturation at 

field 

temperature

mg/1

Percent 

saturation at

field 

temperature

1948 November ...... 10.9 89 .... .... 9.5 80 .... ....

December ...... 11.6 89 .... .... 11.2 84 .... ....

1949 January ....... 12.2 90 .... .... 11.6 85 .... ....

February ...... 11.0 86 .... .... 10.6 84 .... ....

March ......... 11.0 90 .... .... 10.7 72 .... ....

April .......... 11.0 94 .... .... 10.9 94 .... ....

May ........... 10.7 94 11.4 98 10.5 92 9.0 84

June ........... 9.1 87 8.6 84 8.9 85 7.6 80

July ........... 8.2 82 8.1 80 8.1 83 7.1 76

August ........ 6.8 71 6.4 68 6.3 68 5.0 55

September ..... 6.9 69 6.8 69 5.8 61 4.7 52

October ........ 9.6 83 8.9 78 8.4 76 4.8 48

November ...... 10.1 87 9.1 79 9.4 82 5.2 53

December ...... 11.3 89 10.7 84 10.7 85 6.4 62

1950 January ....... 11.9 86 11.8 86 11.9 87 7.5 67

February ...... 11.3 90 11.1 88 11.1 89 8.2 76

March ......... 10.8 88 10.7 86 10.7 88 8.4 78

April .......... 11.0 93 10.9 91 10.7 91 9.2 86

1955 Juhe ......... 10.4 94 .... .... 10.2 95 .... ....

July ........... 9.1 88 .... .... 9.0 88 .... ....

August ........ 7.6 80 .... .... 7.0 77 .... ....

September ..... 7.3 72 .... .... 6.3 67 .... ....

October ........ 9.4 86 .... .... 8.4 78 .... ....

1956 July ........... 8.1 85 7.8 86 8.0 86 7.1 78

August ........ 6.1 67 6.1 67 7.5 82 7.1 77

September ..... 6.1 63 6.0 62 6.1 66 5.7 61

November ...... 11.4 93 11.8 97 10.2 89 6.1 60
DUWAMISH RIVER ESTUARY, WASHINGTON C37

samples at selected stations on the Duwamish River estuary

June-October 1955 by Washington State Pollution Control Commission; July-November 1856 

(written commun., 1957)]

Station 5.6

Surface

mg/1

10.0

10.9

11.3

10.4

10.7

10.8

10.4

8.6

8.4

6.2

5.8

7.9

9.2

10.3

11.6

10.7

10.6

10.7

10.0

8.8

7.0

5.9

8.2

8.2

6.8

5.7

10.2

Percent 

saturation at

field

temperature

85

84

84

84

90

94

92

85

86

68

62

74

82

84

85

86

88

92

93

88

75

62

75

91

78

62

89

Bottom

mg/1

7.9

7.9

7.3

5.5

5.1

5.1

5.5

6.6

7.4

8.0

10.5

8.9

7.5

6.5

5.5

6.1

Percent 

saturation at

field

temperature

79

84

79

61

57

55

56

66

71

76

73

83

82

71

58

60

Station 1.9

Surface

mg/1

8.8

10.3

10.0

10.0

10.5

10.4

10.1

8.4

8.3

6.4

5.7

7.0

8.6

9.9

11.2

10.6

10.6

10.5

9.6

8.3

6.8

5.8

7.4

7.6

6.6

5.2

6.7

Percent 

saturation at

field

temperature

78

85

80

84

89

92

92

85

87

72

62

68

78

83

84

86

88

91

94

86

79

66

73

84

73

56

63

Bottom

mg/1

8.4

8.4

7.6

5.9

5.7

5.5

5,9

7.0

8.0

8.4

9.1

9.5

7.4

6.0

5.4

6.4

Percent 

saturation at

field

temperture

83

89

80

66

64

59

60

69

75

79

83

87

80

65

57

63
C38 ENVIRONMENTAL QUALITY

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DUWAMISH RIVER ESTUARY, WASHINGTON C39

and bottom at seven stations from 50.8 to 1.9 km above the estuary 

mouth at approximately biweekly intervals. The minimum observed 

DO, 4.9 mg/1, again occurred at station 7.7 and in the bottom layer 

(fig. 25). Mean fresh-water inflow during the critical period 

August-September was 258 cfs. Although fresh-water inflow was 

similar, the minimum observed DO was 1.2 mg/1 higher in 1956 

than the minimum observed by the City of Seattle 7 years earlier. 

This difference, however, probably is due mostly to the difference 

in sampling frequency. On the basis of the data available from the 

previous studies, therefore, no progressive decrease in DO content 

in either the upper (fresher) or lower (saline) layers of the estuary 

was apparent.

ANNUAL VARIATION

The present investigation has extended the collection of DO information through the period 1962-66; the data were collected by 

Metro and the Geological Survey. Monthly mean DO values for the 

years in which these data were available, at the sites where automatic water-quality monitors are presently located, are plotted in 

figure 26. The relationship of the DO level to fresh-water inflow 

can be clearly seen in this figure. Minimum DO conditions occur 

during periods of minimum inflow, and conversely, maximum levels 

occur during periods of maximum inflow. Figure 26 also suggests 

that the annual minimum DO concentrations in the study reach 

have generally declined since the pre-1960 studies. For example, 

lesser minimum values of DO are shown for all four sampling locations during 1965-66; even though minimum fresh-water inflow 

rates during those years were about the same as, or slightly greater 

than, those during 1949, 1955, and 1956. However, records are not 

sufficient yet to determine conclusively that the annual minimum 

DO levels in the Duwamish River are declining.

CORRELATION STUDIES

RENTON JUNCTION

The Renton Junction station (sta. 21.0) is the farthest upstream 

sampling point and provides the initial DO information for the 

study reach. Figure 27 shows the daily mean DO and the daily 

standard deviation for the period February-September 1966. The 

mean daily standard deviation is 0.35 mg/1 of DO. The daily variation about the mean DO for this station is smaller than those for 

the other three automatic monitoring sites.

