1000-Road Bridge

US

101

47°33'

47°32'

124°10' 124°05' 124°

South

Fork Saml on

River

Middle Fork S almon River

North Fork Salmon River

Queet s River

Lower Salmon River

North Fork

Middle Fork

South Fork

H

JULY AUG SEPT

1976

1977

1978

1979

DISCHARGE, IN CUBIC

10

100

1,000 CALAWAH RIVER (12043000)

HUMPTULIPS RIVER (12039000)

Watershed Analysis of the Salmon River 

Watershed, Washington: Hydrology

Prepared in cooperation with the 

QUINAULT INDIAN NATION

Water-Resources Investigations Report 03-4204

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Photograph of a section of the Salmon River, on the Olympic Peninsula, Washington. 

Inset shows hydrologic technician and a 5-foot wading rod. Photograph taken by 

William R. Bidlake, U.S. Geological Survey.
Watershed Analysis of the Salmon River 

Watershed, Washington: Hydrology

By William R. Bidlake

Prepared in cooperation with the

QUINAULT INDIAN NATION

Tacoma, Washington

2003

Water-Resources Investigations Report 03-4204

U.S. GEOLOGICAL SURVEY
For additional information write to:

Director, Washington Water Science Center

U.S. Geological Survey

1201 Pacific Avenue – Suite 600

Tacoma, Washington 98402

http://wa.water.usgs.gov

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Suggested citation:

Bidlake, W.R., 2003, Watershed analysis of the Salmon River Watershed, Washington–Hydrology: U.S. 

Geological Survey Water-Resources Investigations Report 03-4204, 34 p.
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

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

Key Hydrology Issues and Key Questions........................................................................................................... 3

Responses to Key Questions ................................................................................................................................ 4

1. What Are the Natural Physical and Biological Features of the Watershed That Control 

Hydrologic Responses? .................................................................................................................... 4

Climate....................................................................................................................................... 4

Physiographic Influences on Local Climate of the Salmon River Watershed........................... 6

Geology and Soils ...................................................................................................................... 7

Vegetation .................................................................................................................................. 8

2. What Is Known About Current Streamflow Characteristics, Including Peak and Low Flows?............ 10

Peak-Flow Characteristics of the Salmon River ........................................................................ 10

Low-Flow Characteristics of the Salmon River......................................................................... 14

Estimating the 5- and 20-Year 7-Day Low-Flow Discharges at the 

1000-Road Bridge Gage Site.................................................................................. 15

Testing the Low-Flow Estimation Procedure Using Long-Term 

Stations on the Calawah and Humptulips Rivers ................................................... 19

Low Flows at Selected Sites on the Salmon River and Its Tributaries 

During August and September 1999 ...................................................................... 22

3. What Evidence Exists That Forest Harvesting and Road Construction Have Altered 

Frequency and Magnitude of Peak and Low Flows? ....................................................................... 25

4. What Is Currently Known About the Distribution and Extent of Wetlands, and 

What Have Been the Impacts of Land Management Activities on Wetlands? ................................ 29

5. How Might Existing Hydrologic Monitoring Be Maintained or Augmented to Help 

Detect Future Hydrologic Change, to Preserve Critical Ecosystem Functions, 

and to Protect Public and Private Property? ..................................................................................... 30

Summary .............................................................................................................................................................. 31

Literature Cited .................................................................................................................................................... 33
iv Figures

FIGURES

Figure 1. Map showing location of the Salmon River watershed in western Washington ............................... 2

Figure 2. Graph showing average monthly precipitation at Clearwater, Washington, during 

water years 1932 to 1998 ................................................................................................................... 5

Figure 3. Graph showing monthly average of daily maximum and minimum air temperature and 

average of maximum and minimum air temperature at Clearwater, Washington, during 

water years 1939 to 1998 ................................................................................................................... 5

Figure 4. Graphs showing annual precipitation at Clearwater, Washington, during water 

years 1932 to 1998............................................................................................................................. 6

Figure 5. Map showing precipitation zones in the Salmon River watershed, delineated by the 

Washington Department of Natural Resources.................................................................................. 9

Figure 6. Map showing hydrologic measurement sites, wetlands, and selected cultural features in the 

Salmon River watershed and vicinity ................................................................................................ 11

Figure 7. Graph showing average monthly discharge of the Salmon River near the 1000-Road 

Bridge during water years 1995 to 1998............................................................................................ 12

Figure 8. Graph showing average monthly precipitation at Clearwater, Washington, during 

water years 1995 to 1998 ................................................................................................................... 12

Figure 9. Graph showing relation between annual unit-area discharge of the Salmon River near 

the 1000-Road Bridge and annual precipitation at Clearwater, Washington, during 

water years 1995 to 1998 ................................................................................................................... 13

Figure 10. Graph showing annual instantaneous peak discharge of the Salmon River at the gaging 

station near the 1000-Road Bridge for water years 1995 to 1999 ..................................................... 13

Figure 11. Graphs showing daily mean discharge of the Calawah River near Forks, Washington, 

(USGS station 12043000) and of the Salmon River near the 1000-Road Bridge during 

July to September of 1995 to 1998 and for which sufficient data are available................................ 17

Figure 12. Graphs showing relation between monthly minimum of daily mean discharge of the 

Calawah River near Forks, Washington, (USGS station 12043000) and concurrent 

discharge of the Salmon River near the 1000-Road Bridge during July to September 

of 1995, 1996, and 1998 .................................................................................................................... 18

Figure 13. Graphs showing annual 7-day low flows of the Calawah River near Forks, Washington, 

(USGS station 12043000) and of the Humptulips River near Humptulips, Washington, 

(USGS station 12039000), as computed from gaging station records and as depicted by 

frequency curves based on the log-Pearson Type III probability distribution................................... 19

Figure 14. Graphs showing daily mean discharge of the Calawah River near Forks, Washington, 

(USGS station 12043000) and of the Humptulips River near Humptulips, Washington, 

(USGS station 12039000) during July to September of years 1976 to 1979..................................... 20

Figure 15. Graph showing relation between monthly minimum of daily mean discharge of the 

Calawah River near Forks, Washington, (USGS station 12043000) and concurrent discharge 

of the Humptulips River near Humptulips, Washington, (USGS station 12039000) for 

July to September of years 1976 to 1979........................................................................................... 21

Figure 16. Graphs showing unit-area discharge of the Salmon River and its tributaries at the 

low-flow measurement sites (table 3), and 24-hour precipitation totaled at 1:00 P.M. 

on the indicated day, at Black Knob, Washington............................................................................. 23

Figure 17. Graph showing discharge of the Salmon River and selected tributaries at selected low-flow 

measurement sites, as related to drainage area .................................................................................. 24
Tables v

TABLES

Table 1. Precipitation zone area, in acres rounded to the nearest 100 acres, by subwatershed ....................... 7

Table 2. Contributing drainage area and computed and estimated 7-day low flow discharge 

for stream gaging stations on the Salmon, Calawah, and Humptulips River .................................... 18

Table 3. Salmon River watershed low-flow discharge measurement sites and measurement dates ............... 22

Table 4. Wetland area, in acres, for each subwatershed of the Salmon River watershed................................ 29

Table 5. Number of intersections of roads with wetlands and mileage of roads within wetlands 

in the Salmon River watershed .......................................................................................................... 29
vi Conversion Factors and Datum

CONVERSION FACTORS AND DATUM

CONVERSION FACTORS

Temperature in degrees Fahrenheit (oF) may be converted to degrees Celsius (oC) as follows:

oC=(oF-32)/1.8.

DATUM

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD of 29).

Multiply By To obtain

Length

inch (in) 2.54 centimeter

foot (ft) 0.3048 meter 

mile (mi) 1.609 kilometer 

Area

acre 4,047 square meter

acre 0.4047 hectare

acre 0.004047 square kilometer

square mile (mi2) 259.0 hectare

square mile (mi2) 2.590 square kilometer

Volume per unit time

cubic foot per second (ft3/s) 0.02832 cubic meter per second

cubic foot per second per square mile [(ft3/s)/mi2] 0.01093 cubic meter per second per square kilometer
Introduction 1

Watershed Analysis of the Salmon River Watershed, 

Washington: Hydrology

By William R. Bidlake

ABSTRACT

The U.S. Geological Survey analyzed 

selected hydrologic conditions as part of a 

watershed analysis of the Salmon River watershed, 

Washington, conducted by the Quinault Indian 

Nation. The selected hydrologic conditions were 

analyzed according to a framework of hydrologic 

key questions that were identified for the 

watershed. The key questions were posed to better 

understand the natural, physical, and biological 

features of the watershed that control hydrologic 

responses; to better understand current streamflow 

characteristics, including peak and low flows; to 

describe any evidence that forest harvesting and 

road construction have altered frequency and 

magnitude of peak and low flows within the 

watershed; to describe what is currently known 

about the distribution and extent of wetlands and 

any impacts of land management activities on 

wetlands; and to describe how hydrologic 

monitoring within the watershed might help to 

detect future hydrologic change, to preserve 

critical ecosystem functions, and to protect public 

and private property.

INTRODUCTION

An analysis of the Salmon River watershed, on 

the western Olympic Peninsula in Washington (fig. 1), 

was conducted by the Quinault Indian Nation in 1999. 

The analysis was initiated to better understand and 

document the state of natural and cultural resources 

and selected ecosystem components within the 

watershed. The U.S. Geological Survey (USGS) 

participated in the watershed analysis by performing 

analyses of selected hydrologic conditions, and by 

writing and contributing a chapter (termed a "module 

report") for the hydrologic analysis to the overall 

Salmon River watershed analysis report (Salmon River 

Watershed Analysis Team, 2002).

The purpose of this report is to present the 

hydrology module report that was prepared for the 

Salmon River watershed analysis report. The 

hydrology module report is organized around the key 

questions for the hydrology issues that were developed 

through consensus among participants in the Salmon 

River watershed analysis.
2 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

Taholah

CLALLAM

JEFFERSON

MASON

GRAYS

HARBOR

PACIFIC LEWIS

THURSTON

PIERCE

KING

SNOHOMISH

SKAGIT

WHATCOM

KITSAP

ISLAND

SAN JUAN

BRITISH COLUMBIA

WASHINGTON

I

5

I

5

US

101

US

101

SR

109

SR

8

US

12

US

101

48o

47o

49o 124o 123o 122o 

wah R.

Nort h Fork

South Fork

Hoh River South F ork

North Fork

Clearwa te r River

Queets

River

Quinault

River

Wes t Fork

East Fork

Humptulips River

Sa lmon River

Neah Bay

Seattle

Olympia

Forks

Queets

Location of

figures 5 & 6

Ocean

Shores

Westport

Hoquiam

Aberdeen

Port Angeles

Clearwater

Black Knob

WASHINGTON

EXPLANATION

STREAMFLOW GAGING STATION

WEATHER STATION

SALMON RIVER WATERSHED

Figure location

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Figure 1. Location of the Salmon River watershed in western Washington.
Key Hydrology Issues and Key Questions 3

KEY HYDROLOGY ISSUES AND KEY 

QUESTIONS

Watershed analysis of the Salmon River 

watershed was conducted using the guide "Ecosystem 

Analysis at the Watershed Scale Federal Guide for 

Watershed Analysis," hereafter, "Federal Guide" 

(Regional Interagency Executive Committee, 1995). 

