Hydrologic Trends Associated with Urban

Development for Selected Streams in the

Puget Sound Basin, Western Washington

Hydrologic Trends Associated with Urban

Development for Selected Streams in the

Puget Sound Basin, Western Washington

Prepared in cooperation with the Washington Department of Ecology

U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 02-4040
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4040

Hydrologic Trends Associated with Urban 

Development for Selected Streams in the 

Puget Sound Basin, Western Washington

Prepared in cooperation with the

WASHINGTON DEPARTMENT OF ECOLOGY

By Christopher P. Konrad, U.S. Geological Survey, Tacoma, WA

and Derek B. Booth, University of Washington, Seattle, WA

Tacoma, Washington

2002
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information write to:

District Chief

U.S. Geological Survey

1201 Pacific Avenue – Suite 600

Tacoma, Washington 98402

http://wa.water.usgs.gov

Copies of this report can be purchased

from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

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.

District Chief

U.S. Geological Survey

1201 Pacific Avenue – Suite 600

Tacoma, Washington 98402

http://wa.water.usgs.gov
Contents iii

CONTENTS

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

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

Description of Study Area.................................................................................................................................... 2

Previous Investigations ........................................................................................................................................ 5

Hydrologic Effects of Urban Development ................................................................................................ 5

Streamflow Variability................................................................................................................................ 5

Alternative Measure of the Hydrologic Effects of Urban Development .................................................... 6

Methods for Identifying Hydrologic Trends Associated with Urban Development............................................ 7

Annual Mean Discharge.............................................................................................................................. 11

Fraction of Year Annual Mean Discharge Was Exceeded.......................................................................... 11

Annual 7-day Low Flow ............................................................................................................................. 11

Annual Maximum Discharge ...................................................................................................................... 12

Continuous Trends ...................................................................................................................................... 12

Error Analysis ............................................................................................................................................. 13

Hydrologic Trends................................................................................................................................................ 24

Trends Over the Entire Period of Record.................................................................................................... 24

Errors in Long-Term Trend Identification Using Short-Term Period of Records...................................... 35

Hydrologic Effects of Urban Development ................................................................................................ 35

Implications of Trend Analysis for Hydrologic Monitoring................................................................................ 37

Summary .............................................................................................................................................................. 38

References Cited .................................................................................................................................................. 38
iv Contents

FIGURES

Figure 1. Map showing location of selected streams in the Puget Sound Basin, western Washington ............ 3

Figure 2. Map showing urban land cover in selected stream basins in the Puget Sound Basin, 

western Washington (based on Landsat Thematic Mapper data by 

Vogelmann and others, 2001)............................................................................................................ 4

Figure 3. Graph showing population density relative to land use for selected streams in the 

Puget Sound Basin, western Washington, 1940 to 2000 ................................................................... 9

Figure 4. Normal quantile plots of the cumulative distributions of annual mean discharge, 

fraction of year annual mean discharge was exceeded, 7-day low flow, and annual 

maximum discharge relative to land use for selected streams in the Puget Sound Basin, 

western Washington........................................................................................................................... 14

Figure 5. Graphs showing annual values of mean discharge (Qmean), fraction of year mean 

discharge was exceeded (TQmean), 7-day low flow (Qmin), and maximum discharge 

(Qmax) relative to land use for selected streams in the Puget Sound Basin, 

western Washington........................................................................................................................... 25

Figure 6. Graph showing fraction of year daily mean discharge exceeds annual mean discharge 

(TQmean) in two rural streams — Issaquah and Newaukum Creeks in the Puget Sound Basin, 

western Washington, water years 1964-2000 .................................................................................... 37
Contents v

TABLES

Table 1. Land use, drainage area, and period of streamflow records for 10 selected streams 

in the Puget Sound Basin, western Washington ................................................................................ 8

Table 2. Mean value and coefficient of variation (CV) for streamflow statistics over period of 

record for 10 selected streams in the Puget Sound Basin, western Washington ............................... 10

Table 3. Hydrologic trends in streamflow statistics over the entire period of record for 10 

selected streams in the Puget Sound Basin, western Washington ..................................................... 35

Table 4. Error rates for comparisons of trends in streamflow statistics for 10-year periods of 

record with trends over the entire period of record for each of the 10 selected streams 

in the Puget Sound Basin, western Washington ................................................................................ 36
vi Conversion Factors

CONVERSION FACTORS AND VERTICAL DATUM

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of

1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929.

Altitude, as used in this report, refers to distance above or below sea level.

Multiply By To obtain

millimeter (mm) 0.03937 inch

meter (m) 3.281 foot 

kilometer (km) 0.6214 mile 

square kilometer (km2) 0.3861 square mile

cubic meter per second (m3/s) 70.07 acre-foot per day 

35.31 cubic foot per second 
Introduction 1

Hydrologic Trends Associated with Urban 

Development for Selected Streams in the 

Puget Sound Basin, Western Washington

By Christopher P. Konrad, U.S. Geological Survey, Tacoma, WA

and Derek B. Booth, University of Washington, Seattle, WA

ABSTRACT

Annual streamflow statistics from 10 

selected streams in the Puget Sound Basin in 

western Washington were analyzed to identify 

possible hydrologic trends associated with urban 

development and to evaluate the effect of record 

length on errors in trend analysis. The analysis 

used three common streamflow statistics (annual 

mean discharge, annual maximum discharge, and 

7-day low flow) and introduced an alternative 

statistic (fraction of year annual mean discharge 

was exceeded) for assessing the hydrologic effects 

of urban development. Although trends were 

identified in each of the four statistics analyzed, 

trends were not consistent in any of the four 

statistics for all selected streams. Instead, trends in 

two statistics – (1) fraction of year annual mean 

discharge was exceeded, and (2) annual 

(instantaneous) maximum discharge – were 

evident in streams with the highest levels of urban 

development over the period of record but not in 

streams with the lowest levels of urban 

development. Trends were not consistent for either 

annual mean discharge or 7-day low flow in urban 

streams. Trends were sensitive to the period of 

analysis for all four statistics, but particularly for 

the 7-day low flow, which showed increasing and 

decreasing trends for 10 subsets of the period of 

record in some streams.

