Measurement of Salt-Wedge 

Excursion Distance in the 

Duwamish River Estuary, 

Seattle, Washington, by Means of 

the Dissolved-Oxygen Gradient

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-D

Prepared in cooperation with the 

Municipality of Metropolitan Seattle
Measurement of Salt-Wedge 

Excursion Distance in the 

Duwamish River Estuary, 

Seattle, Washington, by Means of 

the Dissolved-Oxygen Gradient

By W. A. DAWSON and L. J. TILLEY

ENVIRONMENTAL QUALITY

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1873-D

Prepared in cooperation with the 

Municipality of Metropolitan Seattle

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

ROGERS G. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600252

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

Washington, D.C. 20402 - Price 25 cents (paper cover)

Stock Number 2401-2166
CONTENTS

Page

Abstract ________________________________________ Dl

Introduction _____________________________________________ ______ 1

Description of the estuary _______________________________________ 3

Properties and circulation of Puget Sound water _____ __ ______ 6

Effects of tides ___________________________________ 8

Biological phenomena and hydrodynamics _____________________ 14

Measurement of salt-wedge excursion based on dissolvedoxygen gradients ___________________________________________ 15

Conclusions _____________ ____________________________________ 24

References ____________________________________________________ 26

ILLUSTRATIONS

Page 

FIGURE 1. Map of Duwamish River estuary _______________ D4

2. Map showing location of study area relative to 

Puget Sound and to Straits of Juan de Fuca 

and Georgia ______________________________________ 6

3-11. Graphs showing:

3. Comparison of salinity and temperature 

ranges of top and bottom waters of 

Puget Sound _________________ 7

4. Distribution of salinity on a large-range

tide at high and low tides _ _____ _ _ -_- 11

5. Distribution of salinity on a small-range

tide at high and low tides ____ __ -__ - 13

6. Distribution of salinity at high and low

rates of fresh-water inflow at high tide------ 17

7. Salinities in the salt wedge at three stations upstream from the mouth of the 

estuary _____________________________ 18

8. Dissolved-oxygen gradients in the salt

wedge during periods of low and high

rates of fresh-water inflow _____________ 20

9. Shifts of the dissolved-oxygen gradient in 

the salt wedge for small tidal ranges at 

low and high tides _____________ _ 22

10. Mean dissolved-oxygen gradients in the salt 

wedge at high and at low water for 

selected tidal ranges ________________ ___ 23

11. Variation of salt-wedge excursion distance

with tidal range __________________ 25

ill

ENVIRONMENTAL QUALITY

MEASUREMENT OF SALT-WEDGE

EXCURSION DISTANCE IN THE

DUWAMISH RIVER ESTUARY, SEATTLE,

WASHINGTON, BY MEANS OF THE

DISSOLVED-OXYGEN GRADIENT

By W. A. DAWSON and L. J. TILLEY

ABSTRACT

The Duwamish River estuary has been the object of a series of comprehensive studies 

undertaken to predict the effects of the changing character of waste-water inputs on 

the water quality of the estuary. This report discusses the fresh- and salt-water relations 

of the estuary. The distance that the salt-water wedge in the estuary moves upstream 

and downstream with the tide is measured by a method that utilizes the persistence of 

the longitudinal gradient of dissolved oxygen in the salt water of the wedges. The 

method, though unorthodox, can serve as an independent check on any other measurements of tidal-excursion distance. Typical values obtained were a 1-kilometer excursion for a 1.3-meter tide range and a 3-kilometer excursion for a 3-meter tide range. 

This method of tracing the water movement seems to work because of two unusual 

aspects of the Duwamish River estuary: (1) the channel configuration is simple and 

well-suited to synoptic measurement and (2) the physical properties of the entering 

salt water are nearly constant.

INTRODUCTION

The Duwamish River estuary at Seattle, Wash., like many 

estuaries in urban areas, is undergoing- a change in the patterns 

of its waste-water inputs. The effects of this change are being 

intensively studied. An understanding of how water moves in 

and through the estuary and its rates of movement is fundamental to the study.