Watershed analysis by the Federal Guide is a flexible, 

issue-driven analysis that is focused on specific issues 

or concerns for maintenance or restoration of important 

watershed processes or functions. The key hydrologic 

issues for the Salmon River watershed are:

 A. Maintaining flows in the Salmon River and 

its tributaries that are, to the degree possible, 

beneficial to human uses and that promote 

maintenance or restoration of critical 

ecosystem functions; and

 B. Effects of management on wetland form and 

function.

These issues were addressed through a keyquestion process, which is the principal vehicle 

prescribed by the Federal Guide for advancing through 

watershed analysis. The key questions are:

1. What are the natural, physical, and biological 

features of the watershed that control hydrologic 

responses? 

2. What is known about current streamflow 

characteristics, including peak and low flows? 

3. What evidence exists that forest harvesting and 

road construction have altered frequency and 

magnitude of peak and low flows?

4. What is currently known about the distribution and 

extent of wetlands, and what have been the 

impacts of land management activities on 

wetlands?

5. How might existing hydrologic monitoring be 

maintained or augmented to help detect future 

hydrologic change, to preserve critical ecosystem 

functions, and to protect public and private 

property?

The key questions were researched using the 

scientific literature, a brief field investigation, and 

hydrologic, climatic, and resource mapping databases 

maintained by tribal and governmental agencies. The 

Quinault Indian Nation furnished all watershed maps 

and associated geographical statistics presented in this 

hydrology module report to the author. Data for 

precipitation and air temperature at Clearwater were 

read from a CD-ROM (compact disk-read-only 

medium) produced by Hydrosphere Data Products, Inc. 

(1999). Streamflow data from the gaging station near 

the 1000-Road Bridge were provided by George C. 

Onwumere, Quinault Indian Nation, Taholah, 

Washington (written and electronic commun., August 

1999), and by Don Richardson of Gig Harbor, 

Washington (written commun., August 1999).
4 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

RESPONSES TO KEY QUESTIONS

1. What Are the Natural Physical and Biological 

Features of the Watershed That Control 

Hydrologic Responses?

For this report, hydrologic response is defined as 

a change in a hydrologic variable as a result of changes 

in rates at which water is added to or taken away from a 

hydrologic system. Hydrologic variables include 

streamflow, overland flow rate, water yield, stream and 

lake stage, hydraulic head, hydraulic gradient, groundwater recharge and discharge, evaporation, 

transpiration, ground-water flow rate, soil water 

content, and snow and ice water equivalent.

Climate

Variations of weather and climate are probably 

the most important determinants of hydrologic 

responses in most watersheds. Weather refers to 

meteorological conditions, such as precipitation or air 

temperature, as they occur at any given instant, and 

climate refers to the average weather conditions or to 

recurrent seasonal weather variations during some 

number of years. The location of the Salmon River 

watershed on the west coast of North America at a 

latitude of about 47 degrees N. subjects it to strong 

maritime influences. Westerly trending winds bring 

moist air to the west coast of North America from the 

Pacific Ocean along pathways that vary with the season 

of the year (Hirschboeck, 1991). During summers, the 

pathways are shifted as far north as 60 degrees N. 

latitude and during winter they are shifted as far south 

as 35 degrees N. latitude. Summers are dry as a result 

of the northerly pathways of moisture delivery, 

combined with the proximity of a persistent area of 

high atmospheric pressure in the North Pacific Ocean, 

and the stabilizing effects of the cold California 

Current. During fall and winter, major rain and 

windstorms are brought onto the west coast of North 

America at mid-latitudes by strong westerly winds that 

have shifted to more southerly pathways. These large 

winter storms typically approach the Olympic 

Peninsula from the Pacific Ocean following a 

southwest-to-northeast track because they are spawned 

by systems of low atmospheric pressure over the ocean 

and driven by winds that circulate counterclockwise 

around the pressure systems.

Seasonal variations of precipitation can be seen 

by examining long-term average monthly precipitation. 

Average monthly precipitation measured at nearby 

Clearwater (fig. 1) for water years 1932 to 1998 ranged 

from approximately 3 inches for June to August to 18 

inches for December (fig. 2). The water year begins 

October 1. The annual total of the monthly averages is 

118 inches, which can be considered to be the average 

annual precipitation at Clearwater for the 1932-to-1998 

period. 

January was the coldest month at Clearwater 

from the time of the earliest available air-temperature 

recordings in 1939 to 1998 (fig. 3). The average of 

daily maximum and minimum air temperatures during 

January was 40 degrees Fahrenheit (°F). August was 

the warmest month with an average of daily maximum 

and minimum air temperatures of 60°F. The long-term 

(1939 to 1998) monthly average of daily maximum air 

temperature ranged from 46°F in December and 

January to 71°F in August, which was a seasonal span 

of 25 degrees. The long-term monthly average of daily 

minimum air temperature ranged from 34°F in January 

to 49°F in July and August, which was a season span of 

15 degrees. 

Superimposed on seasonal variations of weather 

in the Pacific Northwest are year-to-year variations and 

variations due to long-term regional climate shifts. 

Year-to-year variations of precipitation at Clearwater 

are depicted in figure 4. In addition, the U. S. 

Geological Survey (1996) has identified two long-term 

climate shifts for Washington State during the 

twentieth century. A relatively dry climate period 

ended in the late 1940s and was followed by a 

relatively wet climate period that ended in 1977. Since 

1977, a relatively dry climate has persisted. The 

climate shifts were determined from analysis of the 

long-term precipitation record, and they reflect 

multi-year climate trends that might or might not be 

reflected by the amount of precipitation in any given 

year. For example, total precipitation during any given 

year of a relatively dry period could be larger or 

smaller than the long-term average precipitation, but 

average annual precipitation for the entire dry period 

will be smaller than the long-term average annual 

precipitation. 
Responses to Key Questions 5

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

0

5

10

15

20

PRECIPITATION, IN INCHES

Figure 2. Average monthly precipitation at Clearwater, Washington, during water years 1932 to 1998.

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

AIR TEMPERATURE, IN DEGREES FAHRENHEIT

20

30

40

50

60

70

80

MINIMUM

AVERAGE

MAXIMUM

Figure 3. Monthly average of daily maximum and minimum air temperature and average of maximum and minimum air 

temperature at Clearwater, Washington, during water years 1939 to 1998.
6 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

1930 1940 1950 1960 1970 1980 1990 2000

0

60

100

140

180

20

40

80

120

160

PRECIPITATION, IN INCHES

WATER YEAR

Figure 4. Annual precipitation at Clearwater, Washington, during water years 1932 to 1998. 

Gaps (for example, 1941 to 1948) indicate sufficient data are not available.

Precipitation at Clearwater reflected the climate 

shift of the late 1970's. Annual (water-year) 

precipitation at Clearwater from 1950 to 1977 averaged 

123 inches, whereas annual precipitation from 1978 to 

1998 averaged 110 inches. Measured annual 

precipitation at Clearwater from 1932 to 1949 averaged 

130 inches, which suggests the climate there became 

drier in the late 1940's instead of wetter as the U. S. 

Geological Survey reported (1996); however, much of 

the precipitation record for Clearwater from 1932 to 

1949 is missing (fig. 4) and this confounds detection of 

the late 1940's climate shift.

Physiographic Influences on Local Climate of the Salmon 

River Watershed

The mountainous nature of the Salmon River 

watershed contributes to the abundant precipitation and 

to the accumulation and melt of snow. The higher 

mountain ridges in the watershed reach altitudes of 

approximately 2,700 feet. When weather systems 

moving in from the ocean encounter such high-relief 

terrain, some of the moisture-laden air within the 

systems is forced to higher altitudes, a process termed 

"orographic lifting," where total atmospheric pressure 

and density decrease. When the pressure on a parcel of 

moist air decreases, temperature of the parcel 

decreases. As temperature of a parcel decreases, its 

ability to hold moisture decreases. For example, a 

parcel of air at a temperature of 40°F can hold only 

about one half as much moisture as can a parcel with a 

temperature of 60°F (Campbell, 1977, p. 150). As a 

result of the lifting and cooling of air, some of the water 

condenses to form clouds and precipitation (Wallace 

and Hobbs, 1977, p. 232). Owing to orographic lifting, 

precipitation amounts probably vary considerably 

within the Salmon River watershed; however, 

precipitation records for the ridges and valleys with 

which to confirm this are not available.

One source for estimates of spatially varying 

precipitation for the Salmon River watershed is PRISM 

(Parameter-elevation Regressions on Independent 

Slopes Model; Oregon Climate Service, 2000). 

PRISM-produced maps of precipitation for 1961 to 

1990 indicate that average annual (calendar year) 

precipitation ranged from 117 inches for most of the 

Lower Salmon River subwatershed to 165 inches for 

the highest terrain of the North Fork and Middle Fork 

subwatersheds. The watershed-average annual 

precipitation from PRISM for 1961 to 1990 was 

132 inches.
Responses to Key Questions 7

Some of the terrain in the Salmon River 

watershed is at high enough altitude that some of the 

winter precipitation falls as snow. Snowmelt can 

contribute to flooding, particularly during winter when 

infiltrability of soils, the ability of soils to accept water, 

is small because the soils are saturated or partially 

frozen. Evaporative demand is small during winter 

months, and much of the water from snowmelt can 

become available for runoff. Some of the water from 

melt in deep snowpacks is stored within the interstitial 

spaces of the pack and is therefore not immediately 

available for runoff. Shallow, transient snowpacks, 

which can melt completely during rain and wind 

storms, are the most efficient snowpacks for yielding 

water for runoff because water from melt is applied 

directly to the soil surface. Water from melt contributes 

to the water available for runoff during rain-on-snow 

events, and the Washington State Department of 

Natural Resources (WADNR) (Washington State 

Department of Natural Resources, 1995) indicates that 

rain-on-snow events are a major cause of flooding in 

Washington State. 

The likelihood of rain-on-snow events apparently 

varies with site altitude, and the WADNR has mapped 

the State of Washington according to 5 altitude zones 

of varying likelihood of rain-on-snow events. The 

altitude zones are termed "precipitation zones." Rainon-snow events, according to the WADNR (1995), are 

most likely in the peak rain-on-snow precipitation 

zone. The rain-dominated and snow-dominated 

precipitation zones, which are immediately below and 

above the peak rain-on-snow zone, have the second 

highest likelihood for rain-on-snow events. 

Collectively, the rain-dominated, peak rain-on-snow, 

and snow-dominated precipitation zones are the three 

precipitation zones with the greatest likelihood for 

flood-producing rain-on-snow events. Finally, the 

lowland and highland zones have the smallest 

likelihood for rain-on-snow events.

Precipitation zones for the Salmon River 

watershed are shown in figure 5. Sixty-six percent of 

the terrain of the Salmon River watershed lies at 

altitudes that encompass the peak rain-on-snow or raindominated precipitation zones (table 1). Of the 13,600 

acres within zones susceptible to rain-on-snow events, 

approximately 18 percent are within the Lower Salmon 

River subwatershed, approximately 21 percent are 

within the North Fork subwatershed, approximately 

33 percent are within the Middle Fork subwatershed, 

and approximately 27 percent are within the South 

Fork subwatershed (table 1).