INTRODUCTION

Urban development in western Washington has 

altered runoff processes in many stream basins 

resulting in modified streamflow patterns. The 

hydrologic consequences of urban development have 

deleterious effects on people and stream ecosystems 

including increased flooding and bank erosion, the 

redistribution of water from periods of baseflow to 

stormflow, and physical disturbance of and changes to 

aquatic habitat (Hammer, 1972; Orser and Shure, 1972; 

Ebisemiju, 1989; Booth, 1990; Konrad, 2000). The 

hydrologic effects of urban development and the 

stormwater-management activities intended to mitigate 

those effects are not easily evaluated because 

variability in streamflow patterns, over time ranging 

from hours to decades, is not necessarily a consequence 

of anthropogenic activities (Lins and Slack, 1999).

A cooperative investigation between the U.S. 

Geological Survey (USGS) and the Washington State 

Department of Ecology (DOE) was initiated in 2001 to 

evaluate methods for monitoring the hydrologic effects 

of urban development and management activities 

intended to mitigate those effects. The results of this 

investigation identify streamflow statistics that exhibit 

trends in response to urban development but, 

otherwise, have relatively low inter-annual variability. 

These statistics may be incorporated as monitoring 

requirements in the National Pollutant Discharge 

Elimination System, Phase I, Municipal General 

Stormwater Permit issued by DOE.
2 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

The purpose of this report is to assess the 

suitability of four streamflow statistics for monitoring 

hydrologic changes associated with urban development 

and, potentially, from activities intended to mitigate 

effects of those land-use changes. The four statistics 

are annual mean discharge, annual maximum 

discharge, annual 7-day low flow, and the fraction of a 

year that annual mean discharge is exceeded. 

Hydrologic changes due to urban development are 

characterized in terms of trends in annual values of the 

statistics over the entire periods of record for 10 

selected streams in the Puget Sound Basin in western 

Washington. Errors in trend identification due to 

hydrologic variability unrelated to land-use changes 

are evaluated by comparing trends over the entire 

period of record to trends over subsets of the entire 

period of record.

DESCRIPTION OF STUDY AREA

The Puget Sound Basin is a 35,000-squarekilometer region of western Washington bounded by 

the Cascade Range and Olympic Mountains (fig. 1). 

The geology and topography of the Puget Lowland 

reflects repeated glaciation with glacial till plateaus, 

outwash plains, and many north-south trending valleys. 

Wetlands and lakes are common on the plateaus with 

streams forming ravines at the margins of the plateaus. 

Many streams flow directly into Puget Sound or Lake 

Washington rather than into the large rivers such as the 

Nisqually, Puyallup, Duwamish, and Snohomish 

Rivers. The region has a maritime climate and annually 

receives about 1,000 mm of precipitation. Much of the 

annual precipitation falls between November and April 

as rain and with occasional snow at high altitudes. 

Streamflow is produced by runoff during frequent 

rainstorms and ground-water discharge from shallow 

aquifers. The combined effect of low rainfall rates and 

deep, permeable soils limit runoff in forested areas 

largely to shallow subsurface flow and saturation 

overland flow.

About 70 percent of the population in 

Washington lives in the Puget Sound Basin, though 

most of the basin is forested (Ebbert and others, 2000).

The population is concentrated in the Puget Lowland 

around the metropolitan centers of Seattle, Tacoma, 

and Everett (fig. 2). Much of the urban development in 

the region has occurred recently: the population of the 

four-county region of King, Kitsap, Pierce, and 

Snohomish Counties has grown 16 to 46 percent per 

decade from 820,000 in 1940 to 3,275,000 in 2000 

(Washington State, 2001).
Description of Study Area 3

S

wamp

Creek

Juanita Creek

May Creek

Issaquah Creek

Soos Creek

Newauk

Le

um Creek

ac h Creek

Big

Beef Creek

Huge Creek

Mercer Creek

123° 122°

48°

47°

CLALLAM 

COUNTY

JEFFERSON

COUNTY

MASON

COUNTY

GRAYS

HARBOR 

COUNTY

THURSTON

COUNTY

KITSAP

COUNTY

SNOHOMISH

 COUNTY

KING

COUNTY

PIERCE

COUNTY

ISLAND

COUNTY

12011200

12069550

12108500

12112600

12119600

12121600

12120000

12120500

12127100

12069550

EXPLANATION

County boundary 

Creek

0

0 20 KILOMETERS 

20 MILES

Study location

WASHINGTON

STREAMFLOW GAGING STATION 

AND NUMBER Streamflow records 

 from these stations were analyzed for 

 trends (see table 1)

12069550

Figure 1. Location of selected streams in the Puget Sound Basin, western Washington.
4 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

123° 122°

48°

47°

Swamp

Creek

Basin

Juanita

Creek

Basin

Mercer

Creek

Basin

May

Creek

Basin

Issaquah

Creek

 Basin

Soos

Creek

Basin

Newaukum

Creek Basin

Huge

Creek

Basin

Big Beef

Creek

Basin

Leach

Creek

Basin

0

0 20 KILOMETERS 

20 MILES

EXPLANATION

County boundary 

Creek

Urban land cover 

Selected basins

Figure 2. Urban land cover in selected stream basins in the Puget Sound Basin, western Washington 

(based on Landsat Thematic Mapper data by Vogelmann and others, 2001).
Previous Investigations 5

PREVIOUS INVESTIGATIONS

The conversion of lowland forests to urban land 

uses has many hydrologic effects with significant 

sociological and ecological ramifications. Hydrologic 

trends resulting from urban development can be 

difficult to identify because development and its effects 

proceed by small increments and variability in 

streamflow over time masks anthropogenic trends. 

Although a number of streamflow statistics are likely 

to change in response to urban development, some 

streamflow statistics also may indicate ecological 

relevant changes.

Hydrologic Effects of Urban Development

Urban development has many potential effects 

on runoff processes, streamflow patterns, and, as a 

result, stream ecosystems (Savini and Kammerer, 

1961; Leopold, 1968; Hollis, 1975; Booth, 1991). 

Urban development modifies hydrologic processes 

when vegetation is cleared from hillslopes, the land 

surface is graded, and building and roads are 

constructed. These changes reduce interception, 

infiltration, subsurface flow, evapotranspiration, 

stormwater storage on hillslopes, and the time required 

for stormwater to travel over and through a hillslope to 

a stream (Savini and Kammerer, 1961; Dinicola, 1990; 

Burges and others, 1998). As a result, streams have 

more frequent stormflow events with high peak 

discharge but rapid stormflow recession (Carter, 1961; 

Harris and Rantz, 1964; Leopold, 1968). Peak 

discharge rates of small, frequent floods in particular 

increase by a greater percentage of pre-development 

rates than large, infrequent floods (James, 1965; Hollis, 

1975). 