Salt water enters from Puget Sound at depth at the mouth 

of the estuary, which is split by Harbor Island (fig. 1), and fresh 

water enters from the Green River at the head. Within the 

estuary these two inputs are mixed, and the mixed water leaves 

the estuary in a surface layer. Relating salt- and fresh-water

Di
D2 ENVIRONMENTAL QUALITY

input to the mixed-water output requires a determination of the 

ratio of salt to fresh water in the output mix as well as a measure 

of the output volume. Although the rate of fresh-water input can 

be and is measured continuously, the total output from the estuary and the salt-water input have been measured only at the 

mouth of the estuary during intensive short-term studies of 

directional flow rates. Therefore, a better understanding of how 

the quality of water in the estuary will be affected by changing 

waste loads requires improved measurements not only of the 

total movement of water through the estuary but also of the 

short term variations in the factors that control water movement. 

The most important factor, the tide, controls both the level of the 

water and the differential rate of movement of the upper, seawardmoving, layer of mixed water and the lower, landward-moving, 

layer of sea water. This report demonstrates the stability and 

persistence of the lower layer and describes its tidal behavior.

All continuous records of water-quality parameters for the 

deeper salt-water layer have indicated that the longitudinal 

movement of salt water was a tidal oscillation, or excursion.

The writers apparently were the first to assume that this small 

estuary 13 kilometers (7 miles) long with a 3-meter (12-ft.) 

tide and a fresh-water inflow rate between 300mVmin (200 cfs) 

and 20,000nr/min (12,000 cfs) would have a persistent deeper 

salt-water layer; therefore, no previous attempts were made to 

measure systematically the movement of such a layer. As this 

presentation demonstrates, the assumption was valid. This 

analysis uses dissolved oxygen (DO), a water parameter not 

usually regarded as stable enough to serve as a tracer of water 

movement, to measure the movement of the salt-water layer.

Use of DO to trace the movement of the salt-water layer seems 

to work because of two unusual aspects of the Duwamish River 

estuary: (1) its configuration is simple and well suited to synoptic measurement and (2) the physical properties of the entering 

salt water are nearly constant year-round, at least in comparison 

with the water of the overlying layers. This second aspect permits both a ready identification of salt-water-input distribution 

in the estuary and a comparison of synoptic measurements made 

in different seasons. The constancy of the Duwamish saltwater input stems from some properties of its parent water body, 

Puget Sound, and the Sound's relationship to the Pacific Ocean. 

These aspects are discussed briefly in subsequent sections of this 

report.
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D3

This report is part of a comprehensive cooperative study of 

the Duwamish River estuary made by the U.S. Geological Survey and the Municipality of Metropolitan Seattle (Metro). The 

data on which the analysis is based were gathered during a study 

of algal growth in the river and estuary, one phase of the cooperative study.

DESCRIPTION OF THE ESTUARY

Prior to 1908, the Duwamish River was fed by the White, 

Black, and Green Rivers. In 1908, the White River was permanently diverted from the Green at Auburn (beyond the area 

shown in fig. 1). In 1916, the lowering of the level of Lake Washington reduced the Black River, which drained the lake, to a 

slough and since then the Black River, which joins the Green to 

form the Duwamish at river km 19, has supplied little water to 

the Duwamish. The Green River is now the main source of the 

Duwamish.

Because tidal rise and fall is observed above the mouth of the 

Black River, the Duwamish may be regarded as a river that is 

estuarine and tidal over its entire length (fig. 1). The estuary 

is quite narrow in proportion to its length, and along most of 

its length its banks are fairly straight and nearly parallel; 

only a few shallow, blind side channels are all that remain 

of old undredged meanders. The estuary is maintained as a 

navigational channel by dredging up to river km 10.0. At 

this point a wide space of deep water is created by the turning 

movements of the dredges. At river km 12.0, a bedrock ledge 

crosses the river and produces an 18-inch drop in the water 

surface at extreme low tides. This location, opposite a drive-in 

theater (fig. 1), is the effective present limit of navigation on 

the estuary at low tide.

The Duwamish River estuary is two-layered from its mouth 

at Elliott Bay (fig. 1) to a distance upstream that varies with 

the amount of fresh water entering its head from the Green 

River. The lower layer has a salinity within a part per thousand 

of that of Elliott Bay. The salinity of the upper layer increases 

downstream, starting with a value at the head equal to that of 

the Green River.