Geology and Soils

The geology of the Salmon River watershed 

affects hydrologic responses in the watershed insofar as 

it controls bed surface slopes and gradients in stream 

channels and as it affects substrate materials that 

control or mediate subsurface flow. Ridges within the 

North Fork, Middle Fork, and South Fork 

subwatersheds are composed of rocks of sedimentary 

origin, such as sandstone and shale (Tabor and Cady, 

1978). The sides of the ridges are very steep. As a 

result, the hydrologic response to storms of many 

smaller, steep-gradient tributaries is prompt. In 

contrast, most of the Lower Salmon River

Table 1. Precipitation zone area, in acres rounded to the nearest 100 acres, by subwatershed

Subwatershed

Precipitation zone

Total

Lowland Rain dominated Peak rain-onsnowSnow dominated Highland

Lower Salmon River 6,900 2,100 400 0 0 9,400

North Fork 0 1,200 1,700 0 0 2,900

Middle Fork 0 1,700 2,800 0 0 4,500

South Fork 0 2,400 1,300 0 0 3,700

Percentage of total area 34 36 30 0 0 20,500
8 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

subwatershed is coastal piedmont consisting of porous, 

unconsolidated deposits of Olympic alpine glaciers, 

including gravels, sands, silts, and clays (Tabor and 

Cady, 1978). As a result of the decreased bed gradient 

and the larger capacity for bank storage in the porous 

materials of the piedmont, the hydrologic response of 

streams in that subwatershed can generally be expected 

to be relatively slow compared to streams of the more 

mountainous subwatersheds, except for isolated 

reaches in the lowlands where the channel is incised to 

sedimentary bedrock, such as between river miles 3.7 

and 4.0 (James E. O Connor, U.S. Geological Survey, 

written commun., December 1999). The composition 

and spatial distribution of geologic materials obviously 

have a major influence on ground-water flow and 

recharge and on interactions between ground- and 

surface-water bodies. Little is known about ground 

water within the Salmon River watershed because, 

apparently, no investigations have been conducted to 

document occurrence, recharge, quality, or flow of 

ground water.

Vegetation

The physical environment of the Salmon River 

watershed, particularly its temperate maritime climate, 

makes it suitable for the establishment and growth of 

stands of long-lived evergreen conifers such as Sitka 

spruce, western hemlock, and Douglas fir. Prior to the 

beginning of logging during the 1900s, the watershed 

was thickly covered with late-successional conifer 

stands, with relatively small patches of younger stands 

that had established following destruction wrought by 

wind storms and fires. Riparian areas that were 

frequently subjected to destructive floods or debris 

flows were inhabited by stands of deciduous hardwood 

trees, primarily red alder. Coniferous forests occupy 

greater than 80 percent of the watershed and the 

remaining watershed area is occupied by hardwood 

forest and by such features as non-forest wetland, rock 

outcrops, human developments, and recent clear-cuts 

(Rodney F. Mayte, U.S. Forest Service, written 

commun., February 2001). 

Vegetation performs a number of functions that 

can influence hydrologic responses. First, terrestrial 

vegetation mediates the exchange of water and energy 

between the atmosphere and the soil and thereby exerts 

control on the amount of water that is stored in the soil 

and available to recharge streams or ground water. For 

example, vascular plants can absorb water from the soil 

through their roots and transport it to their leaves, 

where the receipt of energy from the atmosphere causes 

the water to evaporate. The nature of the leaf surface, 

such as how efficiently it absorbs down welling solar 

radiation, in part determines the magnitude of 

evaporation and, ultimately, the amount of water 

withdrawn from the soil. During rainfall, some of the 

precipitation is intercepted and stored on the surfaces 

of the vegetation. Some of the intercepted water can 

drip from the vegetation to the soil, and some of the 

water evaporates into the atmosphere. The process by 

which water evaporates from wetted vegetation is 

termed "interception loss." In some situations, the 

presence of vegetation can lead to increased inputs of 

water to the soil surface. Vegetation can intercept some 

of the moisture from fog and some of that intercepted 

moisture can drip onto the soil surface.

A second function of vegetation that influences 

hydrologic responses is protection and augmentation of 

soils that can store water and retard runoff. Vegetation 

absorbs impact energy from raindrops that otherwise 

could fall on the soil surface and detach soil particles. 

Vegetation contributes large amounts of fibrous 

materials to the soil, such as leaves, twigs, and stems. 

These materials, by contributing to the overall erosion 

resistance of the soil and to the total volume of the soil, 

can increase capacity of the watershed to store water 

from storms. 
Responses to Key Questions 9

750

1000

1000

1000

1000

1000

1000

1000

2000

2000

2000

1000

2000

2000

2000

1000

1000

1000

1000

2000

2000

2000

2000

1000

1000

Lower Salmon River

North Fork

Middle Fork

South Fork

Salmon

River

South

Fork Salmon

River

Middle Fork

S a ml on River

Nor ht Fork Sa ml on River

Queet s River

500

500

500

500

500

500

500

500

500

500

500

250

250

500

250

250

250

250

250

250

250

250

750

750

750

750

750

750

750

750

750

1250

1250

1250

1500

1500

1250

750

750

1500

1500 1500

1250

1250

1250

1750

1750

1750

1750

750

750

1750

1750

1750

2250

2250

2500

2500

750

750

1250

1250

1250

1500

1500

1500

1250

1500

1500

2500

2500

2500

2250

2250

2250

2250

750

1250

1250

1250

1250

1500

1500

1750

1750

1500

1500

1500

1500

1500

1750

PRECIPITATION ZONES

LOWLAND ZONERAIN DOMINATED ZONE

PEAK RAIN ON SNOW ZONE

CONTOURS– Showing altitude of land surface. Contour interval 250 feet. NGVD 1929

0123

135 2 4 6 KILOMETERS

4 MILES

0

WATERSHED BOUNDARYSUBWATERSHED BOUNDARY

EXPLANATION

Base map modified fromQuinault Indian Nation's Department of Natural Resources,Environmental Protection Division, Watershed Analysis TeamUTM Zone 10, NAD 27, 1:50,000

Figure 5. Precipitation zones in the Salmon River watershed, delineated by the Washington Department of Natural Resources.
10 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

2. What Is Known About Current Streamflow 

Characteristics, Including Peak and Low Flows? 

A streamflow gaging station on the Salmon 

River near the 1000-Road Bridge (fig. 6) has been 

operated since 1994, and streamflow data from that 

station for water years 1995 to 1998 were used for 

computing average monthly and average annual 

discharge. Seasonal variations of discharge near the 

1000-Road Bridge were grossly similar to seasonal 

variations of precipitation at Clearwater during water 

years 1995 to 1998 (figs. 7 and 8), although the 

extremes of average monthly discharge and 

precipitation occurred during different months. The 

minimum average monthly discharge of 40 ft3/s (cubic 

feet per second) occurred during August, which was 

one month later than the precipitation minimum. The 

maximum of average monthly discharge of 595 ft3/s 

occurred during December, which was one month later 

than the precipitation maximum during water years 

1995 to 1998. Average annual discharge near the 1000-

Road Bridge for water years 1995 to 1998 was 

288 ft3/s.

Annual unit-area discharge from the drainage 

area upstream of the gage near the 1000-Road Bridge 

averaged 134 inches for water years 1995 to 1998, and 

annual precipitation at Clearwater averaged 116 inches 

during the same period. Unit-area discharge is 

computed as mean discharge divided by the 

contributing drainage area and can be expressed in the 

same units of measurement as precipitation is 

commonly expressed-inches per unit time. The 

contributing drainage area upstream of the gage site is 

29.2 mi2 (square miles). Annual unit-area discharge 

was correlated with annual precipitation at Clearwater 

during water years 1995 to 1998 (fig. 9). The trend line 

that is given in figure 9 was developed by a statistical 

analysis that also produced the coefficient of 

determination (r2). The coefficient of determination 

indicates the fraction of the variation of annual 

discharge that was explained by precipitation. Given 

that precipitation falling in the watershed is a major 

source for water that appears in the river, the strong 

explanatory power of precipitation related to discharge 

is in accordance with expectations. The fact that annual 

unit-area discharge averaged 16 percent greater than 

annual precipitation at Clearwater was consistent with 

spatially averaged precipitation in the Salmon River 

watershed being greater than precipitation at 

Clearwater, as was discussed previously. 

Peak-Flow Characteristics of the Salmon River

The magnitude and frequency of peak flows bear 

important implications for the design of manmade 

structures in the floodplain, and potentially for fish 

habitat and survival of juvenile fish. Thus, the 

characterization of peak-flow magnitude and frequency 

is important for resource management in the Salmon 

River watershed. The annual peak flow at a point on the 

stream is the maximum instantaneous stream discharge 

at the point during a given water year. Annual peak 

flow of the Salmon River near the 1000-Road Bridge 

for water years 1995 to 1999 is shown in figure 10. 

Annual peak flow is a hydrologic event that can 

be amenable to statistical description and 

characterization. One statistical characterization is the 

frequency curve of annual peak flows-the peak-flow 

frequency curve. A peak-flow frequency curve, which 

relates the magnitude of the annual peak-flow event to 

its frequency of occurrence, can be used to estimate the 

average time interval between occurrences of an annual 

peak flow of a given magnitude or larger. An N-year 

peak flow read from a frequency curve developed for a 

given site is an estimate of the peak-flow magnitude to 

be expected or exceeded on average at the site every N 

years. A peak-flow event with an N-year recurrence 

interval has a probability of 1/N of occurring during 

any year. Development and use of a peak-flow 

frequency curve for annual peak flows at a given site 

assumes those events are statistically independent and 

that the underlying physical factors that lead to peak 

flows remain constant from year to year. Few sites have 

peak flows that strictly meet these statistical 

assumptions because of such factors as climate trends 

or shifts; however, the assumptions are met well 

enough for many sites to make the curves useful tools 

in hydrologic assessment of surface waters. The peakflow frequency curve is the foundation for making 

probabilistic statements concerning magnitude and 

frequency of peak flows at sites with many years of 

flow data. Other peak-flow estimation techniques have 

been developed for estimating N-year events at sites, 

such as the 1000-Road Bridge site, where streamflow 

data are not sufficient to develop a peak-flow frequency 

curve.
Responses to Key Questions 11

Fish Culture

Facility

1000-Road Bridge

US

2

US

101

Salmon River South

Fo

rk

Salmon

River

Middle

Fork

S a ml on River

Nor ht

Fork

Sa ml on

River

Queet s

R vi er

Lower Salmon River

North Fork

Middle Fork

South Fork

A

B

C

D E

F G

H

WETLANDWATERLAKE OR PONDSAND OR GRAVEL IN OPEN WATERUNCLASSIFIEDWATERSHED BOUNDARYSUBWATERSHED BOUNDARYROADSLOW-FLOW DISCHARGE MEASUREMENT SITESTREAMFLOW GAGING STATION

0123

135 2 4 6 KILOMETERS

4 MILES

0

EXPLANATION

Base map modified fromQuinault Indian Nation's Department of Natural Resources,Environmental Protection Division, Watershed Analysis TeamUTM Zone 10, NAD 27, 1:50,000

Figure 6. Hydrologic measurement sites, wetlands, and selected cultural features in the Salmon River watershed and vicinity.
12 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

0

200

400

600

800

DISCHARGE, IN CUBIC FEET PER SECOND

Figure 7. Average monthly discharge of the Salmon River near the 1000-Road Bridge during water years 1995 to 

1998.

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

0

5

10

15

20

PRECIPITATION, IN INCHES

Figure 8. Average monthly precipitation at Clearwater, Washington, during water years 1995 to 1998.
Responses to Key Questions 13

90 100 110 120 130 140 150 160

80

100

120

140

160

180

ANNUAL SALMON RIVER UNIT-AREA DISCHARGE

NEAR 1000-ROAD BRIDGE, IN INCHES

ANNUAL PRECIPITATION AT CLEARWATER, IN INCHES

r2 = 0.92

Figure 9. Relation between annual unit-area discharge of the Salmon River near the 1000-Road Bridge and annual 

precipitation at Clearwater, Washington, during water years 1995 to 1998.