Sawyer (1963) illustrated the redistribution of 

runnoff from baseflow to stormflow for an urban 

stream on Long Island, New York. Simmons and 

Reynolds (1982) estimated baseflow as a fraction of 

annual discharge on a part of Long Island, and found an 

inverse correlation with the amount of urban 

development. The greatest change in baseflow 

occurred with the construction of sanitary and storm 

sewers, which decreased the baseflow fraction of 

streamflow from 95 to 20 percent. When septic systems 

remained in urbanized (storm-sewered) areas, the 

baseflow fraction of streamflow decreased to 84 

percent. In comparison to estimates of recharge for predevelopment conditions on Long Island, New York, 

annual recharge is estimated to have increased by 12 

percent in areas served by infiltration basins but 

decreased by 10 percent where storm sewers collect 

runoff and discharge if to streams or the ocean (Ku and 

others, 1992).

The rapid recession of stormflow in urban 

streams leads to lower baseflow on a unit-area basis 

during the winter wet-season than in suburban streams 

in western Washington (Konrad, 2000). Likewise, 

Klein (1979) studied 27 small catchments in Maryland 

and concluded that wet-season baseflow decreased by 

as much as 90 percent in catchments with 

predominantly impervious land cover. The increase in 

stormflow and decrease in wet-season baseflow 

represent the redistribution of runoff from subsurface 

flow and storage in depressions, the soil column, and 

on vegetation to overland flow and open-channel flow 

in culverts and ditches.

The effects of urban development on dry-season 

baseflow in some cases contrast to the effects of urban 

development on baseflow during the wet season. Harris 

and Rantz (1964) determined that dry-season discharge 

in Permanente Creek, California, increased in response 

to urban development as a result of landscape 

irrigation. 

Streamflow Variability

Streamflow variability at the scale of storms, 

seasons, or years is so great in many streams that it may 

obscure the effects of urban development. As a result, 

trends may not be apparent even over a relatively long 

period of record. Alternatively, variability due to 

climatic conditions may produce trends in streamflow 

that are not necessarily anthropogenic (Lins and Slack, 

1999). Even short, multi-year sequences of wet or dry 

years can produce statistically significant trends, 

particularly when those sequences are at the start or 

end of a period of record (Wahl, 1998). 
6 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

A variety of streamflow statistics, including 

annual maximum discharge, the rate of stormflow 

recession, and the lag time between rainfall and runoff 

can indicate hydrologic effects of urban development 

(Leopold, 1968). As a result, we evaluate the annual 

variability of four different streamflow statistics to 

assess which ones show consistent trends in response 

to urban development but otherwise have low interannual variability. However, the variability of these 

statistics is not necessarily related to land use.

Aquatic organisms are adapted to some forms 

and magnitude of variability in streamflow. For 

example, biological conditions in streams re-establish 

quickly, often within months after periods of extreme 

but short-lived streamflow, such as floods and droughts 

(Boulton and others, 1992; Bayley and Osborne, 1993; 

Jones and others, 1995). In contrast, long-duration 

streamflow patterns are likely to have a persistent 

biological influence on stream ecosystems (Poff and 

others, 1997). 

The hydrologic effects of urban development 

that are most relevant to stream ecosystems may be 

distinguished from other symptomatic effects of urban 

development, in part, by the temporal scale of the 

statistics used to characterize streamflow patterns. The 

intra-annual distribution of runoff between stormflow 

and baseflow has been identified in particular as a 

factor influencing the composition and structure of 

lotic communities (Poff and Ward, 1989; Poff and 

Allen, 1995).

The intra-annual hydrologic variability in a 

stream can be measured in a number of ways including 

the coefficient of variation of daily discharge, the skew 

of daily discharge, or inter-quantile ratios (Helsel and 

Hirsch, 1993). Each of those statistics indicates a 

particular form of hydrologic variability. For example, 

the coefficient of variation and skew of daily discharge 

are sensitive to the magnitude of high but infrequent 

flows. In contrast, the inter-quartile ratio 

(Q0.75/Q0.25) is insensitive to the magnitude of 

extremely high or low flows, but may indicate 

variability in common flows between wet and dry 

seasons.

Alternative Measure of the Hydrologic 

Effects of Urban Development

Intra-annual hydrologic variability is determined 

in the broadest sense by the distribution of annual 

runoff between stormflow and recessional and 

baseflows. These flow conditions are not defined by a 

precise discharge and, moreover, they vary among 

streams and in a stream from year to year and season to 

season. As an alternative, annual mean discharge 

(Qmean), which is typical of the winter baseflow in 

Puget Lowland streams (Konrad, 2000), can be used to 

separate the annual distribution of daily discharge into 

stormflow and recessional and baseflows.

Mean annual discharge has been used to 

normalize and to index discharge of streams in regional 

analyses where annual availability is not an issue. 

Saville and Watson (1933) and Morgan (1936) found 

that the distributions of daily discharges for streams in 

a region were similar when the distribution for each 

stream was normalized by mean annual discharge. 

Likewise, Leopold and Maddock (1953) used mean 

annual discharge of rivers as an index of 

geomorphically effective flows in their hydraulic 

geometry relations because mean annual discharge was 

exceeded on average the same number of days in the 

rivers they analyzed and, thus, provided a consistent 

index of discharge.

For most streams, there are more days when 

discharge is less than Qmean than when discharge is 

greater than Qmean because the distribution of daily 

discharge in a stream generally has a positive skew as a 

result of relatively high discharge rates on a few days in 

a year. However, the distribution of daily discharge 

relative to Qmean is not invariant among streams: daily 

mean discharge is less than Qmean much of the time and 

only occasionally exceeds Qmean in flashy streams 

(high peak discharge, relative to baseflow, rapid 

recession rates), although daily discharge more 

commonly exceeds Qmean in streams with relatively 

large, consistent ground-water inflows. Indeed, Morgan 

(1936, p. 425) observed that the distribution of daily 

flows relative to the mean "is affected by topography, 

arrangement of tributaries with regard to time of 

concentration of surface flow, geologic structure, soil, 

vegetation, weather, and human developments related 

to flow of water." 
Methods for Identifying Hydrologic Trends Associated with Urban Development 7

The redistribution of water from baseflow to 

stormflow decreases the fraction of year daily mean 

discharge in a stream exceeds annual mean discharge 

(TQmean) of streams. Konrad (2000) showed 

statistically significant step trends in TQmean between 

water years 1960 to 1969 and water years 1989 to 1998 

using the non-parametric Wilcoxon rank-sum test for 

three urban streams (Juanita, Leach, and Mercer 

Creeks) in western Washington but only one suburban 

stream (Newaukum Creek).