Green River water temperatures follow the usual annual cycle 

from around 6°C (Celsius) in winter to 24°C in summer. Because even the coldest river water is always less dense than the 

saline Puget Sound water, the fresh water overrides the salt
D4 ENVIRONMENTAL QUALITY

122°20'

47-35'

47°30' -

3 MILES

FIGURE 1. Duwamish River estuary.

water and moves seaward on top of it. The overriding upper 

layer of river water on its way seaward erodes and entrains the 

salt water below. The lower layer of nearly unmixed sea water 

is called a salt wedge because of its shape in side view (figs. 

4-6). The wedge tapers upstream, the bottom of the wedge having the steeper gradient. The toe of the salt wedge is most 

often near river km 12.0. Even at the mouth of the estuary, the
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D5

salinity of the deeper water is always greater than the salinity 

of the water at the surface. This condition is true for all river 

stages, winter and summer, and the salt wedge must be considered a permanent feature of the estuary (figs. 4-6).

The discharge of the Green River varies annually from 

sporadic peak flows of 20,000 mVmin (cubic meters per minute) 

and more to dry-weather minimums of 300-400 mVmin. High 

flows usually occur during the winter and spring concurrent 

with lowland rains and thawing of snow in the upper reaches of 

the Green River in the Cascade Range, and more predictably in 

May, June, or early July, during the main snowmelt period. 

From mid-July through September the flow of the Green River 

tends to be well below 1,000 mYmin.

Stoner (1967) compared the record from a continuous waterquality monitoring station at river km 12.6 with the hydrograph from the nearest Green River gaging station (at river 

km 21.0). The comparison of water quality with rate of inflow 

showed that the salt water that is mixed upward from the salt 

wedge into the river water will be moved upstream on a rising 

tide, reaching a point somewhere above the location of the monitor when the fresh-water inflow rate is less than 1,060 m ! /min. 

Salt water never appears at this monitor station when inflow is 

greater than 1,700 m'/min (fig. 6A).

Data from a second automatic water-quality monitor at river 

km 7.7 show that the salt wedge itself is displaced downstream 

below this point at very high rates of fresh-water inflow. However, this displacement is not shown in figures 4-6, for they do not 

depict longitudinal salinity distribution for this rate of inflow.

As the overriding upper layer erodes the surface of the salt 

wedge, the wedge contributes salt to the river water above but 

is not itself significantly diluted in return. Some salt water is 

mixed upward all the way to the surface, but only negligible 

amounts of fresh water find their way downward into the wedge 

during the net upstream advection of the salt-wedge water. 

During low river flows the vertical salinity gradient is diffuse, 

and a higher concentration of salt is present in the upper layer 

of river water in the estuary than during high flows. If the 

salt wedge at high flow is compared with that at low flow (figs. 

6A, B), the initial impression might be that more mixing occurs 

at low rates of flow; however, figures 6A and B should not be so 

interpreted because they do not show the differing rates of 

flow and, hence, differing residence times.
D6 ENVIRONMENTAL QUALITY

PROPERTIES AND CIRCULATION OF 

PUGET SOUND WATER

One of the better definitions of an estuary is "a semienclosed 

body of water which has a free connection with the open sea 

and within which sea water is measurably diluted with fresh 

water derived from land drainage" (Pritchard, 1967, p. 4). 

Fitting the Duwamish River estuary to this definition requires 

that Puget Sound be its "open sea." The Sound is, in fact, an 

estuary in its own right and is part of a larger estuarine system 

as well (fig. 2) ; it is also well suited for treatment as a seaward

CANADA

BRITISH COLUMBIA_

WASHINGTON 

n Juan Islands

UNITED STATES

FIGURE 2. Location of study area relative to Puget Sound and to Straits 

of Juan de Fuca and Georgia.
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D7

estuarine boundary. Although its surface-to-volume ratio is 

small among the world's estuaries, Puget Sound is far from 

being a fjordlike system with pronounced and persistent stratification. Owing to its bottom configuration a succession of deep 