1995 1996 1997 1998 1999

0

5,000

10,000

DISCHARGE, IN CUBIC FEET PER SECOND

WATER YEAR

12/19/94 11/29/95

3/19/97

10/30/97

11/13/98

Date of peak discharge is indicated at the top of each bar

Figure 10. Annual instantaneous peak discharge of the Salmon River at the gaging station near the 1000-Road 

Bridge for water years 1995 to 1999.
14 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

One such technique is described by Sumioka and 

others (1998), who correlated magnitudes of selected 

N-year peak-flow events from frequency analysis of 

Washington gaging station records with quantifiable 

watershed characteristics (termed "explanatory 

watershed variables"), such as drainage area and mean 

annual precipitation. They developed peak-flow 

regression equations to enable estimates of selected Nyear peak flows to be made for sites where streamflow 

records are not available or not of sufficient duration to 

compute peak-flow frequency curves. Sumioka and 

others' peak-flow regression equations were applied to 

estimate the magnitude of selected N-year peak flows 

at the streamflow gaging site near the 1000-Road 

Bridge. According to the applicable peak-flow 

regression equations from Sumioka and others (1998), 

the largest gaged peak flow at the 1000-Road Bridge 

site, that of March 19, 1997, was approximately a 100-

year event. The regression-predicted, 100-year peak 

flow was 8,300 ft3/s, and the gaged peak flow on 

March 19, 1997 was 8,550 ft3/s. The annual peak flows 

for water years 1995 and 1996 that are also presented 

in figure 10 were approximately 25-year events, 

according to the applicable peak-flow regression 

equation from Sumioka and others (1998). The 

regression-predicted, 25-year peak flow was 6,550 

ft3/s, and the gaged annual peak flows for water years 

1995 and 1996 were 6,290 and 6,140 ft3/s, respectively.

It is considered unlikely that the 5 years of 

available record of annual peak flow at the 1000-Road 

Bridge site captured two 25-year peak-flow events and 

a 100-year peak-flow event as is indicated by the peakflow regression equations. More likely, the peak flows 

were specified as 25- and 100-year events because of 

such factors as the Salmon River peak-flow 

characteristics being statistically aberrant among the 

watersheds that were used to develop the equations or 

because of errors in the gaged peak flows at the 1000-

Road Bridge site. This argument is bolstered by the 

observation that the event of March 19, 1997, was 

approximately a 25-year event, both at the streamflow 

gaging station on the nearby Quinault River at Lake 

Quinault (USGS station 12039500) and at the gaging 

station on the Queets River near Clearwater (USGS 

station 12040500; U.S. Geological Survey, 1998). 

Peak-flow events at these latter two sites were 

characterized by frequency analysis of long-term 

streamflow gaging records (Sumioka and others, 1998). 

Peak-flow frequency analysis of long-term 

streamflow gaging records is in itself subject to errors 

caused by violation of the statistical assumptions that 

were discussed previously and that could be caused by 

such factors as climate shifts or trends. The implication 

of this is the peak-flow magnitudes from frequency 

analysis and the peak-flow regression equations that 

were derived from those magnitudes are sources for 

uncertainty that ultimately could reduce the accuracy of 

the regression-predicted peak flows at the 1000-Road 

Bridge site. The detailed hydrologic analysis that could 

be made to evaluate applicability of those equations for 

the 1000-Road Bridge site, which could include 

analysis for possible precipitation trends or anomalies 

or testing of the regression equations for nearby rivers 

for water years 1995 to 1999, is beyond the scope of 

this report. Uncertainties concerning magnitudes of 

recurring peak-flows at the 1000-Road Bridge site 

could in the future be addressed by additional 

hydrologic analysis. Such an analysis would be aided 

by data from the continued operation of the gaging 

station at that site. 

Low-Flow Characteristics of the Salmon River

Stream low-flow magnitude and frequency bear 

potentially important implications both for the 

uninterrupted operation of the fish culture facility and 

for the survival and growth of juvenile fish in the 

Salmon River and its tributaries. Watershed analysis 

was taken as an opportunity to develop a better 

hydrologic understanding of low-flow characteristics 

of the Salmon River. The three objectives of a brief 

low-flow investigation that was conducted during 

summer and autumn 1999 were:

1. Estimate the magnitudes of the annual 7-day low 

flows for the 5-year and 20-year recurrence 

intervals at the streamflow gaging station that is 

near the 1000-Road Bridge by correlating 

streamflow at that station with streamflow at a 

long-term gaging station on the Calawah River 

(fig. 1);

2. Test the low-flow estimation procedure employed 

for objective (1) using streamflow records from 

long-term stations on the Calawah and Humptulips 

Rivers (fig. 1);

3. Measure and describe low flows at selected sites 

on the Salmon River and its tributaries by 

conducting and interpreting seepage runs during 

the low-flow season. 
Responses to Key Questions 15

A seepage run is a series of near-simultaneous 

measurements of stream discharge that are made at 

different points on a stream. The annual 7-day low flow 

at any point on a stream is the smallest average 

discharge at the point among any 7 consecutive days 

during a single "low-flow" year. The annual 7-day low 

flow is a hydrologic event that is amenable to statistical 

treatment in much the same manner as are events such 

as the annual peak flow. A low-flow recurrence interval 

is a probabilistic statement of how often an N-day low 

flow of a given magnitude or smaller would be 

expected to occur, on average, during a period of many 

years. The low-flow year begins on April 1 and thus is 

distinct from the water year or from the calendar year. 

Estimating the 5- and 20-Year 7-Day Low-Flow Discharges at 

the 1000-Road Bridge Gage Site

The magnitudes of recurring N-day low-flows at 

a given site, such as the annual 7-day low flow, can be 

estimated using low-flow frequency curves that have 

been developed for the site (Riggs, 1972). A low-flow 

frequency curve can be developed for a site where a 

streamflow gaging station exists by statistical analysis 

of the streamflow records. For a curve to be reliable for 

estimating magnitudes of low flows at a site, the low 

flows should meet certain statistical assumptions and 

streamflow records for the site should be of a sufficient 

duration. Two important statistical assumptions are (1) 

the individual N-day low flows are statistically 

independent of each other from time to time and the 

population of those low flows does not change with 

time, and (2) the probability distribution of the N-day 

low flows is compatible with the theoretical 

distribution upon which the frequency curve is based. 

Few populations of low flows strictly meet both of 

these assumptions because of such factors as climate 

shifts or trends that potentially can cause the low flows 

to be correlated from time to time. Nonetheless, 

statistically derived low-flow frequency curves, if they 

are interpreted with the statistical assumptions in mind, 

can be useful tools for estimating frequency and 

magnitude of low flows. 

Generally, 10 or more years of discharge records 

are needed to develop reliable low-flow frequency 

curves for a given station. Riggs (1972) describes a 

procedure that can be used to estimate low-flow 

discharges for a site with less than 10 years of record if 

gaged low flows at the site correlate with concurrent 

flows at a long-term station for which the needed lowflow frequency curves can be developed. The specific 

steps of Riggs’ procedure for estimating low-flow 

characteristics for gaging stations with less than 10 

years of discharge record can be summarized as 

follows.

1. Develop a relation line or equation for predicting 

low flow at the station of interest from low flow at 

an alternative gaging station where a long-term 

record of discharge is available.

2. For the long-term station, compute a low-flow 

frequency curve for each annual N-day low-flow 

event that is of interest, where N could be any 

integer from 1 to 365 but that was 7 for this 

investigation. Use the low-flow frequency curves 

to compute the appropriate annual N-day low 

flows at the long-term station and at the desired 

recurrence intervals.

3. Use the relation line or equation of concurrent low 

flows with the N-day flows from the long-term 

station to estimate the N-day low flows at the 

appropriate recurrence intervals for the station of 

interest.

The gaging station near the 1000-Road Bridge, 

the site of interest, had been operated for less than 10 

years at the time of this analysis, and the technique of 

Riggs was used to estimate the magnitudes of the 5- 

and 20-year 7-day low-flows at that site. The 

alternative long-term station selected for this purpose 

(USGS station 12043000) was on the Calawah River 

near Forks (fig. 1). The area of the Calawah River 

watershed upstream of station 12043000 is 129 mi2, 

approximately four times that of the Salmon River 

watershed upstream of the 1000-Road Bridge. 
16 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

The Calawah station was selected because of physical 

similarities between the Calawah and Salmon River 

watersheds. The Calawah River watershed, like the 

Salmon River watershed, lies on the Pacific slope of the 

Olympic Peninsula and comprises low- to mid-altitude 

forested terrain. The two watersheds are also similar in 

that neither river drains large permanent snowfields or 

glaciers that could augment low flows during late 

summer or autumn. 

Flows gaged near the 1000-Road Bridge and on 

the Calawah River near Forks during July through 

September of years 1995, 1996, and 1998 were 

selected for developing a relation line of concurrent 

flows. The time series of daily mean discharge for the 

two stations for July to September is presented in 

figure 11. No estimated record was used in the analysis, 

and because discharge at the gaging station near the 

1000-Road Bridge was almost entirely estimated 

during 1997, data from that year were not used in the 

analysis. One pair of daily mean flows was selected 

from each month July through September, and each 

pair was selected such that the Calawah River member 

was the smallest daily mean discharge for that station 

for the month. This selection procedure provided a 

degree of selection objectivity and repeatability that 

would not have been realized had the flow pairs been 

selected, for example, from visual inspection of 

figure 11.

Graphical techniques (Riggs, 1972) were used to 

develop a relation line of low flows at the 1000-Road 

Bridge and the Calawah River gaging stations (fig. 12). 

Daily mean discharge averaged 21 ft3/s for the Salmon 

River site among the 9 days that were selected, whereas 

daily mean discharge averaged 82 ft3/s for the Calawah 

River site. That the average low flow of the Calawah 

River near Forks was roughly four times larger than the 

average low flow at the 1000-Road Bridge site could be 

explained by the fact the drainage area for the former 

site is roughly four times larger than the drainage area 

for latter site. 

For the second step, that of computing the lowflow frequency curve for the annual 7-day low flow for 

the Calawah River near Forks, the U.S. Geological 

Survey computer program Surface-Water Statistics 

(SWSTAT; Lumb and others, 1990) was used to 

compute a frequency curve for the annual 7-day low 

flow by fitting a log-Pearson Type III probability 

distribution to the available data for that station. Data 

were excluded from the analysis for years when the 

gaging station records were not complete. Riggs (1968) 

describes the log-Pearson Type III probability 

distribution, and Riggs (1972) describes the application 

of that distribution to the study of stream low flows. 

The computer program SWSTAT is available at this 

time (2003) on the World Wide Web at the universal 

resource locator (URL): http://water.usgs.gov/software/ 

swstat.html. The particular low-flow years for which 

data from the Calawah River station were used for 

fitting the frequency curve, plus the annual 7-day low 

flows for the 5- and 20-year recurrence intervals from 

the fitted frequency curve are presented in table 2. The 

series of annual 7-day low flows was tested for the 

presence of a trend, but no trend was detected. Annual 

7-day low flows computed from the gaged stream flow 

record and the fitted frequency curve for the 7-day low 

flow are presented in figure 13. The low flows specified 

by the frequency curve are themselves estimates of the 

recurring low flows. The specified flows are subject to 

errors that are due to such factors as lack of 

conformance of the gaged low flows to the statistical 

assumptions that were discussed previously, and errors 

of sampling or measurement in the gaged, annual 7-day 

low flows.