The daily mean DO between February and September 1966 was 

regressed against daily mean inflow, logarithm of daily mean inflow,
MAY JUNE JULY AUGUST SEPTEM

FIGURE 25. Fresh-water inflow and DO content at station 7.7, May-October 19

at the bottom; solid line, fresh-water in
DUWAMISH RIVER ESTUARY, WASHINGTON C41

daily mean water temperature, and tidal-prism thickness. The water 

temperature and fresh-water inflow proved to be the most significant variables and are the only two used in the regression equation

Daily mean DO (mg/l=8.79-0.09 temperature (°F)

+2.01 Iog10 inflow (cfs).

The standard error of estimate for this regression analysis is 

0.70 mg/1 DO.

Concentrations of DO at this station have been considered to be 

adequate at all periods of discharge. Concentrations less than 6.0 

mg/1 are rare.

EAST MARGINAL WAY

The station at East Marginal Way (sta. 12.6) is downstream from 

the Renton Treatment Plant. The presence of salt water at this 

location is dependent upon discharge and tide stage. Concentrations 

of DO here are generally less than at Renton Junction, 8.4 km upstream. The daily mean DO curve in figure 28 shows the seasonal 

trend of lower DO concentrations during the late summer months 

when the fresh-water inflow is low. The greater variability during 

late summer, however, is due largely to the influence of salt-water 

intrusion. The minimum DO concentration is rarely less than 4.0 

mg/1. Using the same independent variables (fresh-water inflow 

and temperature) against daily mean DO, regression analysis was 

performed on the data for February to September 1966. The standard error of estimate is 1.04 mg/1 for the regression equation

Daily mean DO (mg/l=5.16-0.08 temperature (°F)

+2.73 Iog10 fresh-water inflow (cfs).

The deviation of the daily mean DO is only slightly greater than that 

obtained for the Renton Junction data.

SIXTEENTH AVENUE SOUTH

DO samples at station 7.7 at 16th Avenue South were taken from 

both top and bottom waters. Surface DO concentrations at this 

station generally are less than those at the upstream stations except 

during periods of phytoplankton bloom, when they may be greater. 

Concentrations as great as 13.2 mg/1 have been recorded at the 

surface during bloom conditions. Minimum surface DO concentrations during the bloom have declined to as low as 2.5 mg/1; mean 

surface DO concentrations, however, are rarely less than 3.0 mg/1. 

Multiple-regression analysis suggests that fresh-water inflow and 

temperature are the major variables contributing to the variations 

in surface DO concentration. Daily mean DO and standard deviation 

for the February-September 1966 data are shown in figure 29.
C42 ENVIRONMENTAL QUALITY

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Station 7.7 

16 Ave. South

O Surface 

 Bottom

I 

Station 1.9 

Spokane Street

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 Bottom

1953 

FIGURE 26. Fresh-water inflow for the period 1948-66, and monthly m

for periods of data collection.
C44 ENVIRONMENTAL QUALITY

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C46 ENVIRONMENTAL QUALITY

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DUWAMISH RIVER ESTUARY, WASHINGTON C47

Deviation of DO concentrations from the daily mean at this station 

was the highest of all of the four surface-sampling stations. Mean 

daily standard deviation here was 0.47 mg/1.

The bottom DO samples taken at this station are from the 

area of minimum DO conditions in the estuary. Daily mean DO 

concentrations of less than 2.0 mg/1 and minimum concentrations 

as low as 0.8 mg/1 have been recorded during the late summer 

months of low fresh-water inflow. The February-September data 

for the bottom samples are plotted in figure 30. The mean daily 

standard deviation (0.79 mg/1) here is the highest recorded for 

all the sampling locations.

Multiple-regression analysis indicates that bottom DO concentrations in this part of the estuary can be estimated from the tidalprism thickness and fresh-water inflow. The standard error of estimate for the regression equation is 0.61 mg/1. 

Daily mean DO (mg/1) = (-12.45)+0.18 tidal-prism thickness (ft)

+5.26 Iog10 inflow (cfs).

SPOKANE STREET

The Spokane Street station, at river kilometer 1.9, is the farthest 

downstream DO monitoring site. Samples are collected for DO 

from both the surface and bottom at this location. Mean surface 

DO concentrations there also generally were less than those at the 

upstream surface-sampling stations. Figure 31 shows a plot of the 

available 1966 daily mean DO data from the surface samples at this 

station. Surface DO concentrations generally followed the annual 

streamflow patterns, with minimums generally occurring during 

the late summer months. Daily mean concentrations of less than 

3.0 mg/1 were rare. As at the other stations, the major factors 

influencing the surface DO concentrations at the Spokane Street 

station were fresh-water inflow and water temperature. The variation about the daily mean DO concentration was of the same order 

as for the Renton Junction and East Marginal Way stations.

Bottom DO concentrations at the Spokane Street station were 

greater than the bottom concentrations upstream at 16th Avenue 

South; summer minimums here usually exceeded 2.0 mg/1 and 

rarely declined to 1.0 mg/1. Daily mean data for February to September 1966 are plotted in figure 32. Here again the bottom DO 

concentrations could be estimated from the fresh-water discharge 

and tidal-prism thickness.

AREA OF CRITICAL DISSOLVED OXYGEN

Minimum DO conditions occurred near the bottom of the estuary 

between river kilometer 7.7 (16th Avenue South) and river kilome-
C48 ENVIRONMENTAL QUALITY

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C50 ENVIRONMENTAL QUALITY

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DUWAMISH RIVER ESTUARY, WASHINGTON C51

ter 10.4 (Boeing Bridge). Very low DO conditions occurred during 

the late summer months only when fresh-water discharge was less 

than 400 cfs. Although streamflow rates play a highly significant 

role, minimum tidal exchange must occur concurrently with low 

flow to produce the very low DO conditions. The length of time that 

the flow rate remains less than 400 cfs is also quite important. 

The longer the flow remains below this rate, the lower will be the 

minimum DO concentration. The mean daily inflow, tidal-prism 

thickness, and minimum daily DO concentration of bottom water at 

river kilometer 7.7 for June to September 1966 are plotted in figure 

33. This figure shows the important influence of the duration of a 

period of inflow at rates less than 400 cfs. The figure also shows that

JUNE JULY AUGUST SEPTEMBER

FIGURE 33. Mean daily fresh-water inflow, tidal-prism thickness, and minimum 

bottom DO, at station 7.7, June-September 1966.
C52 ENVIRONMENTAL QUALITY

the cyclic variation in DO concentration corresponds very well to 

the variation of the tidal-prism thickness, for the lower minimum 

concentrations coincide with the days of minimum tidal exchange. 