(75-100 m) sills and constrictions alternating with large deeper 

(200-300 m) basins and to the large (2-4 m) tidal range, its 

waters are thoroughly mixed. The fresh-water inputs are relatively small and scattered, and the Sound's large volume provides enough thermal inertia so that seasonal variations in temperature are quite small (fig. 3). Refined oceanographic tech4- TO 10- 

METER

DEPTHS V)

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TOP WATERS

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SALINITY, IN PARTS PER THOUSAND

57 9 11 13 15 

TEMPERATURE, IN DEGREES CELSIUS

FIGURE 3. Comparison of salinity and temperature ranges for top and bottom waters of Puget Sound off Seattle, as measured at indicated depths 

January 1956-December 1957. Data from Collias and Barnes (1964), who 

give salinities as accurate to 0.01 ppt (parts per thousand) and temperatures to 0.01 °C.

niques of thermometry and salinity measurement are required to 

detect the stratification that does exist.

The source of Puget Sound water is another factor which helps 

to account for the constancy of its physical properties. Puget 

Sound and the Straits of Georgia and Juan de Fuca (fig. 2) 

form one large estuarine system that has a net transport of mixed 

salt water and land drainage outward at the surface and an inward flow of denser, more saline, sea water at depth (Waldichuk, 1957). The Strait of Juan de Fuca is quite deep (200- 

300 m) at Cape Flattery on the ocean end; the bottom shoals 

gently and continuously inward toward the land. Herlinvaux and 

Tully (1961) showed that the inward flow of ocean water in 

this superestuary is drawn from depths of 100-200 meters
D8 ENVIRONMENTAL QUALITY

below the surface of the Pacific Ocean. At these depths the 

water is not subject to seasonal thermal fluctuations and also has 

been away from the surface so long that it is considerably undersaturated in DO. This water finally surfaces in Admiralty Inlet 

(and in the channels between the San Juan Islands), where the 

strength of the tidal currents acting against the deep-lying sills 

is great enough to produce top-to-bottom mixing at half-tide.

In brief, the salt-wedge water of the Duwamish River estuary comes from a rather large water body which is notable for 

the stability of its temperature and salt content. As a result, 

although new salt water constantly feeds in at the mouth of the 

estuary, the salt-wedge water even as far upstream as river km 

7.7 has nearly the same salinity the year around.

The measurements of salinity (figs. 7A-C) were made in 

every month, at every time of day, and at every stage of the tide. 

Because of the constancy of its physical properties, Puget Sound 

water is (for purposes of analysis) a far more easily measurable input to an estuarine system than are many coastal waters.

Surface waters leave Puget Sound before contact with the atmosphere brings them to saturation with DO. For example, in 

1957 surface waters of central Puget Sound were saturated with 

DO only about 24 percent of the time; water at a depth of 5 

meters in the central Sound was saturated only 17 percent of the 

time, and water at 10 meters was saturated only 11 percent of 

the time. When saturation did occur only in late spring and 

early summer the high degree of oversaturation pointed to 

phytoplankton "blooms" as the source of oxygen rather than 

gas exchange at the water-air interface (Collias and Barnes, 

1964).

EFFECTS OF TIDES

Puget Sound tides are driven by the astronomically timed tide 

wave of the northern Pacnfic Ocean and are strongly influenced 

by resonance properties of the various basins and inlets of the 

Sound. Long-term measurements of the rise and fall of the 

tide at a few points on the Sound and shorter-term measurements at many other points have yielded sufficient data to permit 

publication of predicted tidal heights for any future time. Data 

on the tidal currents in the Sound also have been measured but 

in far less detail, and currents predicted from those data (with
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D9

reference to times of high and low water) are valid only for 

the central Puget Sound basins. Diurnal inequality (the tendency for succeeding highs and lows to vary greatly) is strong. 

Mean tidal ranges increase from 1.5 meters (5 ft) on Juan de 

Fuca Strait to 3 meters (10 ft) in the inlets at the head of the 

Sound. Diurnal ranges (the difference between mean higher 

high and mean lower low tide) increase from 2 meters (7 ft) to 

5 meters (16 ft) between the Strait and the head of the Sound 

(U.S. Coast and Geodetic Survey, 1952, 1967a, b).