For the third step, the relation line for concurrent 

low flows was applied to yield estimates of the annual 

7-day low flow near the 1000-Road Bridge site for the 

5- and 20-year recurrence intervals. The estimates are 

presented in table 2. The estimates appear to be 

plausible. For example, the smallest gaged daily mean 

discharge at the 1000-Road Bridge site during 1995, 

1996, and 1998 was 14.2 ft3/s. This gaged, annual 

1-day low flow was 2 percent larger than the estimated 

annual 7-day low flow at the 5-year recurrence interval 

(13.9 ft3/s) that is presented in table 2.
Responses to Key Questions 17

JULY AUG SEPT

1995

1996

1997

1998

DISCHARGE, IN CUBIC FEET PER SECOND

10

100

1,000

5,000

10

100

1,000

5,000

10

100

1,000

5,000

10

100

1,000

5,000

CALAWAH RIVER (12043000)

SALMON RIVER NEAR THE 1000-ROAD BRIDGE

MEASURED INSTANTANEOUS

 DISCHARGE OF SALMON RIVER

Figure 11. Daily mean discharge of the Calawah River near Forks, Washington, (USGS station 12043000) and of the 

Salmon River near the 1000-Road Bridge during July to September of 1995 to 1998 and for which sufficient data are 

available.
18 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

10

8

20

50

20 50 100 200

DAILY MEAN DISCHARGE, SALMON RIVER NEAR1000-ROAD BRIDGE, IN CUBIC FEET PER SECOND

DAILY MEAN DISCHARGE, CALAWAH RIVER NEAR FORKS, IN CUBIC FEET PER SECOND

Figure 12. Relation between monthly minimum of daily mean discharge of the Calawah River near Forks, Washington, 

(USGS station 12043000) and concurrent discharge of the Salmon River near the 1000-Road Bridge during July to 

September of 1995, 1996, and 1998.

Table 2. Contributing drainage area and computed and estimated 7-day low flow discharge for stream gaging stations on the Salmon, Calawah, 

and Humptulips River

Station

Contributing 

drainage area 

(square miles)

Period(s) of 

station record 

used to develop 

low-flow 

frequency 

function

Annual 7-day low flow computed 

from frequency function

(cubic feet per second)

Annual 7-day low flow estimated 

using correlation with alternative, 

long-term station 

(cubic feet per second)

Recurrence interval (years) Recurrence interval (years)

5 20 5 20

Salmon River near the 

1000-Road Bridge

29.2 None Not computed Not computed 113.9 110.8

Calawah River near 

Forks (USGS station 

12043000)

129 Low-flow years 

1899 to 1901, 

1977 to 1980, 

1985 to 1998

48.6 34.4 Not estimated Not estimated

Humptulips River near 

Humptulips (USGS 

station 12039000)

130 Low-flow years 

1944 to 1979

118.3 95.5 164.2 140.8

1Alternative station was Calawah River near Forks (USGS station 12043000).
Responses to Key Questions 19

10

100

1,000

95 90 80 70 50 30 20 10 5

1.05 1.11 1.25 1.43 2 3.33 5 10 20

DISCHARGE, IN CUBIC FEET PER SECOND

ANNUAL NONEXCEEDANCE PROBABILITY, IN PERCENT

RECURRENCE INTERVAL, IN YEARS

Humptulips River

Calawah River

GAUGED 7-DAY LOW FLOW

FREQUENCY CURVE

Figure 13. Annual 7-day low flows of the Calawah River near Forks, Washington, (USGS station 12043000) 

and of the Humptulips River near Humptulips, Washington, (USGS station 12039000), as computed from gaging 

station records and as depicted by frequency curves based on the log-Pearson Type III probability distribution.

Testing the Low-Flow Estimation Procedure Using Long-Term 

Stations on the Calawah and Humptulips Rivers

To provide a sense of how reliably the annual 

7-day low flows in mid-altitude, forested watersheds 

on the Pacific slope of the Olympic Peninsula can be 

estimated for short-term stream flow stations using the 

Riggs (1972) procedure, the procedure was tested for 

two long-term stream flow gaging stations on different 

rivers in the vicinity. The alternate station was again on 

the Calawah River near Forks (USGS station 

12043000), and the station for which the 7-day low 

flow was estimated was a discontinued gaging station 

on the Humptulips River near Humptulips (USGS 

station 12039000; fig. 1) that was last operated during 

water year 1979. Testing was done using the long-term 

station on the Humptulips River because this permitted 

the low flows estimated by the Riggs procedure to be 

compared with low flows from the more reliable 

frequency curve for that station. Selection of 

concurrent flows and development of a relation line for 

those flows were performed as was described 

previously. The Humptulips River watershed upstream 

of station 12039000 is a forested, low- to mid-altitude 

watershed of 130 mi2 on the Pacific slope of the 

Olympic Peninsula. The Humptulips River, like the 

Salmon and Calawah Rivers, does not drain large 

permanent snowfields or glaciers that would augment 

low flows during late summer. The time series of daily 

mean discharge for the Calawah and Humptulips River 

stations overlap for July to September of years 1976 to 

1979. The overlapping time series are presented in 

figure 14.
20 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

JULY AUG SEPT

1976

1977

1978

1979

DISCHARGE, IN CUBIC FEET PER SECOND

10

100

1,000

5,000

10

100

1,000

5,000

10

100

1,000

5,000

10

100

1,000

5,000

CALAWAH RIVER (12043000)

HUMPTULIPS RIVER (12039000)

Figure 14. Daily mean discharge of the Calawah River near Forks, Washington, (USGS station 12043000) and of the 

Humptulips River near Humptulips, Washington, (USGS station 12039000) during July to September of years 1976 to 

1979.
Responses to Key Questions 21

The annual 7-day low flows for the period of 

record for the Humptulips River station, exclusive of 

data from years when the gage record was not 

complete, were used to develop a frequency curve in 

the manner that was described previously. The series of 

annual 7-day low flows also was tested for the presence 

of a trend, but no trend was detected. The particular 

low-flow years for which data from the Humptulips 

River station were used for fitting the low-flow 

frequency curve, plus the curve-specified annual 7-day 

low flows for the 5- and 20-year recurrence intervals 

are presented in table 2. Annual 7-day low flows 

computed from the gage record and the fitted frequency 

curve for the annual 7-day low flow are plotted in 

figure 13.

The relation between concurrent flows at the 

Calawah River and Humptulips River gaging stations is 

presented in figure 15. The Riggs procedure produced 

estimates of the annual 7-day low flows of the 

Humptulips River near Humptulips for the 5- and 20-

year recurrence intervals that compared poorly with 

flows prescribed by the low-flow frequency curve 

(table 2). The low-flow estimate for the 5-year 

recurrence interval (64.2 ft3/s) was 46 percent smaller 

than the corresponding low flow prescribed by the 

frequency curve (118.3 ft3/s), and the low-flow 

estimate for the 20-year recurrence interval (40.8 ft3/s) 

was 57 percent smaller than the corresponding flow 

prescribed by the frequency curve (95.5 ft3/s). 

50

50

100 200 300

DAILY MEAN DISCHARGE, CALAWAH RIVER NEAR FORKS, IN CUBIC FEET PER SECOND

DAILY MEAN DISCHARGE, HUMPTULIPS RIVER NEAR

HUMPTULIPS, IN CUBIC FEET PER SECOND 100

500

1,000

Figure 15. Relation between monthly minimum of daily mean discharge of the Calawah River near Forks, Washington, 

(USGS station 12043000) and concurrent discharge of the Humptulips River near Humptulips, Washington, (USGS 

station 12039000) for July to September of years 1976 to 1979. 
22 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

That the estimates of low flows for the 

Humptulips River near Humptulips that were computed 

by the Riggs procedure agreed poorly with the lowflow estimates from the frequency curve for that site 

could have been caused by at least two factors. The first 

of these potential factors is the lack of data for flows 

small enough to accurately define the relation between 

concurrent flows at the two sites at the magnitudes of 

the 5- and 20-year low-flow events. Another possible 

factor is climatically induced differences in the lowflow frequency curves for the Calawah River and the 

Humptulips River sites. The low-flow frequency curve 

for the Humptulips site was computed from a period 

(table 2) that encompassed the relatively wet period of 

the late 1940s to 1977 that was described previously. 

Conversely, most of the years (17 out of 21 years) used 

to compute the low-flow frequency curve for the 

Calawah River site were during the relatively dry 1978-

to-1998 period. Thus, the underestimation of low flows 

at the Humptulips River site could have resulted from 

estimating those flows using the frequency curve for 

the Humptulips River that was developed based on a 

relatively wet climatic regime. 

Low Flows at Selected Sites on the Salmon River and Its 

Tributaries During August and September 1999

Understanding temporal and spatial variations of 

the evolution of low flows is important for 

understanding the relative importance of various 

stream reaches in contributing to low flows and for 

describing and understanding interactions among 

ground water and surface waters. Seepage runs were 

made on the main stem of the Salmon River and its 

major tributaries during August and September 

1999 to describe variations of flow gains or losses on 

selected reaches of the river. Four seepage runs were 

conducted during which discharge was measured at 6 

to 8 sites from the mountainous subwatersheds to near 

the 1000-Road Bridge (sites A-H, fig. 6, table 3). 

Discharge at each site was measured with a verticalaxis type current meter (Smoot and Novak, 1968). An 

attempt was made to complete each seepage run during 

a relatively brief period of 24 hours and during a period 

without precipitation; however, precipitation occurred 

during the seepage runs of August 24-25 and 

September 22-23. Although the rain on September 23 

probably had minimal effect on discharge (fig. 16), the 

flows measured on August 25 on the Middle and South 

Forks near the confluence of those streams and on the 

main stem immediately downstream of the confluence 

appeared to have been affected by rain that fell the 

previous night, and those flows likely consisted of 

surface runoff as well as low flow. Despite the 

difficulties in interpreting flows that were affected by 

precipitation, the seepage runs provided information 

about low flows for some major streams in the Salmon 

River watershed.

Table 3. Salmon River watershed low-flow discharge measurement 

sites and measurement dates

Point on 

Figure 6 Description Discharge 

measurement dates

A South Fork Salmon River at 

a point approximately 

150 feet upstream of its 

confluence with the 

Middle Fork

Aug. 9 and 25, 1999

Sept. 9 and 23, 1999

B Middle Fork Salmon River at 

a point approximately 

200 feet upstream of its 

confluence with the South 

Fork

Aug. 9 and 25, 1999

Sept. 9 and 23, 1999

C Main stem Salmon River 

approximately 50 feet 

downstream of confluence 

of the South and Middle 

Forks

Aug. 9 and 25, 1999

Sept. 9 and 23, 1999

D North Fork Salmon River 

approximately 200 feet 

upstream of its confluence 

with the Main stem

Aug. 9 and 24, 1999

Sept. 9 and 22, 1999

E Main stem Salmon River 

approximately 250 feet 

downstream of its 

confluence with the North 

Fork

Sept. 22, 1999

F Main stem Salmon River near 

Forest Road Number 040

Aug. 10 and 24, 1999

Sept. 9 and 21, 1999

G Main stem Salmon River near 

fish culture facility

Aug. 10 and 24, 1999

Sept. 10 and 22, 1999

H Main stem Salmon River 

approximately 300 feet 

downstream of the 

1000-Road Bridge

Aug. 24, 1999

Sept. 10 and 22, 1999
Responses to Key Questions 23

Firstly, the South Fork contributed substantially 

less flow per unit drainage area than did the other two 

mountainous forks, the Middle and North Forks. For 

example, on August 9, the unit-area discharge of the 

South Fork near its confluence with the Middle Fork 

was 27 percent less than the unit-area discharge of the 

Middle Fork, and it was 29 percent less than the unitarea discharge of the North Fork near the confluence of 

that fork with the main stem (fig. 16). On September 9, 

the unit-area discharge of the South Fork was 20 

percent less than the unit-area discharge of the Middle 

Fork, and it was 19 percent less than the unit-area 

discharge of the North Fork. The underlying causes of 

the relatively small contribution by the South Fork are 

not known at this time but could be related to variations 

of geologic composition or to variations of road 

construction or timber harvesting practices or harvest 

extent among the mountainous subwatersheds. 