Furthermore, a comparison of mean values of 

TQmean for 23 streams in the Puget Lowland for water 

years 1989 through 1998 indicated an inverse relation 

between TQmean and urban development as measured 

using road density (Konrad, 2000). TQmean was 

inversely correlated with road density (Pearson 

correlation coefficient, ρ = -0.50; probability that ρ is 

significantly different than 0, p = 0.01). The mean 

value of TQmean for 11 urban streams (defined as a road 

density greater than 6 km/km2) was 0.29 while it was 

0.34 for 12 suburban streams (road density less than 6 

km/km2). The difference was statistically significant (p

< 0.01 using a two-tailed Student's t-test among groups 

with equal variance). TQmean was greater than or equal 

to 0.32 in "suburban" streams with one exception 

(Huge Creek). TQmean was less than or equal to 0.31 in 

"urban" streams with one exception (Clover Creek). 

Independent of urban development, large streams 

typically have more attenuated streamflow patterns 

than small streams and, as a consequence, high values 

of TQmean. However, an analysis of the 10-year mean 

values of TQmean of urban and suburban streams with 

drainage areas greater than 20 km2 still showed an 

inverse correlation between TQmean and road density 

(ρ = -0.51, p = 0.01) (Konrad, 2000). Thus, TQmean is a 

reliable indicator of hydrologic differences resulting 

from urban development when comparing streams with 

similar drainage areas and, presumably, other 

physiographic factors.

METHODS FOR IDENTIFYING 

HYDROLOGIC TRENDS ASSOCIATED 

WITH URBAN DEVELOPMENT

Hydrologic trends associated with urban 

development were assessed by (1) identifying changes 

in annual streamflow statistics for selected streams in 

western Washington over time during periods of urban 

development, and (2) comparing the variation in the 

annual streamflow statistics with the population density 

of representative communities in each stream basin. 

Because all large river basins in the region are mostly 

rural or forested with mountain headwaters, the 

analysis was limited to small streams (drainage area 

less than 200 km2) in the Puget Lowland (altitudes less 

than 200 m above sea level). The focus on small, 

lowland streams limits the extent to which 

physiographic differences (topography, hypsometry, 

geology, and drainage area) are likely to produce 

differences in hydrologic processes (runoff mechanism 

and channel routing) or hydrologic trends. Continuous 

streamflow records spanning at least 30 years (not all 

records are continuous) from 10 USGS gages were 

analyzed to characterize the hydrologic effects of urban 

development and to assess the likelihood of errors in 

identifying trends from using relatively short periods of 

record (table 1). Discharge records at all of the gaging 

stations were rated good with the exception of Juanita 

and Big Beef, which were rated fair. The gaging station 

on May Creek was operated by King County Division 

of Surface-Water Management for the period for 1989 

to 1998.

Streamflow has been regulated to varying 

degrees in the selected basins (Kimbrough and others, 

2001). Mercer Creek basin has numerous detention 

ponds to limit flooding. High flows in Leach Creek 

have been regulated since 1962 by a large detention 

pond and, since 1994 by an interbasin bypass intended 

to limit peak discharge to less than 1.7 m3/s. Low 

flows in Leach Creek have been supplemented by 

ground-water pumping since June 1993 to maintain 

aminimum flow of 0.43 m3/s for water-quality 

concerns.
8 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

The hydrologic effects of these flood-control and flowaugmentation systems are anticipated to reduce the 

magnitude of trends related to increased stormflow and 

decreased baseflow. Other basins may have structures 

regulating streamflow, but they are likely to have little 

effect on the streamflow records.

In the urban and most suburban stream basins 

(Leach, Mercer, Juanita, Swamp, and May), much of 

the domestic water is supplied via interbasin transfers 

and much of the wastewater is collected in a regional 

wastewater treatment system and discharged to Puget 

Sound or the Duwamish River. In most rural stream 

basins (Huge, Big Beef, and Newaukum), domestic 

water is supplied by ground-water pumping and 

wastewater is discharged back to the ground. In parts 

of the Issaquah Creek and Soos Creek basins, however, 

water is supplied by ground-water pumping and 

wastewater is collected, treated, and discharged out of 

the basin. The combination of ground-water pumping 

and interbasin wastewater transfers may deplete 

streamflow in these creeks as a result.

Hydrologic trends are likely to coincide with 

periods of the greatest rate of development in terms of 

the area of a basin graded for construction and 

connected to storm sewers or other drainage systems 

including road networks. Streamflow records do not 

necessarily span these periods in each basin, so 

hydrologic trends may not be evident where records 

began after extensive urban development (Leach 

Creek) or where development only recently has 

expanded (1990-2000) (Issaquah Creek).

Urban development was characterized by 

population growth of representative communities and 

road density in each stream basin. Population growth 

and urban development during the 20th century has not 

been uniform in western Washington (fig. 3).

. Table 1. Land use, drainage area, and period of streamflow records for 10 selected streams in the Puget Sound 

Basin, western Washington

[km2, square kilometer; km/km2; kilometer per square kilometer]

Stream USGS 

Station No.

Drainage area 

(km2)

Road density 

(km/km2)

Period of streamflow record

Mean daily

discharge

Annual maximum 

discharge

Urban

Leach Creek 12011200 12 9.9 1958-85

1989-2000

1958-99

Juanita Creek 12120500 17 11.3 1964-90 1964-91

Mercer Creek 12120000 31 9.1 1956-2000 1956-99

Suburban

Swamp Creek 12127100 25 7.9 1964-90 1964-90

May Creek 12119600 32 5.0 1965-71

1989-90

1993-99

1955-79

1989-98

Soos Creek 12112600 171 4.7 1961-2000 1961-99

Rural 

Huge Creek 12069550 17 2.5 1947-69

1978-2000

1948-98

Big Beef Creek 12069550 35 2.1 1970-81

1996-99

1970-81

1996-99

Newaukum Creek 12108500 70 2.6 1945-50

1953-2000

1945-99

Issaquah Creek 12121600 145 2.4 1964-2000 1964-99
Methods for Identifying Hydrologic Trends Associated with Urban Development 9

1940 1950 1960 1970 1980 1990 2000

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

(PEOPLE PER SQUARE KILOMETER)

CALENDAR YEAR

Swamp Creek

Soos Creek

May Creek

SUBURBAN STREAMS

Issaquah Creek

Big Beef and Huge Cre

Newaukum Creek

RURAL STREAMS

EXPLANATION

Juanita Creek

Mercer Creek

URBAN STREAMS

Leach Creek

Figure 3. Population density relative to land use for selected streams in the Puget Sound Basin, western 

Washington, 1940 to 2000.