The Duwamish tides, on the other hand, are hydraulic in the 

sense that they are driven by the difference between tidal stage 

of the Sound at the estuary mouth and river stage at the head. 

When the water level at the mouth is higher than the water 

levels in the remainder of the estuary, Puget Sound water flows 

in, raising water levels in the estuary. At these levels the water 

in turn brakes and accelerates the flow velocity of the river. 

For most small estuaries the tidal phenomena are of this hydraulic type; that is, the effect of the tide on the estuarine circulation and the amount of tidal water entering the estuary are 

rigidly controlled on the one hand by the volume and configuration of the estuary basin its length, width, taper, and geographic position and on the other hand by the entering streamflow. Some hydraulic tides are predictable, as, for example, in 

straits between large islands and the mainland, where the hydraulic heads producing the tides will be largely a function of 

differences in phase (arrival times) of the astronomic tides at the 

ends of the strait. But the hydraulic tide of the Duwamish 

River estuary, as for any small estuary, will be only as predictable as the entering streamflow.

If the tidal range on the Duwamish River estuary were near 

zero, the volumes of water in the estuary, in the salt wedge, and 

in the outgoing layer of mixed fresh and salt water would be 

relatively easy to measure. On such a tideless estuary changes 

in the position and volume of the wedge would follow changes 

in the rate of inflow of fresh water from the river, and when 

the discharge of the river was not changing rapidly, ample time 

would be available for making vertical profiles of the salinity 

distribution in the estuary.

In the presence of a strong tide with a pronounced diurnal 

inequality, however, the problem of mapping the wedge is more 

complex. The wedge position fluctuates constantly and its shape 

probably changes with the tide (figs. 4, 5). The period of time
D10 ENVIRONMENTAL QUALITY

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MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE Dll

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MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D13

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D14 ENVIRONMENTAL QUALITY

when the wedge is not changing shape or position too rapidly 

to be mapped is limited to an hour or two near the turn of the 

tide. With at most 2 hours available for measuring vertical profiles along 15 km of estuary, the choices of parameters and points 

to measure had to be made carefully for example, the number 

of DO measurements had to be limited to two at each site 

selected, although 10 measurements per site would have been 

preferable.

Flow reversal, observed even at river km 12.6 (Stoner, 1967, 

p. 253) and certainly for some distance above, indicates that 

the tidal-height fluctuations along the estuary are by no means 

attributable only to a pileup of fresh water as the instantaneous 

flow rates in the lower part of the estuary approach zero on a 

rising tide. Instead, the increase in water volume during a 

rising tide is contributed partly by salt water and partly by 

fresh. Not only has the salt wedge itself been observed to move 

upstream (fig. 4), but the mixed water on top of the wedge also 

flows upstream for short periods as well. There must be, of 

course, for each rising tide at low rates of river flow a moving 

point somewhere between km 12.6 and km 21.0 (where reversal 

has never been observed) where the river-flow velocity is zero. 

During high rates of fresh-water inflow, the movement of the 

upper-layer waters through the entire estuary probably is downstream at all stages of the tide (fig. 6A).

BIOLOGICAL PHENOMENA AND HYDRODYNAMICS

There is ample ground for belief that all biological phenomena 

in the estuary water are strongly influenced by the shift from a 

nonreversing (continuously outward) flow pattern during high 

river flow to a reversing pattern during low flow. Welch (1969) 

related both the "blooms" of phytoplankton (algae in suspension) 

and the low DO concentrations in the salt wedge to low-flow 

conditions and consequent increased retention times for both 

upper-layer water and salt-wedge water. Although he lacked the 

supporting evidence of retention-time estimates, he was able to 

show that the timing of the "blooms" was related to minimum 

tidal ranges and that the gradual decline in fresh-water inflow 

from midsummer to early autumn was closely matched by correspondingly lower minimum DO concentrations in the saltwedge water. The relation between water movement and DO 

concentration was described by Welch (1969, p. 43) as follows:
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D15

Concentrations of DO in the saline-water wedge decrease during late summer apparently because retention time of that water increases as discharge 

and consequent flushing rate decrease. Water in the part of the wedge that 

is farthest upstream has been retained in the estuary for the longest time 

without reaeration. ***There is a net upstream movement of bottom water, 

and ***the only escape for that water is through mixing and entrainment 

within the brackish surface-water layer. As retention time of the bottom 

water increases, the DO concentration decreases because the BOD [biochemical oxygen demand! has a longer time to act.