Additional investigation of contrasting road 

construction or timber harvesting practices or harvest 

extent in the uppermost reaches could help identify 

whether or not low-flow yields are controllable through 

forest management practices.

A

B

D

F

G

H

SITE

C

E

AUG 10 AUG 24 SEPT 9 SEPT 22

0

0.2

0.4

0.8

0.6

1.50

1.25

1.00

0.75

0.50 PRECIPITATION, IN INCHES

UNIT-AREA DISCHARGE, IN

CUBIC FEET PER SECOND PER SQUARE MILE

Figure 16. Unit-area discharge of the Salmon River and its tributaries at the low-flow measurement sites (table 3), 

and 24-hour precipitation totaled at 1:00 P.M. on the indicated day, at Black Knob, Washington.
24 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

Secondly, low-flow discharge increased with but 

was not linearly related to drainage area throughout the 

Salmon River and its tributaries, and trends of low-flow 

gains with drainage area changed with discharge. 

Excluding the precipitation-affected seepage run of 

August 24 and 25, and given that measured discharge 

was subject to errors of approximately 10 percent, 

discharge between low-flow measurement sites A and 

F increased in roughly fixed proportion to drainage 

area (fig. 17). During the two final seepage runs, when 

flows overall were smallest, discharge between sites F 

and G increased relatively little, and discharge between 

sites G and H increased sharply. 

The causes of the variations of low flows in the 

upper and lower reaches of the Salmon River are not 

known with certainty; however, variations of watershed 

geology and physiography are consistent with the 

following possible explanations. Most of the 

mountainous reaches of the Salmon River are probably 

gaining reaches during the low-flow season because 

precipitation amounts are relatively large at the higher 

altitudes, soil moisture stores are depleted later in the 

season, and flows tend to remain in the channel due to 

the confining bedrock. A gaining reach of a stream is 

one where the flow is greater at the downstream end of 

the reach than it is at the upstream end. Thus, low flows 

in the Salmon River and its tributaries probably 

accumulate in the headwater reaches during late 

summer. As was stated previously in this report, the 

lower Salmon River channel lies mostly within porous 

unconsolidated deposits where the potential for the 

surface waters to exchange with the regional groundwater flow system is high. Thus, reaches in these 

deposits, such as are found between sites F and G 

(fig. 6), can be either gaining or losing reaches 

depending on the degree of hydrologic connectivity 

and hydrologic gradients between the surface- and 

ground-water systems. Finally, between sites G and H 

the river becomes confined by bedrock. The increases 

in low flows that were observed in that reach might 

have been caused by the river's encounter with the 

bedrock. For example, the bedrock might have 

prevented channel losses to the regional ground-water 

system and inflows from tributaries therefore remained 

in the channel and increased the river's flow.

0 5 15 10 20 25 30

0

10

20

30

DISCHARGE, IN CUBIC FEET PER SECOND

DRAINAGE AREA, IN SQUARE MILES

August 9-10

August 24-25

September 9-10

September 22-23

A C E F G H

Figure 17. Discharge of the Salmon River and selected tributaries at selected low-flow measurement sites, as related to 

drainage area.

Letters on upper X-axis refer to low-flow measurement sites given in table 3.
Responses to Key Questions 25

3. What Evidence Exists That Forest Harvesting 

and Road Construction Have Altered Frequency 

and Magnitude of Peak and Low Flows?

Timber harvesting and associated road 

construction might have affected one or more 

hydrologic responses in the Salmon River watershed. 

The Salmon River watershed has been extensively 

harvested throughout, although the timing of 

harvesting has varied for different parts of the 

watershed. Clearcut logging began in the easily 

accessed Lower Salmon River subwatershed during the 

1930's (Rodney F. Mayte, U.S. Forest Service, written 

commun., February 2001). Clearcut logging began in 

the mountainous North Fork, Middle Fork, and South 

Fork subwatersheds during the 1960s. The resulting 

mosaic of forest stands represent different stages of 

forest succession. Almost all of the conifer stands and 

many of the hardwood stands that currently are at early 

to middle successional stages likely resulted from 

forest harvesting and related human activities of the 

twentieth century. Stands at early to middle 

successional stages occur in approximately 90 percent 

of the Lower Salmon River subwatershed, 

approximately 50 percent of the North Fork 

subwatershed, approximately 60 percent of the Middle 

Fork subwatershed, and approximately 80 percent of 

the South Fork subwatershed (Rodney F. Mayte, U.S. 

Forest Service, written commun., February 2001).

Available hydrologic data for the watershed are 

currently not sufficient to document the possible effects 

of timber harvesting and associated road construction 

on any hydrologic variables. Given the relatively short 

record from the streamflow gaging station near the 

1000-Road Bridge and the lack of streamflow data for 

preharvest conditions, statistical examination of 

timber-harvesting effects on hydrologic responses was 

not possible during this watershed analysis. Because of 

the lack of hydrologic data for impacted areas within 

the watershed, no attempt was made in this analysis to 

quantify effects of timber harvesting on hydrologic 

responses.

Instead, questions concerning effects of 

vegetation manipulation can be addressed qualitatively 

to provide insight into the possible change mechanisms 

for hydrologic responses and to point out the types of 

investigations and hydrologic data that will be needed 

to determine effects of forest clearing and associated 

road construction on hydrologic responses. The 

primary resource for making qualitative evaluations of 

effects of timber harvesting and road construction on 

hydrologic response is the scientific literature 

describing investigations in other impacted watersheds. 

The brief review of that literature that follows is not 

intended to be comprehensive, but rather to give the 

reader some examples of documented harvest effects 

from the Pacific Northwest and to provide insights into 

how questions about the possible hydrologic effects of 

harvesting and road construction in the Salmon River 

watershed can be addressed through site-specific 

investigations.

Numerous scientifically rigorous investigations 

of forest hydrology and of hydrologic effects of forest 

harvesting and road construction have been conducted 

on the forested lands of the Pacific Northwest during 

the past century. The particular hydrologic response 

variables that have been investigated most commonly 

in conjunction with harvest and road construction are 

total watershed surface-water yield; magnitude of 

stream peak flows; and duration and magnitude of 

stream low flows. Surface-water yield is the unit-area 

surface-water discharge from a given watershed or 

subwatershed, and it is commonly expressed in inches 

per month or inches per year. 

Investigations of surface-water yield conducted 

in small, low- to mid-altitude mountain watersheds in 

the western Oregon Cascades and the Oregon Coast 

Range indicated that annual and summer yield 

generally increased immediately following extensive 

harvesting of dense forest stands (Harris, 1977, Harr, 

1982, 1983; Hicks and others 1991). Harr (1983) and 

Hicks and others (1991) reported that most of the 

increase in annual water yield occurred during the 

winter rainy season. Postharvest increases in surfacewater yield have been attributed to reductions in 

evapotranspiration caused by clear-cutting (Hicks and 

others, 1991). The magnitude of yield changes, 

expressed in inches per year, has varied somewhat 

among the studied watersheds. For example, annual 

water yield for the first eight years following 100-

percent harvesting of a small (0.27 mi2), hardwooddominated watershed on the west slope of the Oregon 

Coast Range averaged 57 inches, which was 19 inches 

larger than the yield predicted from a control watershed 

(Harris, 1977). Annual precipitation for that watershed 

was about 95 inches. Annual water yield of a nearby 

watershed with forest cover that initially was about 60 

percent Douglas-fir and 40 percent red alder averaged 
26 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

52 inches following patch-clear-cut harvesting of about 

25 percent of the watershed. Annual postharvest water 

yield from this latter watershed was not statistically 

different from the annual yield of 48 inches that was 

predicted from the control watershed. In another study, 

annual water yield of a small (0.37 mi2) watershed in 

the western Oregon Cascades increased by an average 

of 14 inches as compared to water yield predicted from 

a control watershed for the eight years following the 

beginning of 100 percent harvesting of dense stands of 

mature Douglas-fir (Hicks and others, 1991). Some of 

the variability among the watersheds in the change of 

annual water yield was probably caused by variations 

in composition of the preharvest forests, by the extent 

and type of watershed harvest and roads, and by 

differences among the watersheds with respect to local 

geological, soil, or climate conditions.

Increases in annual surface-water yield following 

harvesting, where they occurred, generally decreased 

during the decade following harvesting (Harris, 1977; 

Harr, 1983; Hicks and others, 1991). Recovery 

(decrease) of surface-water yield toward the preharvest 

yield was attributed by Hicks and others (1991) to 

regrowth of vegetation that reestablished evaporative 

water losses from the watershed toward preharvest 

amounts. Timing and extent of postharvest yield 

recovery varied considerably among the watersheds, 

and although specific causes for these variations are 

elusive, they may be related to differences in geological 

conditions or climate or to differences in water-use 

characteristics of the dominant postharvest plant 

species in the watersheds.

Although changes in annual surface-water yield 

are an integrated measure of the effects of harvesting 

and road construction, it is probably the extreme lowand peak-flow events that are of most interest in the 

management of watersheds such as the Salmon River 

watershed. Decreased stream discharge and changes in 

channel geometry during low flows could pose a threat 

to the fishery resource, and increased peak flows can 

cause flooding. Scientifically rigorous investigations of 

the effects of timber harvesting and road construction 

on low and peak flows have been conducted for a 

number of watersheds in the Pacific Northwest, 

including those watersheds discussed previously. For 

the small, low- to mid-altitude mountain watersheds in 

the western Oregon Cascades, forest harvesting 

generally was associated with postharvest increases in 

summer low flows (Rothacher, 1965, Hicks and others, 

1991), at least during the initial postharvest years. In 

contrast, Harris (1977) reported that no statistically 

significant changes in summer low flows occurred 

following clear-cut harvesting in two low-altitude 

mountain watersheds in the Oregon Coast Range. Also, 

Harr (1982) reported a small decrease in summer low 

flows as a consequence of clear-cut harvesting in a 

small, high-altitude watershed in the Oregon Cascades. 

Harr proposed that eliminating trees decreased 

interception of fog and subsequent drip to the soil, and 

thereby removed a summer source of water to augment 

low flows. Some investigations in coastal forests of 

Oregon (Isaac, 1946) and northern California (Azevedo 

and Morgan, 1974) have indicated that fog drip can be 

a significant water-balance component. Fog does occur 

over the low-lying coastal terrain of the Pacific slope of 

the Olympic Peninsula (Capoeman, 1990, p. 24), and 

removal of fog-intercepting conifers might reduce the 

amount of water that is available to augment low flows 

of the Salmon River, at least for a few years following 

harvesting. The effects of timber harvesting on fog drip 

and summer low flows in the Salmon River watershed 

can be known only if appropriate investigations of 

streamflow, fog drip, and related water-balance 

components are conducted in the watershed.