The minimum possible DO concentrations apparently are related 

to the retention time. As described previously, the bottom water is 

removed by entrainment into the upper layer with little or no mixing 

of the surface layer downward. This method precludes any reaeration by downward mixing. When the lower rates of fresh-water 

inflow coincide with minimum tidal exchange, removal of bottom 

waters by entrainment is reduced, and retention time of the contained oxygen-demanding materials is increased. However, because 

the quantitative relation between minimum DO and retention time 

in the estuary has not been firmly established, this relation is 

undergoing further investigation.

PHYTOPLANKTON

Frequently the introduction of nutrients (nitrogen and phosphorus compounds) into an aquatic environment has caused 

nuisance blooms of phytoplankton. A study of the factors controlling 

phytoplankton growth in the estuary was made to evaluate the 

influence of the effluent from the Renton Treatment Plant. The following material on phytoplankton is summarized from a comprehensive report by Welch (1969).

Phytoplankton blooms occurred in August 1965 and 1966 after 

the discharge of effluent from the Renton Treatment Plant started 

in June 1965. A pre-effluent bloom in 1963 is indicated by supersaturated oxygen values that occurred at station 7.7 in the euphotic 

zone (upper, light-receiving zone). No bloom occurred in 1964 when 

summer fresh-water inflow was twice as large as the summer inflow 

during bloom years. Chief factors observed to control the occurrence of a bloom in the estuary are fresh-water inflow rate, tidalprism thickness, water temperature, and solar radiation.

Nutrient concentrations do not control the occurrence of the 

bloom inasmuch as nitrogen and phosphorus compounds always 

are present in sufficient quantities for a bloom to exist. Also, because 

no noticeable decrease in nutrient concentration was observed during 

the bloom, it is apparent that nutrient limitation was not the cause 

of the bloom decline. An increase in nurients, however, may increase 

the biomass produced during the bloom.

Retention time during the warm season probably is more significant in triggering a bloom. A low fresh-water inflow rate and 

small tidal-prism thickness increase retention time. If this combination happens during a period when solar radiation is optimum,
DUWAMISH RIVER ESTUARY, WASHINGTON C53

the surface-water temperature increases and causes thermal stratification. The stratification decreases the mixing depth and increases 

the amount of light available for plankton cells, and a bloom occurs. 

Welch and Isaac (1967) demonstrated that chlorophyll a is a 

good indicator of phytoplankton productivity in the Duwamish 

River estuary. Figure 34 shows the hydrographic conditions during 

the 1965 and 1966 blooms, which are indicated by large chlorophyll a 

concentrations at station 7.7. Figure 34 shows that the same hydroJUNE JULY AUGUST SEPTEMBER

1500

1000

500

1500

1000 UJ

500

FIGURE 34. Relation of phytoplankton bloom-time, as indicated by 

chlorophyll a concentration to fresh-water inflow and tidal-prism 

thickness, during 1965 and 1966 (after Welch, 1969).
C54 ENVIRONMENTAL QUALITY

graphic conditions that occurred in August of both years were repeated in September of both years, but the small chlorophyll a 

values in September 1965 and 1966 indicate that no blooms occurred 

during either September. The lack of a September bloom probably 

was due to the fact that solar radiation and surface-water temperature both were much less in September than in August.

Welch (1969) concluded that effluent from the Renton Treatment 

Plant probably was not the sole cause of the phytoplankton blooms 

but that the increase in nutrients from the plant could increase 

the biomass of the blooms. The relations he described between 

chlorophyll a and BOD show that phytoplankton both alive and 

dead may be an important source of BOD in the estuary. If the 

effluent from the sewage treatment plant increases the bloom biomass, it could result in a greater BOD, and the greater BOD would 

further decrease the DO content of the bottom water.

The present (1967) program is oriented toward a better understanding of what environmental factors control the magnitude and 

timing of the phytoplankton bloom. Some of these factors, such as 

tidal-prism thickness, fresh-water inflow, solar radiation, and water 

temperature, are relatively easy to determine but are difficult to 

relate to the blooms, particularly to their influence on the magnitude of the bloom. The role of the nutrients from the Renton Treatment Plant is made even more obscure, of course, because those 

nutrients are superimposed upon the various nutrients from other 

sources that are present in the estuary.

BIOCHEMICAL OXYGEN DEMAND 

EXPERIMENTS

As stated in the discussion of methods and equipment, a part of 

this investigation was devoted to experiments seeking methods of 

determining BOD values that would closely represent conditions 

in the estuary.

Because the four automatic-monitor water-quality stations have 

a continuous source of river water, incubation of BOD samples at 

river temperature was determined to be more nearly representative 

than the laboratory procedure. Incubation troughs were constructed 

and installed at the four monitor stations. (See section on "Special 

Equipment.") Many of the initial experiments, however, failed 

because of pump failure, and there was some difficulty in making the 

BOD bottles lightproof.

In the tests a 4-liter sample was taken in a sampler and subdivided 

into an initial-DO sample, 10 BOD samples, and a second initial-DO 

sample at the end of sample drawoff. For the next 5 days the DO 

content was determined on a sample air at 1-day intervals. The
DUWAMISH RIVER ESTUARY, WASHINGTON C55

average DO concentration for the two samples then was used to 

plot the DO uptake for 5 days. The graphical method of Thomas 

(1960) for determining the ultimate first-stage BOD demand was 

tried, but the plots of DO uptake versus time were poor, mainly 

because BOD values were small in comparison to possible errors 

in determining the residual DO in the BOD sample.