Without more detailed knowledge of the rate of this tidally 

pulsed upstream flow, it becomes nearly impossible to make 

useful interpretations of any measurements of biochemical DO 

consumption.

Upper-layer retention times are obviously more directly related to fresh-water inflow rate than are the retention times of 

the salt-wedge water; the relation is probably a continuous function. It is equally obvious that the onset and growth of flow reversal in the upper layer with declining river flow could cause 

an abrupt increase in retention time. Although figures 6A, B 

show a much greater volume of fresh water in the estuary at high 

rates of inflow than at low rates, they do not show the different 

rates of flow of the water through the estuary and therefore cannot be used to determine retention time.

MEASUREMENT OF SALT-WEDGE EXCURSION BASED 

ON DISSOLVED-OXYGEN GRADIENTS

One possible approach to measurement of wedge excursion 

distance might be to locate the toe of the wedge at different times 

during the tidal cycle. Spot probing with a salinometer can do 

this quite easily. Nevertheless, this approach was not pursued 

because the toe of the wedge is likely to be the most unstable part 

 the smallest volume fraction and the most vulnerable to 

entrainment and mixing.

Measuring the excursion distance of the submerged saltwedge water by the method here described requires only that 

some property of the wedge water be distributed in a longitudinal gradient which is both steep enough and persistent 

enough to define displacement. As a careful comparison of the 

centroid values of figure 7A-C will show, the salt-wedge salinities have a definite tendency to decline in an upstream direction
D16 ENVIRONMENTAL QUALITY

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RIVER KILOMETERS

FIGURE 6. Schematic longitudinal profile of estuary showing distribution of sal

water inflow at high tide. Numbered line represents depth distribution of 

sand. Dots indicate points of individual measurements, which were made" at 
D18 ENVIRONMENTAL QUALITY

2 -

3 -

4 -

5 -

6 -

7

8 -

- 9

100% (1453)

A. River Km 1.9 

(Spokane Street)

5 10 15 20 25 

SALINITY, IN PARTS PER THOUSAND

30

FIGURE 7. Salinities in the salt wedge (below 2-m depth) 

at three stations upstream from the mouth of the estuary. 

Number outside parentheses shows percentage of measurements that fall within outlined area. Number within 

parentheses shows number of plotted measurements represented by outlined areas.

at a rate of about 0.2 ppt (part per thousand) per km. Although 

this gradient has the required steepness, the scatter of the 

salinity values (fig. 7) weakens their usefulness as an index of 

the position of the wedge in the estuary. The same is true of 

temperatures (not shown). The individual points around which 

the envelope lines of figure 7 were constructed represent synoptic data measured with a portable instrument to a precision of 

0.1 ppt. Continuous salinity data from fixed-position recorders 

have the same (or better) precision; so the possibility exists 

that a systematic examination of continuously recorded salinities 

might indeed yield usable estimates of excursion distance.
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D19

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SALINITY, IN PARTS PER THOUSAND

FIGURE 7. Continued.

The rate of decline of DO in an upstream direction along the 

length of the wedge was found to vary less from periods of low 

to high rates of fresh-water inflow (fig. 8) than was expected
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MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D21

Avenue) are shown by the curves of figure 8. Although winter 

DO-decline curves slope less (fig. SB) than summer curves (fig. 

8A), the differences in slope are smaller than expected.

The possible use of the DO data to extend the knowledge of 

the hydrodynamics of the estuary became apparent when the 

DO-decline curves for consecutive sampling periods one flood or 

one ebb apart were found to be similar. The similarity permitted an estimation of the distance a tide change of a given range 

causes the salt wedge to move upstream or downstream. All the 

measurements used for this purpose were made in the daytime; 

the first set on the first daylight turn of the tide, and the second 

set on the next succeeding turn of the tide. Repeated measuring 

throughout the year yielded data for nearly 24 pairs of saltwedge conditions, each pair representing a consecutive relation 

of one high-water and one low-water wedge condition. Tide 

range for each pair was calculated as follows:

1. "Midpoint time" was calculated for each measurement run 

from which wedge data were plotted. The midpoint time 

was half the time difference between arrival at the first 

profiling station (km 1.9) and departure from the last 

(km 10.4 or 12.6).