(Population density data from Washington State, 2001).
10 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

Table 2. Mean value and coefficient of variation (CV) for streamflow statistics over period of record for 10 

selected streams in the Puget Sound Basin, western Washington

[For Qmax, summary statistics are geometric mean values in cubic meters per second and coefficient of variation assuming a two-parameter

log normal distribution of annual maximum discharge]

Stream

Streamflow statistics (in cubic meters per second)

Annual mean 

discharge

(Qmean)

Fraction of year annual 

mean discharge was 

exceeded (TQmean)

7-day low 

flow 

(Qmin)

Annual 

maximum 

discharge (Qmax)

Urban 

Leach Creek Mean 0.13 0.25 0.04 2.0

CV 0.25 0.16 0.33 0.47

Juanita Creek Mean 0.31 0.30 0.07 4.3

CV 0.21 0.11 0.20 0.31

Mercer Creek Mean 0.64 0.29 0.17 8.6

CV 0.21 0.14 0.23 0.39

Suburban

Swamp Creek Mean 0.96 0.31 0.13 12.6

CV 0.21 0.11 0.20 0.31

May Creek Mean 0.66 0.33 0.09 6.5

CV 0.27 0.11 0.14 0.45

Soos Creek Mean 3.49 0.39 0.72 20.4

CV 0.27 0.10 0.20 0.42

Rural

Huge Creek Mean 0.31 0.27 0.11 3.7

CV 0.28 0.18 0.15 0.63

Big Beef Creek Mean 1.22 0.28 0.10 18.0

CV 0.35 0.21 0.21 0.26

Newaukum Creek Mean 1.70 0.35 0.42 18.9

CV 0.24 0.10 0.22 0.45

Issaquah Creek Mean 3.77 0.35 0.68 40.0

CV 0.26 0.09 0.22 0.47

Populations expanded rapidly during the latter 

half of the 20th century in urban basins with low, 

though increasing growth rates in the suburban and 

rural basins (King County, 2001; Washington State, 

2001). The largest cities grew quickly and attained 

higher population densities than the surrounding areas, 

while growth in smaller cities lagged. For example, the 

population density around Leach Creek in Tacoma had 

reached a relatively high level (1,100 people/km2) by 

1950 while the population around Mercer Creek, which 

drains Bellevue, did not reach this density until 1990 

(Washington State, 2001). Hydrologic trends are 

expected to be most pronounced in Juanita and Mercer 

Creeks where population growth has been greatest 

during periods with streamflow records.

Road densities (about 1995) were used to 

classify the basins as urban, suburban, or rural. Leach, 

Juanita, and Mercer Creeks are "urban" with road 

densities greater than 9 km/km2. Swamp, May, and 

Soos, Creeks are "suburban" with road densities of 4 to 

8 km/km2. Huge, Big Beef, Newaukum, and Issaquah 

Creeks are "rural" with road densities of less than 3 

km/km2. Road densities (table 1) correlate well with 

population densities of representative communities 

from each basin (table 2).

Four annual streamflow statistics were 

investigated to characterize the range of streamflow 

patterns that may be affected by urban development: 

annual mean discharge, the fraction of year annual 

mean discharge was exceeded, 7-day low flow, and 

annual maximum (instantaneous) discharge.
Methods for Identifying Hydrologic Trends Associated with Urban Development 11

Annual Mean Discharge 

Annual mean discharge (Qmean) provides a 

central measure of the range of discharge in a stream 

and indicates the annual runoff volume. Changes in 

annual Qmean without corresponding changes in timing 

of runoff would effectively change the aquatic habitat 

available in a stream. A decrease in annual Qmean

would decrease stream depth and wetted surface area 

and an increase in annual Qmean would increase stream 

depth and wetted surface area. The productivity and 

diversity of aquatic communities generally vary with 

habitat area or other measures related to Qmean such as 

drainage area (Welcomme, 1979, Allan, 1995). Thus 

biological conditions of a stream could be expected to 

vary directly with trends in annual Qmean. However, 

urban development is not anticipated to have a 

systematic effect on Qmean, unless (1) irrigation in a 

basin is extensive with water imported from other 

basins or pumped from deep aquifers, (2) rates of 

consumptive water use of surface water or ground 

water from shallow aquifers that would otherwise 

discharge to the stream are high, or (3) interbasin 

transfers of stormwater are large.

Fraction of Year Annual Mean Discharge 

Was Exceeded

The fraction of year annual mean discharge was 

exceeded (TQmean) provides a temporal measure of the 

distribution of annual runoff in a stream between 

stormflow periods and recessional and baseflow 

periods. TQmean is high in streams with sustained 

stormflow periods with gradual recession rates and 

relatively high baseflow. TQmean is low when 

stormflow periods are brief but with high magnitudes 

and rapid recession rates ("flashy" hydrographs). 

TQmean is calculated as the fraction of days in a given 

water year that daily discharge exceeds that year's 

Qmean. TQmean may be low in years with large floods 

when Qmean is high but not necessarily the number of 

days when daily discharge exceeds Qmean. However, 

TQmean is less sensitive to the magnitude of large floods 

than other statistics such as the coefficient of variation 

or skew of daily discharge rates that are derived from 

the difference between daily and annual mean 

discharge rates. 

An increase in hydrologic variability is likely 

important to the structure and composition of lotic 

communities (Shelford and Eddy, 1929; Odum, 1956; 

Horwitz, 1978; Poff and Allan, 1995). The increased 

variability specifically referenced by TQmean, which is 

an increase in stormflow volume relative to winter 

baseflow, is likely to represent an increase in the 

frequency of streambed disturbance and diminished 

extent of ambient aquatic habitat. The biological effects 

of this variability in Puget Lowland streams was 

evident in the variation of diversity of benthic 

macroinvertebrate assemblages with TQmean (Konrad, 

2000). TQmean is expected to decrease in streams 

during periods of urban development as runoff is 

redistributed from periods of stormflow recession and 

baseflow to stormflow.