Large differences in BOD values between the two samples of a 

pair were noted occasionally. One reason for this could be that while 

the samples were being drawn off, some settling of particulate organic material could take place, so that some samples received more 

biodegradable substances than did others. An experiment was made 

to determine what influence the order of sample withdrawal from 

the sampler had on the BOD value found. A sample was collected 

at Renton Junction (sta. 21.0) and, after agitation, eight consecutive BOD samples were drawn off and incubated for 5 days. The 

time taken to draw off the eight BOD samples was less than 2 minutes and, except for very heavy material, only a very small amount 

of settling could have taken place during that short time. Samples 

1, 3, and 8 have BOD values above the median (0.30 mg/1), and 

samples 4, 5, and 7 had values below the median. The maximum 

BOD was 0.90 mg/1 (sample 1), and minimum was 0.10 mg/1 (sample 4). The results did not show any correlation of BOD with the 

order of sample withdrawal.

The negative results of these tests led to another experiment 

wherein the samples were collected from nine stations in the estuary 

rather than from just one. As before, the sample was divided into 

an initial-DO sample, eight consecutive BOD samples, and another 

initial-DO sample. In addition, a sample was taken at the same 

time for a determination of specific conductance. Results were similar to those from the previous test and show what variation in BOD 

values occur when a 4-liter sample is subdivided. Figure 35 shows 

the minimums, maximums, and means for the nine sets of samples. 

The greatest variation between subdivided samples occurred in the 

sample collected at station 17.9 and the least variation occurred 

in the sample collected at station 1.9 from 3 feet above the bottom. 

Although the specific conductance increased progressively in a downstream direction, no correlation was apparent between BOD and 

specific conductance.

One influence on BOD was found to be the initial DO in the BOD 

sample. Aerated and unaerated samples gave different results. In 

unaerated samples which had an initial DO of less than 4 mg/1, 

the average BOD was 1.5 mg/1. The average BOD for aerated samples was 2.6 mg/1. Zobell (1940, p. 218) suggested that this type 

of relation was due to the fact that, during the period of incubation,
C56 ENVIRONMENTAL QUALITY

DISTANCE ABOVE MOUTH, IN KILOMETERS 

4 8 12 16 20 24

Z X

<o

Su.2

5<

UJCC 1 

10

y^j O-J 

005

0

5 10 

DISTANCE ABOVE MOUTH, IN MILES

15

FIGURE 35. Variation of minimum, mean, and maximum 

BOD to distance above river mouth, at a mean daily inflow 

of 710 cfs. Black circles, surface samples; black squares, 

bottom samples; open circles and squares, mean values for 

surface and bottom samples, respectively.

the bacteria were using oxygen faster than it could diffuse into 

localized microspheres of oxidizable material.

The preceding discussion indicates that accurate data on the 

BOD actually occurring in the river are very difficult to obtain; 

therefore, the data presented in this section should be considered 

as only semiquantitative.

LONGITUDINAL PROFILES

Figure 36 shows selected longitudinal BOD profiles for rates of 

fresh-water inflow ranging from 293 to 1,760 cfs.

The figure indicates that below station 21.0 surface BOD increases downstream and generally reaches a maximum near or 

between stations 12.6 and 17.9. The increase can be attributed to 

waste discharges into the river upstream from station 12.6. The 

increase is small, with the maximum observed (1.9 mg/1) occurring 

between stations 21.0 and 12.6. Downstream from station 12.6 the 

surface BOD decreases or remains about the same when freshwater inflow rates are large (figs. 36 A, F). When fresh-water inflow
DUWAMISH RIVER ESTUARY, WASHINGTON C57

DISTANCE ABOVE MOUTH,

IN KILOMETERS 

04 8 12 16 20 24

Higher high water, January 11 

Fresh-water inflow, 1,760 cfs

LJ

52

O

m

I

Higher high water, July 1 

Fresh-water inflow, 710 cfs

0 5 10 15

DISTANCE ABOVE MOUTH,

IN MILES

C

DISTANCE ABOVE MOUTH,

IN KILOMETERS 

4 8 12 16 20 24

Lower low water, August 22 

Fresh-water inflow, 293 cfs

< 

O

u 0

I 

O

o

CO

Higher high water, August 23 

Fresh-water inflow, 293 cfs

_____________I I

E

Lower

i r

low water, (Jetober 28

Fresh-water inflow, 1,710 cfs

0 5 10 15

DISTANCE ABOVE MOUTH,

IN MILES

F

FIGURE 36. Longitudinal distribution of BOD at higher high water or 

lower low water at various rates of fresh-water inflow, JanuaryOctober 1966. Circles, surface BOD; squares, bottom BOD.
C58 ENVIRONMENTAL QUALITY

rates are large and water temperatures are cold, the bottom BOD 

values are small (figs. 36 A-C, F). When inflow rates are moderate 

or small, the effect of tides is apparent.

Figures 365 and C show the effect of tide when fresh-water inflow 

is moderate. At low tide (fig. 365) the surface BOD shows a slight 

decrease downstream from station 12.6. Figure 36C shows a fairly 

rapid downstream increase in the surface BOD, reflecting a "compression" effect of the high tide. Under the conditions represented 

by that figure, the maximum surface BOD occurred at station 16.2, 

whereas at low tide (fig. 365) the maximum occurred 3.6 km 

farther downstream at station 12.6. The relatively large surface 

BOD (about 2.4 mg/1) at station 1.9 (fig. 36C) represents water 

returning from the previous ebbtide that contained wastes discharged into the estuary by industries downstream from station 

1.9. To a lesser degree this also is shown in figure 36-4.

Figures 36D and E show the increase in BOD values observed 

during the die-off stage of a phytoplankton bloom. (See fig. 34.) 

Although part of the BOD increase is due to planktonic material 

from the bloom, the decreased fresh-water inflow during late summer also is responsible for part of the observed increase. The maxi10

20

25

30

I

£

LJ

6 Q

 9

024 6 8 10 12 

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

FIGURE 37. Vertical distribution of DO (squares) 

and BOD (circles) at station 9.5 during a phytoplankton bloom, August 9,1966.
DUWAMISH RIVER ESTUARY, WASHINGTON C59

mum BOD in the bottom layer (about 3 mg/1) was observed at station 12.6 (fig. 36E1 ). Maximum bottom values occurred when minimum tidal-prism thickness coincided with low fresh-water inflow; 

under these conditions, water velocities are not large enough to 

prevent the organic detritus from settling to the bottom. A device 

has been constructed for sampling the amount of detritus that settles to the bottom. Hopefully, the analyses of samples thus collected will define the amount of BOD in detritus and will help to 

delineate the hydrographic conditions needed for the maximum settling rate of detritus.