The literature describing investigations of effects 

of harvesting and road construction on peak flows in 

the Pacific Northwest is more difficult to interpret than 

that for surface-water yield and low flows. Part of the 

reason for this is that different investigators have used 

different techniques for classifying and analyzing peakflow events. Some have considered all peaks above an 

arbitrary base, some have classified peaks as being 

either "high" or "low" peaks, while others have 

attempted to classify peaks based on whether or not 

they were associated with rain-on-snow events. In 

addition, the runoff-generating mechanisms for peak 

flows appear to be more variable in time and space than 

those for yield and low flows. For example, rain-onsnow might be the runoff-generating mechanism for 

major flow events in a watershed consisting largely of 

mid-altitude terrain, whereas rain without snow might 

be the major flood producer in low-altitude watersheds. 

One result of these complications is that general 

conclusions about whether or not forest harvesting 

increases peak flows are difficult to reach. 
Responses to Key Questions 27

One of the most recent published investigations 

of peak flows in the Pacific Northwest at the date of 

this literature review (2000) employed inferential 

statistics to examine effects of forest harvesting and 

road construction on stream peak-discharge in eight 

small (<1 mi2) and large (20 to 250 mi2) watersheds in 

the western Cascades of Oregon (Jones and Grant, 

1996). Specifically, Jones and Grant examined 34 years 

of streamflow records for three small research 

watersheds. The small watersheds included a control, 

where no timber harvesting or road construction was 

conducted, a treated watershed that was entirely clearcut and slashed-burned during 1962 to 1966 without 

construction of roads, and a treated watershed in which 

1.7 miles of road were constructed in 1959 and 25 

percent of the area of which was patch clear-cut during 

1963. Baseline comparisons of peak discharges among 

the control and treated watersheds were made during 

1955 to 1961 and prior to the treatments. Jones and 

Grant classified peak discharge magnitudes according 

to ranges in the statistically derived event recurrence 

interval. For example, they classified peaks (events) 

with a recurrence interval of less than 0.125 year as 

"small," and they classified peaks with a recurrence 

interval of from 0.4 to 100 years as "large." 

Jones and Grant (1996) developed a statistical 

model to compare paired peak-discharge events among 

the control and treated watersheds. They then used 

inferential statistics and probability theory to judge 

whether or not the magnitudes of peak-discharges from 

each treated watershed changed relative to peak 

discharge from the control watershed by more than 

could be explained by random chance. If the treatmentrelated changes were larger than could be explained by 

chance, Jones and Grant reasoned, the treatments 

(forestry operations) must have been the cause of the 

changes.

Among Jones and Grants' conclusions were (1) 

the average magnitude of all events increased 

significantly following 25 percent patch clear-cutting 

with roads; (2) the average magnitude of small events 

and all events increased significantly following 100 

percent clear-cutting without roads; however, the 

average magnitude of large events did not increase 

significantly following 100 percent clear cutting 

without roads; (3) the largest percentage increase in 

peak discharge was as much as 50 percent; and (4) 

increases in the magnitude of peak discharge were 

persistent for up to 25 years following 25 percent patch 

clear-cutting with roads and for up to 16 to 22 years 

after 100 percent clear-cutting without roads. 

Experimental conditions in the large watersheds 

studied by Jones and Grant were not as carefully 

controlled as were conditions in the small research 

watersheds because the former were managed for 

multiple uses, including fiber production, rather than 

for research. As a result, Jones and Grant (1996) 

developed a statistical regression model to examine 

whether or not relative differences in paired-event, 

peak discharges among the large watersheds were 

statistically linked to differences in cumulative 

percentage of harvested watershed area. Their 

regression analyses indicated the difference between 

the paired watersheds in magnitude of peak discharge 

was significantly correlated with the difference 

between cumulative percentage of the watershed area 

that had been harvested. Furthermore, Jones and Grant 

concluded that forest harvesting had increased the 

magnitude of peak discharge from the large watersheds 

by as much as 100 percent during the 50 years prior to 

their investigation.

In a subsequent investigation that employed 

much or all of the same data used by Jones and Grant 

(1996), Thomas and Megahan (1998) criticized the 

methods and conclusions of the former investigators. 

As for the small watersheds, Thomas and Megahan 

rejected both the statistical model and the rationale that 

Jones and Grant applied in choosing the model. 

Thomas and Megahan selected and applied a different, 

regression-based statistical model, termed an "analysis 

of covariance model," that allowed them to group all 

events together regardless of magnitude. Among the 

conclusions reached by Thomas and Megahan for the 

small watersheds was that the magnitude of peak 

discharge was significantly increased for 20 years 

following harvest in the watershed that was 100 percent 

clear-cut without roads and for 10 years following 

harvest in the watershed that was 25 percent patch 

clear-cut with roads. Secondly, the percentage increase 

in peak discharge decreased with peak-discharge 

magnitude. For example, for the 100 percent clear-cut 

watershed, the smallest peaks were increased by 90 

percent, whereas the largest peaks with statistically 

detectable increases were increased by 25 percent. 

Thirdly, the magnitude of the peak-flow increase 

decreased exponentially with time in both watersheds.
28 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

Thomas and Megahan (1998) also were critical 

of the methods and conclusions presented by Jones and 

Grant (1996) for the large watersheds. Thomas and 

Megahan questioned Jones and Grants' choice for the 

dependent variable (differences in paired-event, peak 

discharges among watersheds) and for the independent 

variable (difference in cumulative percentage of 

watershed area harvested). Thomas and Megahan also 

objected to the statistical regression model that Jones 

and Grant employed. As was the case with the small 

watersheds, Thomas and Megahan analyzed the peakdischarge data for the large watersheds using a 

statistical model of their own derivation. Thomas and 

Megahan concluded that the data for one watershed 

pair failed to show that magnitude of peak discharge 

responded to harvesting. As for the other two largewatershed pairs, where harvest effects were small but 

statistically significant, Thomas and Megahan stated, 

"the 'usefulness' of these relationships is questionable." 

(p. 3401). The conclusions of Thomas and Megahan for 

the large watersheds stand in contrast to the conclusion 

stated by Jones and Grant (1996) that forest harvesting 

has caused large percentage increases in the magnitude 

of peak flows both in small and large watersheds.

The contrasting findings reported by Jones and 

Grant (1996) and Thomas and Megahan (1998) point to 

some of the difficulties in attempting to predict the 

effects of forest harvesting and road construction on 

hydrologic responses in the Pacific Northwest. One 

point of agreement between the two pairs of 

investigators is that more information and investigation 

of the potential hydrologic effects of forest harvesting 

are needed to identify when hydrologic changes caused 

by harvesting have occurred and to identify the 

important mechanisms of those changes. 

Results of the above investigations of hydrologic 

effects of forest harvesting and road construction in 

Pacific Northwest watersheds, taken as a whole, do not 

support a conclusion that these practices have resulted 

in long-term or large-magnitude changes in annual 

water yield, low flows, or peak flows in the studied 

watersheds. As for the Salmon River watershed, it is 

considered probable that periods of extensive 

harvesting have yielded low flows and peak flows on 

the Salmon River and its impacted tributaries that were 

of slightly larger magnitude than would have occurred 

had the forest been left intact. This statement is 

qualified by the observation that hydrologic responses 

to forest practices on the Pacific slope of the Olympic 

Peninsula might differ substantially from documented 

responses in investigated watersheds, owing principally 

to climatic differences, and lessons taken from 

investigations of remote watersheds are best accepted 

cautiously as they are applied to the specific case of the 

Salmon River watershed.
Responses to Key Questions 29

4. What Is Currently Known About the 

Distribution and Extent of Wetlands, and What 

Have Been the Impacts of Land Management 

Activities on Wetlands?

A preliminary map of current wetlands and 

related hydrologic features of the Salmon River 

watershed was produced by the staff of the U.S. Forest 

Service, Olympic National Forest (fig. 6). The staff 

combined spatial wetland and hydrology databases 

(layers) from several sources, including the National 

Wetlands Inventory (NWI) layer (U.S. Fish and 

Wildlife Service, 1979; and U.S. Fish and Wildlife 

Service, 2000), the WADNR Hydrology Layer, and the 

U.S. Forest Service Geometronics Service Center 

(GSC) hydrology layer (Robin Stoddard, U.S. Forest 

Service, written commun., March 1997). Area of 

current wetlands is shown in table 4. Almost all of the 

wetland area (96 percent) is in the Lower Salmon River 

subwatershed. The inventory of current wetlands 

indicates the Middle Fork subwatershed accounts for 

the remaining 4 percent of the wetland area. 

The preliminary map of current wetlands 

provides a good starting point for discussion of 

wetlands issues within the watershed. Because the map 

was derived by combining different wetland surveys, 

the uniformity of wetland mapping and classification 

throughout the watershed has not been established. 

Classification and mapping uniformity could be 

established by conducting a single, comprehensive 

survey of wetlands within the watershed. 

Description of the change of the distribution and 

extent of wetlands would require a map of historical 

wetlands against which the current map could be 

compared. A map of historical wetlands was not found 

during this watershed analysis, and a sufficiently 

detailed historical wetland map probably does not 

exist. Whether or not a sufficiently accurate and 

detailed map of historical wetlands could be produced 

from historical aerial photographs is not known, and 

the investigation and application of the requisite 

photogrammetric techniques for producing such a map 

is beyond the scope of this report.

Forest harvesting, if it alters the water balance by 

reducing evaporative loss, could increase wetland water 

depth or the duration of inundation of seasonal 

wetlands. Construction and use of the transportation 

network have impacted wetlands within the Salmon 

River watershed. Roads have the potential for 

impacting wetlands by altering wetland hydrology 

(water depth and velocity, and duration of inundation 

for seasonally flooded wetlands), by disrupting wetland 

wildlife habitat, and by occupying lands that have 

previously been wetlands. An attempt was made in this 

analysis to provide a sense of the scope of wetland 

disruption related to the construction and use of the 

transportation network. To accomplish this, a map of 

existing roads was drawn on the map of current 

wetlands and the number of intersections of roads with 

wetlands and the total mileage of roads within wetlands 

were computed. The results of this analysis are 

presented in table 5. The only known impacts of roads 

on wetlands have occurred in the Lower Salmon River 

subwatershed (fig. 6).

Table 4. Wetland area, in acres, for each subwatershed of the Salmon 

River watershed

Subwatersheds

Lower 

Salmon 

River

North Fork Middle Fork South Fork Total

298 0 11 0 309

Table 5. Number of intersections of roads with wetlands and mileage 

of roads within wetlands in the Salmon River watershed

Subwatershed Number of 

intersections

Mileage within 

wetlands

Lower Salmon River 12 0.7

North Fork 0 0

Middle Fork 0 0

South Fork 0 0
30 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

5. How Might Existing Hydrologic Monitoring 

Be Maintained or Augmented to Help Detect 

Future Hydrologic Change, to Preserve Critical 

Ecosystem Functions, and to Protect Public and 

Private Property?

Many opportunities exist for monitoring to 

increase overall understanding of and predictive 

capability for hydrologic responses and processes 

within the Salmon River watershed. These include:

1. Maintaining the streamflow gaging station near 

the 1000-Road Bridge. This station had been 

operated for about 5 years at the time this 

hydrologic analysis was conducted. Continued 

operation will, in time, yield sufficient data for 

refining estimates of peak- and low-flow 

magnitudes and frequencies that are described in 

this report. 

2. Measuring precipitation at high and low altitudes. 

Precipitation is quantitatively the most important 

hydrologic input to the Salmon River watershed, 

and understanding and prediction of hydrologic 

responses, such as flows measured at the 1000-

Road Bridge gaging station, could be enhanced by 

determining precipitation within the watershed. 