Annual 7-day Low Flow

The 7-day low flow (Qmin) provides a measure of 

the lowest magnitude of streamflow during summer 

baseflow conditions in selected streams in western 

Washington. Qmin is calculated as the minimum mean 

daily discharge for 7 consecutive days. Qmin provides a 

measure of the severity of annual droughts in a stream, 

although the biological significance of a drought may 

depend on other conditions such as water depth or 

temperature. Even though aquatic communities may 

recover rapidly from a severe drought (Bayley and 

Osborne, 1993), a decreasing trend of low flows in a 

stream may signify a decrease in the habitat available 

to fish and other aquatic organisms during the summer 

with consequent reductions in the diversity and 

complexity of lotic communities in habituating that 

stream (Stehr and Branson, 1938; Rabeni and Wallace, 

1998). The effects of urban development on Qmin

warrants further examination in western Washington 

streams given the conflicting responses observed in 

other regions (Sawyer, 1963; Harris and Rantz, 1964; 

Klein, 1979).
12 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

Annual Maximum Discharge 

Annual maximum (instantaneous) discharge 

(Qmax) provides a measure of the magnitude of the 

largest flood in a stream during a year. By transporting 

and depositing bed material, floods represent a form of 

ecological disturbance. As with droughts, aquatic 

communities can recover rapidly from floods (Stehr 

and Branson, 1938; Fisher and others, 1982). In 

alluvial streams, an increase in the magnitude of floods 

expected from urban development may cause only a 

transient increase in the frequency extent of biological 

disturbances during periods of channel expansion 

(Booth, 1990). Once a stream channel has expanded, it 

may accommodate frequent foods without extensive 

erosion. However, streams in urban areas generally are 

not free to erode their banks, thus even frequent floods 

may represent a persistent form of disturbance. Indeed, 

Orser and Shure (1972) suggested that the decrease in 

salamander populations in urban streams in Georgia 

was a consequence of the increased frequency and 

magnitude of floods. 

Qmax increases in response to urban development 

in a stream basin, particularly for smaller magnitude 

events, which can increase by as much as an order of 

magnitude (Hollis, 1975). More typical increases for 

less frequent (but more potentially damaging) storms in 

western Washington, evaluated using the Hydrologic 

Simulation Program Fortran (HSPF), show two- to 

three-fold increases following urban development 

(King County, 1991). This is approximately the same 

range of variability imposed by fluctuations in year-toyear weather conditions, even in the absence of any 

changes in a drainage basin. As a consequence, trends 

in Qmax may be statistically significant only for long 

records spanning a period of urban development.

Continuous Trends

Trends in the streamflow statistics were 

evaluated using nonparametric and parametric tests. 

The nonparametric tests for correlation between each 

statistic and time used Kendall's correlation coefficient 

(Helsel and Hirsch, 1993):

(1)

where

Kendall's t has a value of 1 where the value of a 

statistic increases each year and has a value of -1 where 

the value decreases each year. The statistical 

significance of trends were evaluated in terms of the 

probability (p) of observing the statistic S, where S = I - 

D, given the null hypothesis of no trend. This provided 

a "two-tailed" test for either an increasing or decreasing 

trend. If p was less than 5 percent, the trend was 

identified as significant. S-PLUS 2000 (MathSoft, 

2000) was used to perform the tests. 

The parametric test for trends assessed the 

correlation between each statistic and population 

density of the representative communities for each 

stream using the Pearson correlation coefficient (ρ)

(Helsel and Hirsch, 1993):

(2)

where

The significance of the correlation was assessed 

using a two-tailed Student's t distribution where t was 

calculated by (Helsel and Hirsch, 1993):

(3)

with n-2 degrees of freedom where n is the number of 

years of record. If p < 0.05, then the correlation is 

significant, though its strength depends on the value of 

τ ρ. 2( ) I D–

n n( ) – 1 = --------------------

I = Number of pairs of annual values of a statistic 

that have an increasing value over time,

D = Number of pairs of annual values that have a 

decreasing value over time, and

n = Total number of annual values of the statistic.

xi is population density for year i,

x is the mean population density,

sx is the standard deviation of population density,

yi is the streamflow statistic for year i,

y is the mean streamflow statistic, and

sy is the standard deviation of the streamflow 

statistic.

ρ

1

n – 1 -----------    

i = 1

n

∑ xi – x

sx

------------     yi – y

sy

------------     =

t ρ n – 2

1 ρ

2 –

= -------------------
Methods for Identifying Hydrologic Trends Associated with Urban Development 13

Normal quantile plots were used to evaluate the 

distributions of the four statistics (fig. 4). A normally 

distributed variable will plot as a straight line on a 

normal quantile plot. Qmean, TQmean, and Qmin have 

approximately normal distributions in most streams. 

Qmax generally is skewed with the largest floods in a 

stream having greater magnitudes than are expected for 

a normal distribution (fig. 4) so it was plotted on a logscale. In this case, Qmax will plot as a straight line if it 

has a log normal distribution.

Error Analysis

Tests for anthropogenic trends in streamflow 

patterns are likely to produce a relatively high error 

rate because of the high annual variability in 

streamflow and the incremental effects of urban 

development on streamflow patterns over time. Shortterm hydrologic trends can emerge over periods of a 

few years to a decade as a result of climatic conditions, 

but are unlikely to be effects of urban development, 

which tend to be monotonic and permanent. If a trend 

due to climatic variability is mistakenly identified as a 

"consequence of urbanization," it represents a Type I 

statistical error (false positive). Alternatively, statistical 

tests may not detect trends in an annual streamflow 

statistic where the incremental change each year is 

small relative to annual variability or where the trend 

occurred outside of the period of record. In such cases, 

a statistical test would not recognize the importance of 

the short-term change and so produce a Type II error 

(false negative).

The use of "error" in this report, then, is limited 

to cases where the results of testing a portion of the 

period of record for trends diverge from the results of 

testing the entire period of record for trends. Errors are 

identified by either a trend during a portion of a period 

of record that did not persist over the period of record 

(Type I) or the lack of a trend during a portion of the 

period of record when there was a trend over the entire 

period of record (Type II).

The error rates of trend tests for each individual 

stream were analyzed by comparing the results of trend 

tests for selected intervals to the results of the test for 

the entire period of record. Kendall's t test was applied 

to 10-year periods of record beginning in 1960, 1965, 

1970, 1975, 1980, 1985, and 1990. Type I errors (false 

positives) were identified for any stream that had a 

statistically significant trend over the 10-year period 

but did not have a trend, or had a trend of the opposite 

direction, over the entire period of record. Type II 

errors (false negatives) were identified for any stream 

that showed no statistically significant trends during 

any of 10-year periods but did have a trend based on a 

Kendall's t test over the entire period of record.