Although BOD values are generally greater for the surface layer 

than for the bottom layer, the DO in the surface layer is considered 

adequate for the maintenance of aquatic life. Figure 37 shows the 

relation of DO and BOD to depth at station 9.5.

The bottom water, on the other hand, receives virtually no additional DO except for small quantities that diffuse downward from 

the water above. Therefore, even though BOD is small in the bottom 

water, the DO can be depleted to values considered detrimental to 

aquatic life because the bottom water is essentially unaerated. 

(See p. C47.)

CHEMICAL CHARACTER OF THE FRESH WATER

MAJOR CONSTITUENTS

In a river basin in its pristine state the dissolved-solids content 

of the waters traversing the basin is controlled largely by the geology of the basin and by the precipitation. In general, man's activities 

in a watershed increase the dissolved-solids content by such influences as municipal and industrial waste disposal, changes in overland drainage, and changes in land-use patterns.

Chemical-quality data have been collected routinely at four locations in the Duwamish River basin (fig. 3) and have been published 

by the U.S. Geological Survey (1960-63, 1964,1965,1966). Selected 

comprehensive analyses of fresh-water samples from those stations 

are presented in table 4.

The analyses of the water of Green River at Palmer (table 4) 

indicates that the water comes from a near-pristine watershed. The 

water is a dilute calcium bicarbonate type. Silica is the major constituent, and nitrate and phosphate values are generally about what 

could be expected from an unpolluted stream (Hem, 1959, p. 117).

It can be seen from table 4 that mineralization increases progressively between the two downstream sampling stations (Green 

River near Auburn and Duwamish River at sta. 16.2). A similar 

downstream increase occurred between the Green River stations 

at Palmer to near Auburn. At the Auburn sampling site, chemical
C60 ENVIRONMENTAL QUALITY

v.

50

I

e

1 

C)

-

IIs s 

s I

-sj

7

O0«0 SB)

(OoOST W uoi^BJOdBAa 

uo anpisai) spips paA[OssiQ

( Od8B)

ON)

(ID) apwoRO

( fOS) a^BJing

8OOH) «»»«<

(H)

uinipog

(BO) ranp

(°ois) *onis

11

eOT)i«>SMicO'*l i-ieoe4C4eoe4e4'*e9e<a'*

OOoOOOO OOOOO i-1 04 iH i-1

C4C4OOOOoqu9OUJ>aoOC4OOU9OC4C4 "^ ^ 0°

U9ou9ooioo o 10 10 o iao<±>cou3

§5

-ooosoeqeoNosr-ii-ioot-NOS'-iiHOOt-Nosi-iiHW A

D9 te« 

«
DUWAMISH RIVER ESTUARY, WASHINGTON C61

analyses of the 70 samples collected monthly since July 1959 show 

that the dissolved solids range in concentration from 33 to 71 mg/1 

and consist principally of calcium and bicarbonate.

The addition of surface-water inflow between Auburn and station 

21.0 is chiefly from Big Soos Creek, which enters the Green River 

about 500 feet downstream from the Auburn sampling site. The 

discharge from Big Soos Creek averages less than 10 percent of 

the flow of Green River at Auburn. Although the dissolved-solids 

concentration in water of Big Soos Creek is larger than that in the 

Green River, the water also is of the calcium bicarbonate type, similar to that of the Green River (table 4). Although small in terms 

of tons, nitrate concentrations are generally greater in Big Soos 

Creek, because of considerably more human activity in that watershed than in the Green River watershed upstream from the Auburn 

station.

A long-term chemical-quality sampling site is at station 16.2, 

downstream from the outfall of the Renton Treatment Plant and on 

the reach of the Duwamish River that is affected by salt water. 

Data from station 16.2 show the water there to be slightly more 

mineralized than that from the Auburn station. During periods of 

low fresh-water inflow and high tides, traces of salt water (specific 

conductance values greater than 250 micromhos) have been detected 

as far upstream as station 17.9.

The salt water found in the downstream reaches of the estuary 

may be represented simply as various percentages of Elliott Bay 

water diluted by Green River water. A graph of specific conductance 

values versus chloride values (fig. 38) shows the linear relation 

between the two. The graph was used to estimate chloride values 

for correcting the DO values used in calculating percentage saturation of DO.

Nitrate in water is one of the indicators of pollution caused by 

man's activities and is a nutrient for the microorganisms in the estuary. Inspection of nitrate values (table 4) indicates that nitrate 

concentrations increase significantly between Auburn and station 

16.2. To determine whether the increase was real or attributable 

to differences in rate of streamflow, the nitrate values were weighted 

with water discharge by the equation

2 (mg/1 N03 ) 1 (cfs) 1+(mg/l N03 ).2 (cfs) 2+***+ (mg/1 N03 )n(cfs) n 

2 (cfs) 1+(cfs) 2+***+(cfs) n

where n=the number of samples.

A plot of the yearly nitrate values for the two sampling stations 

(fig. 39) indicates that nitrate values have increased slightly in
C62 ENVIRONMENTAL QUALITY

20,000

10,000

5000

2000

1000

500

200

100

0.5 10 20 50

SPECIFIC CONDUCTANCE, IN THOUSANDS OF MICROMHOS 

PER CENTIMETER AT 25° C

FIGURE 38. Relation of chloride concentration to specific conductance in 

the Duwamish River estuary.

the Green River near Auburn since 1962 and progressively in the 

Duwamish River at station 16.2 since 1964. The indicated increase 

in nitrate concentration is attributed principally to wastes from 

increased industrial activity and to increased urbanization along 

this 46.7-km reach of the river, rather than from the Renton Treatment Plant effluent. Data from Metro (G. D. Farris, written commun., 1969) show that the average discharge of effluent from the 

plant during 1966 was 18.7 cf s and that the effluent had a dischargeweighted average nitrate concentration of 3.1 mg/1. At this rate 

and concentration, the effluent could have increased the dischargeweighted average nitrate concentration at station 16.2 by only 

0.05 mg/1. Because the observed increase in nitrate concentration 

in the river between Auburn and station 16.2 was 1.4 mg/1 in 1966,
DUWAMISH RIVER ESTUARY, WASHINGTON

3

C63

Q d

£ z

5 o

uj I- 1 o 

FIGURE 39. Increase in discharge-weighted average 

nitrate concentrations. Circles, upstream concentrations in the Green River near Auburn; triangles, downstream concentrations in the Duwamish River at station 16.2.

the contribution from the Renton Treatment Plant could have accounted for less than 5 percent of the increase.