Because precipitation probably varies with landsurface altitude, precipitation-measurement 

stations at both low and high altitudes would be 

useful for detecting onset, duration, and intensity 

of precipitation events that drive many of the 

watershed's hydrologic responses. 

3. Describing the evolution of water available for 

runoff throughout the watershed. Hydrologic 

change due to forest harvesting and road 

construction in the Pacific Northwest has been 

linked to changes in the quantity and timing of 

water available for runoff during rain-on-snowevents (WADNR, 1995). Water available for 

runoff is the quantity of water per unit time or per 

storm that is applied to the soil surface from 

snowmelt and precipitation. Apparently, little is 

known about distribution of water available for 

runoff during these events in Pacific Northwest 

watersheds because watershed-scale surveys to 

compute water available for runoff have not been 

made. Such a survey in the Salmon River 

watershed for one or more storms could enhance 

understanding of runoff-generating rain-on-snow 

events in the Salmon River watershed and similar 

watersheds. 

A survey of water available for runoff for a 

single storm would consist of measurements to 

compute water available for runoff at several different 

sites that vary with respect to altitude, vegetation type, 

or aspect direction. The snowpack snow-water 

equivalent would be measured before and after the 

storm, as would the amount of water from precipitation 

that is applied to the snow surface or bare soil (no 

snow) surface during the storm. Total storm water 

available for runoff at each site would then be 

computed from an equation for the site water balance. 

An equation for the water balance is

, (1)

where

If the magnitude of E is small compared to the 

magnitude of (P - ∆SWE) during rain-on-snow events 

(van Heeswijk and others, 1996), water available for 

runoff could be estimated from the equation

. (2)

WAR is the total water available for runoff for the 

storm, in inches;

P is the quantity of water from precipitation 

that is input to the snow or bare soil surface 

during the storm, in inches;

∆SWE is the change of snow water equivalent 

during the storm, in inches; and

E is the total evaporative loss during the storm, 

in inches.

WAR P SWE E = – ∆ –

WAR P SWE ≈ – ∆
Summary 31

A survey of water available for runoff would be 

an intensive effort that would require several teams of 

people to react to an incoming storm in time to make 

the necessary pre-storm measurements and 

preparations and to return to the survey sites to make 

post-storm measurements. The number of sites to be 

incorporated would be decided upon as part of the 

survey design. Potentially, the following types of 

questions could be addressed by properly designed and 

conducted surveys.

 A. Where in the watershed is most of the water 

available for runoff generated during rain-onsnow events?

 B. How do precipitation and water available for 

runoff amounts vary with land-surface 

altitude during rain-on-snow events?

 C. How important is pre-storm snow-water 

equivalent on the generation of water 

available for runoff during rain-on-snow 

events?

 D. How much of the water available for runoff is 

derived from precipitation and how much is 

from snowmelt during rain-on-snow events? 

Do the relative contributions of precipitation 

and snowmelt vary with land-surface 

altitude?

 E. What is the effect of clear-cutting on 

prestorm snow-water equivalent and on 

generation of water available for runoff 

during rain-on-snow events?

 F. Is site directional aspect important for 

determining prestorm snow-water 

equivalent?

 G. Do the deepest snowpacks contribute to water 

available for runoff during rain-on-snow 

events?

4. Enhance the existing, preliminary map of wetlands 

through wetland surveys. An intensive mapping 

and classification of wetlands in the Salmon River 

watershed would be useful for checking and 

updating the preliminary map of current wetlands 

(fig. 6). The preliminary map of current wetlands 

could be compared to future wetland inventories 

for the purpose of evaluating status and trends in 

wetland resources. 

SUMMARY

The U.S. Geological Survey analyzed selected 

hydrologic conditions of the Salmon River watershed 

on the western Olympic Peninsula, Washington, as part 

of a watershed analysis conducted by the Quinault 

Indian Nation. The analysis of selected hydrologic 

conditions was motivated by five key questions that 

were developed by watershed analysis participants. 

1. What are the natural, physical, and biological 

features of the watershed that control hydrologic 

responses? 

2. What is known about current streamflow 

characteristics, including peak- and low-flows? 

3. What evidence exists that forest harvesting and 

road construction have altered frequency and 

magnitude of peak and low flows?

4. What is currently known about the distribution and 

extent of wetlands, and what have been the 

impacts of land management activities on 

wetlands?

5. How might existing hydrologic monitoring be 

maintained or augmented to help detect future 

hydrologic change, to preserve critical ecosystem 

functions, and to protect public and private 

property?

The key questions were researched using the 

scientific literature, a brief field investigation, and 

hydrologic, climatic, and resource mapping databases 

maintained by tribal and governmental agencies. 

The natural, physical, and biological 

characteristics that control hydrologic responses in the 

Salmon River watershed include the climate, the 

watershed physiography, geology, and vegetation. 

Climate is influenced by the watershed's location on 

the west coast of North America at a latitude of about 

47 degrees N. Westerly trending winds bring moist air 

to the west coast of North America from the Pacific 

Ocean. The strong maritime influence results in 

abundant precipitation. Annual precipitation measured 

at nearby Clearwater, Washington, averaged 118 inches 

during 1932 to 1998, most of it falling as rain during 

the winter months. 
32 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology

The mountainous nature of the Salmon River 

watershed contributes to the abundant precipitation and 

to the accumulation and melt of snow. The higher 

mountain ridges in the watershed reach altitudes of 

approximately 2,700 feet. Average annual precipitation 

in the Salmon River watershed during 1961 to 1990 

was estimated to range from ranged from 117 inches 

for low-altitude parts of the watershed to 165 inches for 

the highest terrain. 

The terrain of the Salmon River watershed was 

classified according to system of five precipitation 

zones intended to portray the likelihood for floodproducing rain-on-snow events. Sixty-six percent of the 

terrain of the Salmon River watershed lies at altitudes 

that encompass the peak rain-on-snow or raindominated precipitation zones, which are two of the 

three precipitation zones with the greatest likelihood 

for rain-on-snow events. The remainder of the 

watershed terrain is in the lowland precipitation zone, 

which has a relatively small likelihood for rain-onsnow events.

The geology of the Salmon River watershed 

affects hydrologic responses in the watershed insofar as 

it controls bed surface slopes and gradients in stream 

channels and as it affects substrate materials that 

control or mediate subsurface flow. Vegetation 

potentially performs a number of functions that can 

influence hydrologic responses, including taking up 

and transpiring soil water, intercepting water in 

precipitation that is then evaporated back into the 

atmosphere, intercepting water in fog that drips to the 

soil surface, and protecting and augmenting soils that 

can store water and retard runoff. 

Current streamflow characteristics of the Salmon 

River were investigated by examining the streamflow 

record from a gaging site on the river, by estimating 

stream peak flows using a regional regression equation, 

by estimating low flows by correlating flows at the 

Salmon River gage site with flows of a nearby river for 

which a low-flow frequency curve could be developed, 

and by measuring and interpreting flow of the Salmon 

River at selected points during the low flow season. 

Annual unit-area discharge from the drainage 

area upstream of the gage near the 1000-Road Bridge 

averaged 134 inches for water years 1995 to 1998, and 

annual precipitation at Clearwater averaged 116 inches 

during the same period. The contributing drainage area 

upstream of the gage site is 29.2 mi2. Annual unit-area 

discharge was correlated with annual precipitation at 

Clearwater during water years 1995 to 1998.

Annual peak discharge at the gage near the 1000-

Road Bridge ranged from 3,570 to 8,550 ft3/s during 

water years 1995 to 1999. One of the five gaged annual 

peak discharges was greater than the 100-year peak 

discharge for the site that was predicted using a 

regional regression equation, and two of the gaged 

annual discharges were almost as large as the predicted 

25-year event. It is considered unlikely that the 5 years 

of available record of annual peak flow at the 1000-

Road Bridge site captured three such rare events. More 

likely, the peak flows were specified as 25- and 100-

year events because of such factors as the Salmon River 

peak-flow characteristics being statistically aberrant 

among the watersheds that were used to develop the 

equations or because of errors in the gaged peak flows 

at the 1000-Road Bridge site.

Annual low flows at the 1000-Road Bridge site 

were estimated by correlating flows at that site during 

1995, 1996, and 1998 with flows at a long-term site on 

the nearby Calawah River for which a low-flowfrequency curve could be developed. The estimated 5- 

and 20-year 7-day low flows at the 1000-Road Bridge 

site were 13.9 and 10.8 ft3/s, respectively.

Discharge of the Salmon River and its tributaries 

were measured during four seepage runs made during 

August and September 1999. The seepage runs 

indicated the South Fork of the Salmon River 

contributed substantially less flow per unit drainage 

area than did the other two mountainous forks, the 

Middle and North Forks. On August 9, the unit-area 

discharge of the South Fork near its confluence with 

the Middle Fork was 27 percent less than the unit-area 

discharge of the Middle Fork, and it was 29 percent 

less than the unit-area discharge of the North Fork near 

the confluence of that fork with the main stem. Also, 

low-flow discharge increased with but was not linearly 

related to drainage area throughout the Salmon River 

and its tributaries, and trends of low-flow gains with 

drainage area changed with discharge.

Timber harvesting and associated road 

construction might have affected one or more 

hydrologic responses in the Salmon River watershed. 

The Salmon River watershed has been extensively 

harvested throughout, although the timing of 

harvesting has varied for different parts of the 

watershed. Available hydrologic data for the watershed 

are currently not sufficient to document the possible 

effects of timber harvesting and associated road 

construction on any hydrologic variables. Therefore, 

the primary resource for making qualitative evaluations 
Literature Cited 33

of effects of timber harvesting and road construction on 

hydrologic response is the scientific literature 

describing investigations in other impacted watersheds 

in the Pacific Northwest. Based on a brief review of 

such scientific literature, it is considered probable that 

periods of extensive harvesting have yielded low flows 

and peak flows on the Salmon River and its impacted 

tributaries that were of slightly larger magnitudes than 

would have occurred had the forest been left intact. 

This conclusion is qualified by the observation that 

hydrologic responses to forest practices on the Pacific 

slope of the Olympic Peninsula might differ 

substantially from documented responses in 

investigated watersheds, and lessons taken from 

investigations of remote watersheds are best accepted 

cautiously as they are applied to the specific case of the 

Salmon River watershed.

A preliminary map of current wetlands that was 

produced by staff of the U.S. Forest Service indicates 

wetlands area in the Salmon River watershed totals 309 

acres, with 96 percent of the wetlands area in the 

Lower Salmon River subwatershed. Impacts of land 

management activities on wetlands are generally not 

known, except analysis of current wetlands and roads 

indicates there currently are 12 intersections of roads 

with wetlands within the Lower Salmon River 

subwatershed and a total of 0.7 mile of roads within 

wetlands. 

Many opportunities exist for monitoring to help 

detect future hydrologic change, to preserve critical 

ecosystem functions, and to protect public and private 

property. Monitoring opportunities discussed in this 

report include (1) maintaining and operating the 

streamflow gaging station that is near the 1000-Road 

Bridge, (2) measuring precipitation at high and low 

altitudes within the watershed, (3) investigating the 

storm-driven evolution of water available for runoff 

throughout the watershed, and (4) enhancing the 

existing, preliminary map of wetlands through 

intensive mapping and classification of wetlands in the 

watershed.

LITERATURE CITED

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789.

Harr, R.D., 1983, Potential for augmenting water yield 

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western Oregon: American Water Resources 

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WRIR 03–4204 Watershed Analysis of the Salmon River Watershed, Washington: Hydrology Bidlake

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