The error analysis for the parametric tests 

proceeded as with the nonparametric tests. Type I 

errors were identified for streams where the correlation 

between the value of a statistic and population density 

for a 10-year period was significant, but the correlation 

between the statistic and population density was not 

significant over the entire period of record. Type II 

errors were identified for streams where the correlation 

between the value of a statistic and population density 

for any 10-year period was not significant, but the 

statistic was significantly correlated with population 

density over the entire period of record.
14 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

0

0.02

0.04

0.06

0.08

0.10

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

0

0.05

0.10

0.15

0.20

0.25

LEACH CREEK

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

A. Urban Streams

100

101

102

2

3

4

5

7

2

3

4

5

7

Figure 4. Normal quantile plots of the cumulative distributions of annual mean discharge, fraction of year annual mean 

discharge was exceeded, 7-day low flow, and annual maximum discharge relative to land use for selected streams in the 

Puget Sound Basin, western Washington.
Methods for Identifying Hydrologic Trends Associated with Urban Development 15

0

0.02

0.04

0.06

0.08

0.10

0

0.1

0.2

0.3

0.4

0.5

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

-2 -1 0 1 2 -2 -1 0 1 2

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

A. Urban Streams.--Continued

0

0.05

0.10

0.15

0.20

0.25

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

100

101

102

2

3

4

5

7

2

3

4

5

7

JUANITA CREEK

Figure 4.—Continued.
16 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

0

0.05

0.10

0.15

0.20

0.25

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

0

0.2

0.4

0.6

0.8

1.0

1.2

MERCER CREEK

A. Urban Streams.--Continued

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

Figure 4.—Continued.
Methods for Identifying Hydrologic Trends Associated with Urban Development 17

0

0.05

0.10

0.15

0.20

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

0

0.4

0.8

1.2

1.6

SWAMP CREEK

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

B. Suburban Streams

Figure 4.—Continued.
18 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

0

0.02

0.04

0.06

0.08

0.10

0.12

0

0.1

0.2

0.3

0.4

0.5

-2

0

0.2

0.4

0.6

0.8

1.0

1.2

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

B. Suburban Streams.--Continued

-1 0 1 2 -2 -1 0 1 2

MAY CREEK

Figure 4.—Continued.
Methods for Identifying Hydrologic Trends Associated with Urban Development 19

0

0.02

0.04

0.06

0.08

0.10

0

0.1

0.2

0.3

0.4

0.5

1

2

3

4

6

5

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

100

101

102

2

3

4

5

7

2

3

4

5

7

-2 -1 0 1 2 -2 -1 0 1 2

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

B. Suburban Streams.--Continued

SOOS CREEK

0

Figure 4.—Continued.
20 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

0

0.04

0.08

0.12

0.16

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

0

0.1

0.2

0.3

0.4

0.5

0.6

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

C. Rural Streams

HUGE CREEK

Figure 4.—Continued.
Methods for Identifying Hydrologic Trends Associated with Urban Development 21

0

0.05

0.10

0.15

0.20

0

0.1

0.2

0.3

0.4

0.5

-2

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

-1 0 1 2 -2 -1 0 1 2

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

C. Rural Streams.--Continued

0.5

1.0

1.5

2.0

2.5

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

BIG BEEF CREEK

0

Figure 4.—Continued.
22 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

0

0.1

0.2

0.3

0.4

0.6

0.5

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

C. Rural Streams.--Continued

0

0.5

1.0

1.5

2.0

2.5

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

100

101

102

2

3

4

5

7

2

3

4

5

7

NEWAUKUM CREEK

Figure 4.—Continued.
Methods for Identifying Hydrologic Trends Associated with Urban Development 23

0

0.2

0.4

0.6

0.8

1.0

0

0.1

0.2

0.3

0.4

0.5

-2 -1 0 1 2 -2 -1 0 1 2

0

1

2

3

4

6

5

FRACTION OF YEAR ANNUAL MEAN 

DISCHARGE IS EXCEEDED

7-DAY LOW FLOW 

(CUBIC METER PER SECOND)

ANNUAL MAXIMUM DISCHARGE

(CUBIC METER PER SECOND)

ANNUAL MEAN DISCHARGE

(CUBIC METER PER SECOND)

NORMAL DISTRIBUTION NORMAL DISTRIBUTION

100

101

102

2

3

4

5

7

2

3

4

5

7

C. Rural Streams.--Continued

ISSAQUAH CREEK

Figure 4.—Continued.
24 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

HYDROLOGIC TRENDS

Although hydrologic trends were identified in 

each of the four statistics analyzed, trends were not 

consistent for any of the four streamflow statistics in all 

streams. Trends in TQmean and Qmax over the entire 

period of record emerged consistently in urban streams 

using nonparametric and parametric tests. The 

nonparametric tests for trends over 10-year periods 

frequently produced results inconsistent with those for 

trends for the entire period of record. 

Trends Over the Entire Period of Record

Each of the four streamflow statistics exhibited 

trends in some of the streams. Only TQmean and Qmax

consistently exhibited trends in the three urban streams: 

Leach, Juanita, and Mercer Creeks (fig. 5). Qmax

increased significantly (p < 0.001 of no trend) over the 

entire period of record in all three urban streams using 

the nonparametric tests and was significantly correlated 

with population density using the parametric Pearson 

tests (ρ ranged from 0.28 to 0.52, p < 0.001) (table 3). 

Qmax also was significantly correlated with population 

density in Big Beef Creek (ρ = 0.54, p = 0.025). 

TQmean decreased significantly over time (p < 0.001 of 

no trend) in Juanita and Mercer Creeks using the 

nonparametric tests. TQmean was correlated with 

population density in Juanita Creek (ρ = -0.42, p = 

0.043), Mercer Creek (ρ = -0.71, p = 0.043), and 

Swamp Creek (ρ = -0.53, p = 0.023). Trends in Qmax or 

TQmean using either parametric or nonparametric tests 

were not significant in any of the suburban or other 

rural streams.