Plottings of discharge-weighted values of phosphate, another 

nutrient compound, failed to give significant results.

TRACE ELEMENTS

Trace elements, as denned in this discussion, are minor elements 

that usually are present in fresh or sea water in concentrations so 

low that they are measured in micrograms per liter. Only in recent 

years have investigators looked into the role that these minor constituents have in biological processes of fresh- and salt-water phytoplankton. Most studies have been in laboratories under controlled 

conditions and have used pure cultures of algae to determine 

whether a minor constituent is essential for algal growth. Wiessner 

(1962) presented an excellent sumary of what he called inorganic 

micronutrients (minor constituents). He points out the extreme 

difficulty of proving that a minor constituent is essential for algal 

growth.

Durum and Hafty (1961) presented trace-element data for prin-
C64 ENVIRONMENTAL QUALITY

cipal rivers of the United States and Canada, and Livingstone (1963) 

gave trace-element data for rivers and lakes of the world. Their 

data indicate that large ranges in trace-element concentrations exist 

in many rivers. Sverdrup, Johnson, and Fleming (1942, p. 176-177) 

summarized trace-element data for sea water. The variation in concentrations of minor constituents in river water makes it extremely 

difficult to relate their occurrence to environmental parameters, such 

as precipitation.

In spite of the difficulties involved, it was felt that work should 

be done to determine if some relation existed between phytoplankton 

and minor constituents in the Duwamish River. As initially set up, 

the program of sampling for the minor constituents in the fresh 

water entailed defining the quantity and distribution of these constituents and how they relate to the microorganisms in the estuary. A 

series of four weekly sampling trips was made during the summer of 

1964, and surface samples were collected for chlorophyll a, b, and c, 

numbers of phytoplankton, specific conductance, and trace elements. 

Evaluation of the data leads to the conclusion that the minorconstituent concentrations are so large that phytoplankton uptake 

cannot be detected.

TABLE 5. Range in concentration of trace elements in the Green-Duwamish 

River prior to discharge of effluent from the Renton Treatment Plant

[In micrograms per liter]

Station 1.9 Station 7.7 Station 12.6

Element1

Nickel

Bismuth ........

Minimum

<14

.... 4

.... 2.4

.... < .50

.... 1.0

^"\ o

.... < .50

<^1 9

^ 9>i

.... < .25

Maxi- Minimum mum

13 

24

76

7 

13

5

16 

5

3,9 

< .50 

4 .66 

. < .50

4 <1.2 

. < .25 

< .25 

7 < .25

Maxi- Minimum mum

24 1.7 

138 2.6 

100 <1.2 

.... < .50

4.7 .43 

8.1 <1.2 

.... < .50 

2.3 <1.2

.... < .25 

3.6 < .25 

.94 < .25

Maximum

58 

>70 

33 

1.8 

6.0 

2.4 

5.4 

^5.7

6.9

3.8 

.89

Station 21.0

Minimum

1.6 

3.4

< .50 

< .39 

< .50

< .25 

< .25 

< .25

Maximum

37 

>70 

18 

1.4 

6.0 

2.5 

10 

11 

9.7

.71 

.85

1 Abo detected lead, beryllium, gallium, and germanium.

The trace elements (summarized in table 5) can generally be separated into three categories: (1) elements, such as lead, that were 

detected but in concentrations not large enough for quantitative 

definition, (2) the element molybdenum, the only one whose occurrence could be readily observed as relatable to specific conductance
DUWAMISH RIVER ESTUARY, WASHINGTON C65

10.0

7.5

5.0

o: 

g2.5

o: 

o

o: h5 i.o o 

z 

o 

o 

_ 0.75

0.50

0.25

0.20

X

X

X

X

X

X

X

Detection limit

2.5 10 25 50

SPECIFIC CONDUCTANCE, IN THOUSANDS OF 

MICROMHOS PER CENTIMETER AT 25° C

FIGURE 40. Relation of molybdenum concentration to specific conductance in the Duwamish River estuary.

(fig. 40), and (3) elements, such as aluminum, iron, manganese, 

copper, nickel, and vanadium, whose concentrations appear to have 

been a random distribution with respect to specific conductance.

In the downstream part of the estuary, the occurrence of some 

minor constituents can be related to specific conductance. For many 

of the elements, however, the relations are obscured by the addition 

of other variables, such as wastes from municipal and industrial 

sources.

The trace-element concentrations in the fresh-water samples 

showed no correlation to rate of fresh-water inflow. Also, the longi-
C66 ENVIRONMENTAL QUALITY

tudinal distribution of trace-element concentrations in the river 

exhibited no clear correlation with tide.

After the Renton Treatment Plant had been in operation for about 

8 months, a set of samples was collected to determine which trace 

elements were in the treated effluent and if they could be detected 

in the river. Samples were collected for the measurement of trace 

elements and specific conductance at stations 12.6, 17.9, 21.0, and 

at the treated-effluent source (sta. 20.5).

TABLE 6. Analyses of special trace-element samples collected at selected 

stations, showing theoretical versus actual concentrations

[In micrograms per liter]

Station

Element

12.6 17.9 20.5

Calculated

for Percent 

 station difference 

21.0 17.9

Iron _ __ _

Manganese ___ _

Chromium __ __

Nickel __ __ ___

Copper ___ __

Zinc _ __ _ ______

Vanadium __ __

__ __ 16

_ __ >70

___ 33

______ 6.0

__ __ 1.3

4.9

<5 7

6 9

____ .4

16

>70

21

2.7

1.8

5.1

<5.7

6.6

.3

114

>70

20

177

5.4

34

30

223

.4

13

>70

18

6.0

1.6

10

11

<l-4

.3

19

18

16

1.8

11

12

14

.3

4-19

+ 17

-^90

0

 116

 100

112

0

The samples were analyzed for the 17 elements listed in table 5, 

and nine elements were found in quantitative amounts (table 6). 