2. From the two midpoint times for each pair, a "sampled tide 

range" was determined by interpolation on the predicted 

tide curve for Elliott Bay. Measurement runs varied in 

duration. During the runs precise knowledge of the actual 

stage of the tide was not available, but if delays extended 

the run too far beyond slack water, the increased flow 

velocity of the estuary gave warning, and the run was 

abandoned.

3. The wedge-pair curves were sorted according to the sampled tide range and were divided into six groups, each 

group containing six curve pairs (chosen without regard to 

season of the year or rate of stream inflow).

4. Within each group the curves were sorted and composited on 

two graphs one for high and one for low tide and mean 

DO values from each sampling site were calculated for 

each graph. The line joining these mean DO values is 

called the mean gradient. Figures 9A and B show the position of the mean gradient for the group that represents the 

smallest tide ranges.
D22 ENVIRONMENTAL QUALITY

100

90

80

70

60

50

40

30

Q. 20

i I I i I I I i I i I

A. Low tide

0123456789 10 11 12

80

70

60

50

40

30

20

B. High tide

i i i

0123456789 10 11 12 

RIVER KILOMETERS

FIGURE 9. Dissolved-oxygen gradients in the 

salt wedge for small tidal ranges (0.2-0.8 m) 

at low tide (A) and high tide (B). Dashed 

line represents mean gradient for the corresponding set of gradient curves.
i

3

(D fO

SD jo

o a

13" i Ore?

<

COW 

o v> $ *'

co co

DISSOLVED OXYGEN, IN PERCENT SATUR

>o w *» 01 o> ^j 

o o o o o o 
D24 ENVIRONMENTAL QUALITY

these plots of mean DO decline one for high and one 

for low tide for each of the tide-range groups.

6. The horizontal displacements between each curve pair were 

measured at four points, as shown by the arrow pairs in 

figure 10F. The horizontal distance between the meangradient curves for each graph of figure 10 should then 

give a rough indication of salt-wedge excursion distances 

for the corresponding tidal ranges; for example, for tidal 

ranges between 3.0 meters and 4.2 meters, an excursion of 

about 3.6 km might be expected (fig. 10F). A plotting of 

all the displacements indicated by figure 10 against their 

mean tidal range yielded the graph in figure 11; from this 

graph an estimate of salt-wedge excursion for any tidal 

range can be determined.

Strictly speaking, the path of any water particle through a 

tidal excursion should be such that the volume of water between 

the water particle and a hypothetical upstream point of zero 

tide is a constant (Thames Survey Committee and Water Pollution Research Laboratory, 1964, p. 6-7). The effect of this type 

of water movement in a tapering estuary would be a divergence 

in the upstream direction between the line pairs of gradients 

depicted in figures 9 and 10. Apparently the natural taper of 

the estuary has been adjusted by man to such a small value that, 

in a first approximation, such a divergence can be neglected. 

Accordingly, only one excursion value; the mean of four measurements, was taken from each of the plots of figure 10.

CONCLUSIONS

A close relationship between tidal excursion (longitudinal 

water movement) and tidal range (rise and fall of water level) 

is only to be expected in a nonestuarine embayment. Estuaries 

with their irregular fresh-water inputs are known to be less 

predictable in their tidal behavior. The data on which this 

analysis of hydrodynamics of the Duwamish River estuary is 

based were gathered over a wide range of fresh-water inflow 

rates, and the writers, who made these observations, have had 

ample first-hand experience with the changes which these inflow 

variations can produce in the estuary as a whole. One discovery 

is that one of the estuary's components, the salt wedge, seems to 

have more regularity to its behavior than had been suspected.