Data on the water discharged by the Renton Treatment Plant and 

the discharge of the river allowed a calculation of the theoretical 

amounts of trace elements that should be found in the sample collected at station 17.9. The calculated theoretical concentrations of 

nickel and vanadium compared very well with the concentrations 

actually found, and the differences between the calculated and actual 

concentrations of aluminum and manganese were not great enough 

to be significant. The calculated concentrations for cadmium, chromium, copper, and zinc suggest a loss of these elements from solution. It was felt that the comparisons, particularly for increased 

concentrations, could not be extended downstream to station 12.6 

because of possible additions of trace elements through storm drains 

and minor outfalls in the reach between stations 12.6 and 17.9.
DUWAMISH RIVER ESTUARY, WASHINGTON C67

FLUVIAL SEDIMENTS

As a part of the comprehensive study, suspended-sediment samples were obtained once daily or of tener at Renton Junction kilometer 21.0. The sediment is derived principally from natural processes 

along- the channel and alluvial plain of the Green River below Eagle 

Gorge, but in recent years appreciable amounts of sediment have 

been added to the lower Green River as a result of industrial construction between Kent and Renton Junction.

From station 0 to station 7.7 the river-bottom material is always 

an estuarine ooze. A transitional zone, dependent on river flow, 

exists between stations 7.7 and 10.4, during much of the high-flow 

winter months the bottom there consists of sand, whereas during 

the low-flow summer period the bottom consists of a typical estuarine ooze of mud and organic detritus. Upstream from station 10.4 

the estuary is more typical of a meandering sand-bottom river.

Total-carbon analysis cannot be used to quantify the amount of 

organic material present in the bottom deposits because of the 

large amounts of coal in the bottom deposits. In the upstream reach 

between Palmer and Black Diamond, numerous coal seams, interbedded with shale or sandstone (Evans, 1912, pi. 20), have been 

exposed by erosion of the Green River channel. The coal seams contribute to the sediment load of the Duwamish River; for example, 

a bed-material sample taken at mile 12.6 contained, in addition to 

smaller-sized particles, a piece of coal that measured three-quarters 

of an inch square by one-quarter of an inch thick. Analysis of bottom deposits collected during periods of high flow in the winter and 

low flow during early fall show almost no difference in the percentage of carbon.

Daily mean suspended-sediment concentrations at Renton Junction usually ranged from 20 to 200 mg/1; however, during periods 

of high streamflow mean daily concentrations of 1,000 mg/1 have 

been observed. The maximum observed daily suspended-sediment 

load was 31,000 tons on January 30, 1965, whereas minimums of 

less than 20 tons per day occur frequently during the low-flow 

periods in the summer. Yearly suspended-sediment loads vary with 

peak discharge, as can be seen from the following table:

Water

year

1964

1965

1966

Maximum

discharge

during the

water year

(cfs)

. __ 6,000

._ _ 12,000

4,800

Annual

suspendedsediment, load

( tons pei- water

year)

125,000

375,000

57,000
C68 ENVIRONMENTAL QUALITY

Appreciable quantities of sediment, too heavy to be carried in 

suspension, are moved along the riverbed. This bedload of a river 

(usually expressed as a percentage of the suspended sediment) 

increases as water discharge decreases. Two measurements of bedload at Renton Junction show that it amounted to 40 and 20 percent 

of the suspended-sediment load when streamflow was 1,880 and 

11,600 cfs, respectively.

Scour and fill play an important part in sediment transport and, 

during peak discharges, the streambed may be deepened locally 

as much as 7 feet, as shown in figure 41. The importance of channel 

scour, particularly in the lower estuary, is still under investigation. 

It has been suggested that a buildup of oxygen-consuming deposits 

would result if there were little or no channel scour from one year 

to the next. Studies are being made to determine what rates of 

fresh-water inflow are needed to scour the oxygen-consuming deposits from the lower estuary.

100

WIDTH. IN METERS 

200 300 400 500 600

CD 20

100 

WIDTH,IN FEET

200

FIGURE 41. Comparison of channel cross-section areas at station 12.6 during two rates of fresh-water inflow. Dashed line, area at 12,700 cfs; 

solid line, area at 1,910 cfs.

Particle-size analyses of suspended-sediment samples, collected 

when fresh-water inflow ranged from 540 to 12,000 cfs, show that 

the suspended sediment consists of 8-28 percent clay particles 

(<0.004 mm), 11-57 percent silt (0.004-0.062 mm), and 16-75 percent sand (0.062-2.0 mm). The unit weight of sedient transported
DUWAMISH RIVER ESTUARY, WASHINGTON C69

past the daily station is estimated to be 1 ton per cubic yard. The 

reader is referred to the annual reports of the U.S. Geological Survey (1960-63) for more detailed information on daily sediment 

concentrations and particle size in the Duwamish River.

CONTINUING AND FUTURE STUDIES

During later phases of this investigation, emphasis will be on 

developing a more precise quantitative understanding of the hydrodynamics of the estuary and on extending the biologic aspects of 

the study. Additional subjects of study and work planned or already 

underway include the following:

1. The mixing-length theory of tidal flushing proposed by Arons 

and Stommel (1951) will be tested for application to the Duwamish River estuary. Salinity-velocity studies will be made 

during periods of different rates of fresh-water inflow and 

during selected tidal cycles.

2. During a period of low fresh-water inflow, the residence time of 

the salt-water wedge will be determined by using a fluorescent 

dye.

3. DO data from the water-quality monitors will continue to be supplemented by field data to determine the severity of the DO 

sag during the low fresh-water inflow period. The severity of 

the DO sag for each year will be compared to the historical 

(pre-1962) DO data to determine if the sag is becoming more 

severe for comparable low-flow periods.

4. The conditions that control the occurrence of phytoplankton 

blooms need better definition, especially the roles played by 

solar radiation and water temperature. When a bloom does 

occur, its magnitude and duration will be determined and compared to previous bloom data for indications of increased 

biomass.