It might be assumed that the excursion-range relationship was 

also an excursion-stage relationship. This assumption implies
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D25

O 2

z

2 3 

TIDAL RANGE, IN METERS

FIGURE 11. Variation of salt-wedge excursion distance with tidal range. 

Each point is the mean of four measurements; vertical bars show excursion-distance range for a given tidal range.
D26 ENVIRONMENTAL QUALITY

that the wedge moves as far on one foot of tidal rise (or fall) as 

the average change in excursion with a one-foot difference in 

tidal range. If this were true for all values of fresh-water inflow, the task of formulating the hydrodynamics of the Duwamish 

would be vastly simplified. However, it is far more likely that 

variations in fresh-water inflow induce corresponding temporary changes in the wedge's behavior. The formal description 

of the estuary will require that these changes be quantified by 

means of further hydrodynamic studies.

The indicated relationships between tidal ranges and saltwedge excursions have almost certainly been rendered simpler as 

a result of man's modification of the channel and of the freshwater flow of the Duwamish River. Similarly, these and other 

relationships and processes within the estuary could be changed 

by further channel modifications. For example, an extension of 

the present navigation channel farther upstream would also 

lengthen the salt wedge. Conceivably, an increased wedge length 

could result in an increase in wedge-water erosion rate and 

entrainment of salt water by the upper-layer water, and the increased entrainment might produce some benefit by increasing 

the dilution of any pollutants in the upper layer. On the other 

hand, a lengthened salt wedge would mean increased traveltime 

of the wedge water to the upstream end and would thereby give 

oxygen-consuming processes more time to act. In this manner, 

the apparently unrelated action of dredging would cause a local 

degradation of water quality that would decrease the ability of 

the estuary to assimilate wastes and support fish life.

Thus the results of upsetting the delicate balance by arbitrarily 

altering the estuary can be beneficial or detrimental. Before 

prediction of all the imbalances and their ultimate consequences 

is possible, however, hydrodynamic analysis of the estuary must 

progress considerably beyond the present state of knowledge.

REFERENCES

Collias, E. E., and Barnes, C. A., 1964, Physical and chemical data for 

Puget Sound and approaches. September 1956-December 1957: Univ. 

Washington [Seattle] Dept. Oceanog. Tech. Kept. 110, p. 1-167.

Herlinvaux, R. H., and Tully, J. P., 1961, Some oceanographic features of 

Juan de Fuca Strait: Jour. Fisheries Research Board Canada, v. 18, 

no. 4, p. 1027-1071.

Pritchard, D. W., 1967, What is an estuary? in G. H. Lauff, ed., Estuaries: 

Am. Assoc. Adv. Sci. Pub. 83, p. 3-5.
MEASUREMENT OF SALT-WEDGE EXCURSION DISTANCE D27

Stoner, J. D., 1967, Prediction of salt-water intrusion in the Duwamish 

River estuary, King County, Washington, in Geological Survey research, 

1967: U. S. Geol. Survey Prof. Paper 575-D, p. D253-D255.

Thames Survey Committee and Water Pollution Research Laboratory, 1964, 

Effects of polluting discharges on the Thames Estuary: London, Dept. 

Sci. and Indus. Research, Water Pollution Research Tech. Paper 11, 

609 p.

U. S. Coast and Geodetic Survey, 1952, Tidal current charts, Puget Sound, 

Northern part [2d ed.l: Washington, D. C., U. S. Govt. Printing Office.

 1967a, Tide tables, high and low water predictions, 1968, west coast, 

North and South America Washington, D. C., U. S. Govt. Printing 

Office, 224 p.

 1967b, Tidal current tables, Pacific Coast of North America and Asia, 

1968: Washington, D. C., U. S. Govt. Printing Office, 254 p.

Waldichuk, M., 1957, Physical oceanography of the Strait of Georgia, British 

Columbia: Jour. Fisheries Research Board Canada, v. 14, no. 3, p. 

321-486.

Welch, E. G., 1969, Factors initiating phytoplankton blooms and resulting 

effects on dissolved oxygen in Duwamish River estuary Seattle, Washington: U. S. Geol. Survey Water-Supply Paper 1873-A, 62 p.

U.S. GOVERNMENT PRINTING OFFICE. 1972