5. Work on BOD is continuing. A differential respirometer has been 

acquired; it is felt that this apparatus will provide data on 

rates of BOD more accurately than is possible with the equipment used to date (1967).

6. An attempt will be made to develop a mathematical model of the 

estuary that satisfactorily represents the relationship of circulation and mixing to the distribution of dissolved oxygen in 

the estuary.
C70 ENVIRONMENTAL QUALITY

SUMMARY AND CONCLUSIONS

Major preliminary results and conclusions from the various 

phases of the study completed to date (1967) are:

1. The Duwamish River estuary can be classed as well stratified 

to partially mixed, depending on the rate of fresh-water inflow. The inflowing fresh water and any contained pollutants 

remain mostly in the upper 5-15 feet of the estuary and are 

underlain by a wedge-shaped body of salty water throughout 

most of the length of the estuary. The degree of stratification 

increases in an upstream direction and is more distinct during 

high rates of inflow than during lower rates of inflow. Where 

stratification is evident, the thickness of the upper (fresher) 

water also varies directly with the rate of fresh-water inflow. 

The volume of the estuary ranges from 205 to 592 million 

cubic feet for the lowest and highest recorded tides, respectively.

2. The distribution of salinity in the estuary varies with the rate 

of fresh-water inflow and with tide stage. For a given inflow, 

the salt-water wedge migrates upstream and downstream 

with the tide stage; for similar tide conditions, the wedge 

advances farthest upstream during low rates of inflow and 

retreats downstream during greater inflow. A fresh-water 

inflow greater than 1,000 cfs prevents salt-water movement 

to station 12.6 regardless of tide height, but during low 

fresh-water flows, salt water has been detected upstream 

beyond station 16.2. At virtually any station on the estuary, 

the salinity at a given depth is laterally constant.

3. Fluorescent-dye studies confirmed that there is virtually no 

mixing of the fresh-water downward into the salty-water 

layer. Most of the mixing of fresh and salty water in the estuary takes place at the interface, where salty water is entrained 

upward into the overriding fresh water. The erosion of salty 

water is balanced by a net upstream movement of the water 

in the salt-water wedge.

4. Three methods that use salinity to calculate flushing rates were 

tried and abandoned. A comparison of results of dye studies 

during succeeding years (1965 and 1966) indicates that some 

fraction of a pollutant introduced upstream at station 21.0 

would remain in the lower estuary a minimum of 7 days. Dispersion coefficients for the upper layer have been determined 

to be on the order of 100-400 square feet per second.
DUWAMISH RIVER ESTUARY, WASHINGTON C71

5. DO in the surface layer decreases almost linearly in a downstream direction, but this layer usually contains adequate 

amounts for the maintenance of aquatic life. DO in both the 

upper and lower layers increases with increasing fresh-water 

inflow and diminishes with decreasing inflow. The minimum 

DO concentrations in the bottom layer vary also with tidalprism thickness, although the rate of fresh-water inflow is 

the dominant factor. Bottom DO concentrations decrease in 

an upstream direction. The minimum recorded DO values, 

obtained during late summer, consistently occur in the bottom 

layer at station 7.7; however, results from manual sampling 

showed that a seasonal DO sag oscillates between stations 

10.4 and 7.7. At low rates of fresh-water inflow and minimum 

tidal exchange, DO content in the minimum reach has been 

less than 1 mg/1 at times.

6. Information available from previous studies failed to indicate 

a decrease in DO content in either the upper (fresh-water) 

or lower (salty water) layer during the period 1949-56. Data 

from this study suggest a slight general decrease in the 

annual minimum DO concentrations since the pre-1960 studies, but additional data are needed to evaluate this suggested 

change.

7. Phytoplankton blooms occur in the estuary only when combinations of hydrographic and climatic conditions are favorable. The DO of the surface layer is increased as a result of 

photosynthesis of the plants composing the bloom, but the 

DO of the bottom layer is decreased as the plants die and 

settle.

8. The chief factors controlling whether or not a phytoplankton 

bloom will occur are: (a) fresh-water inflow rate, (b) tidalprism thickness, (c) water temperature, and (d) solar radiation. Determination of the relative importance and critical 

ranges of these factors must await further study during subsequent blooms. Concentrations of nutrients (nitrogen and 

phosphorous) in the upper layer do not control the occurrence 

of a bloom, because sufficient nutrients are always present. 

Nutrients from the effluent of the Renton Treatment Plant 

may increase the biomass produced by the phytoplankton 

blooms, but previous data showing saturation values of DO 

indicate that a phytoplankton bloom occurred prior to installation of the Renton Treatment Plant. Trace-element studies 

have not defined any possible role that these elements play 

in algal growth.
C72 ENVIRONMENTAL QUALITY

9. Extensive experiments and collection of data on BOD have 

failed to produce a method that was satisfactory for determining quantitatively the actual BOD in the estuary. In 

general, BOD was greater in surface samples than in bottom 

samples for most tide conditions and seasons; however, the 

bottom samples showed the greater BOD during some periods 

of low fresh-water inflow.

10. The inflowing fresh water is low in total dissolved solids and is a 

calcium bicarbonate-type water. The general chemical character of the salty water in the estuary may be represented simply as various percentages of the fresh water of the Duwamish 

River and the salt water of Elliott Bay. Once-monthly analyses suggest that the nitrate concentrations in the inflowing 

fresh water increased during the period 1960-66. Nitrate 

concentrations at station 16.2 increased to a greater degree 

than could be attributed to the Renton Treatment Plant, 

which began discharging effluent in 1965.

11. Daily mean concentrations of suspended sediment in the inflowing fresh water usually range from 20 to 200 mg/1 at station 

21.0; however, during periods of high inflow daily mean concentrations of 1,000 mg/1 have been observed. A maximum 

sediment load of 31,000 tons per day occurred on January 30, 

1965, whereas minimum amounts of less than 20 tons per 

day occur frequently during low-flow periods in the summer. 

The streambed material is eroded during periods of peak flow, 

and the bed may be deepened locally as much as 7 feet during 

such times. Bottom deposits contain large amounts of coal 

particles from upstream sources, and this coal prevents the 

quantitative determination of other organic material by carbon analysis.

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 & U.S. GOVERNMENT PRINTING OFFICE: 1972 0 441-242