Trends in Qmean and Qmin were not consistent for 

urban, suburban, or rural streams (fig. 5). For urban 

streams, a significant increase (p < 0.001) in Qmean was 

detected in Leach Creek but not in Juanita or Mercer 

Creeks using the nonparametric tests. Qmean was 

correlated with population density in Leach Creek 

(ρ = 0.58, p < 0.001). The increase in Qmean for Leach 

Creek was possibly due to low-flow augmentation or 

the particularly wet years of 1996, 1997, and 1999, 

though the higher than average streamflow during 

those years did not produce significant trends in Qmean

in other streams. A significant decrease (p < 0.001) in 

Qmean was evident in Issaquah Creek using the 

nonparametric test and Qmean was inversely correlated 

with population density (ρ = -0.64, p < 0.001). The 

decrease in Qmean may be associated with the 

combination of ground-water pumping and wastewater 

collection in the Issaquah Creek basin. Qmin increased 

over the entire period of record in Mercer Creek (p < 

0.001) and correlated with population density (ρ = 

0.73, p < 0.001). In contrast, Qmin decreased over time 

in Issaquah Creek (p = 0.008) and negatively correlated 

with population density in Issaquah (ρ = -0.64, p < 

0.001) and Newaukum Creeks (ρ = -0.52, p = 0.003).
Hydrologic Trends 25

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

LEACH CREEK

WATER YEAR

A. Urban Streams

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5. Annual values of mean discharge (Qmean), fraction of year mean discharge was exceeded (TQmean), 7-day low flow 

(Qmin), and maximum discharge (Qmax) relative to land use for selected streams in the Puget Sound Basin, western Washington.
26 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

WATER YEAR

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

JUANITA CREEK

A. Urban Streams--Continued DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
Hydrologic Trends 27

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

MERCER CREEK

WATER YEAR

A. Urban Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
28 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

WATER YEAR

B. Suburban Streams

SWAMP CREEK

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
Hydrologic Trends 29

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

MAY CREEK

WATER YEAR

B. Suburban Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
30 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

SOOS CREEK

WATER YEAR

B. Suburban Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
Hydrologic Trends 31

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

HUGE CREEK

WATER YEAR

C. Rural Streams

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
32 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

BIG BEEF CREEK

WATER YEAR

C. Rural Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
Hydrologic Trends 33

1940 1950 1960 1970 1980 1990 2000

0.1

1

10

100

0

0.1

0.2

0.3

0.4

0.5

NEWAUKUM CREEK

WATER YEAR

C. Rural Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

Figure 5.—Continued.
34 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

STREAMFLOW STATISTICS, 

 IN CUBIC METERS PER SECOND

EXPLANATION

Annual mean discharge (Qmean)

Annual maximum discharge (Qmax)

7-day low flow (Qmin)

Fraction of time that mean discharge 

 rate is exceeded (TQmean)

1940 1950 1960 1970 1980 1990 2000

0.1

1

10

100

1,000

0

0.1

0.2

0.3

0.4

0.5

ISSAQUAH CREEK

WATER YEAR

C. Rural Streams--Continued

FRACTION OF YEAR MEAN DISCHARGE IS EXCEEDED

(TQmean)

DISCHARGE (CUBIC METER PER SECOND)

Figure 5.—Continued.
Hydrologic Trends 35

Errors in Long-Term Trend Identification 

Using Short-Term Period of Records

Trends in streamflow statistics over 10-year 

periods were compared to trends over the entire period 

of record for each stream. Results of analyzing 10-year 

periods for trends frequently diverged from the result 

of analyzing the entire period of record (table 4). The 

nonparametric Kendall correlation tests produced 

lower Type I errors (false positives) but slightly higher 

Type II errors (false negatives) than the parametric 

Pearson correlation tests. Thus, neither the 

nonparametric nor the parametric tests confers a clear 

advantage for identifying long-term hydrologic trends 

based on a 10-year period of record. The high rate of 

Type I errors (90 percent of streams) produced with the 

Pearson correlation coefficient indicates that a lower 

significance level than p = 0.05 may be appropriate for 

identifying long-term trends when using 10-year period 

of records but as a result, Type II error rate would 

increase.

Hydrologic Effects of Urban Development

The hydrologic effects of urban development in 

western Washington are manifested as changes in 

annual streamflow patterns: in particular, TQmean

decreased while Qmax increased in urban streams. 

These trends represent the redistribution of runoff from 

stormflow recession and baseflow to stormflow that is a 

result of urban development. Trends in Qmean and Qmin

were not consistent in urban streams and, in fact, trends 

in these statistics were observed for some suburban and 

rural streams lacking extensive changes in land use at 

the scale of a drainage basin over the entire period of 

record. Although urban development may affect Qmean

or Qmin in some streams, relations between 

development and these statistics were not consistent.

Table 3. Hydrologic trends in streamflow statistics over the entire period of record for 10 selected streams in the Puget 

Sound Basin, western Washington

[Hydrologic trends in streamflow statistics were evaluated with nonparametric and parametric tests. Kendall’s τ: Nonparametric test of correlation 

between streamflow statistic and time. Pearson ρ: Parametric test of correlation between streamflow statistic and population density. ns, not significant 

(probability of no correlation <0.05)]

Land use Stream

Trends in Streamflow Statistics

Mean annual 

discharge (Qmean)

Fraction of year 

annual mean 

discharge was 

exceeded (TQmean)

7-day low flow

(Qmin)

Log max annual 

discharge (Log 

Qmax)

Kendall's 

τ

Pearson 

ρ

Kendall's 

τ

Pearson 

ρ

Kendall's 

τ

Pearson 

ρ

Kendall's 

τ

Pearson 

ρ

Urban Leach Creek 0.4 0.58 ns ns ns ns 0.43 0.52

Juanita Creek ns ns -0.31 -0.42 ns ns 0.48 0.59

Mercer Creek ns 0.27 -0.46 -0.71 0.43 0.73 0.47 0.28

Suburban Swamp Creek ns ns ns -0.53 0.43 ns ns ns

May Creek ns ns ns ns ns ns ns ns

Soos Creek ns ns ns ns ns -0.64 ns ns

Rural Huge Creek ns ns ns ns ns ns ns ns

Big Beef Creek ns ns ns ns ns ns ns 0.54

Newaukum Creek ns ns ns ns -0.27 -0.52 ns ns

Issaquah Creek -0.3 -0.43 ns ns -0.45 -0.64 ns ns
36 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

Although trends were not consistent for any 

statistic in all 10 of the selected streams in western 

Washington, trends in Qmin were identified in five 

streams over the entire periods of record. Trends in 

Qmin demonstrated the greatest sensitivity to the 10-

year period of record selected for analysis with a mix 

of upward and downward trends occurring over 

different periods in the same stream. These findings are 

in general agreement with the results of Lins and Slack 

(1999) who found decreasing discharge rates at all 

quantiles from minimum to maximum daily discharge 

in many, but not all, Pacific Northwest rivers. 

Although Qmin was not consistently affected by 

urban development in the Puget Lowland streams, the 

increasing trends in Mercer and Swamp Creeks could 

be responses to urban development as water may be 

supplied to these basins by interbasin transfers. 

Likewise, the decreasing trends in Issaquah and 

Newaukum Creeks may be responses to withdrawal of 

shallow ground water in these basins. Changes in 

hydrologic processes, such as infiltration and recharge, 

resulting from urban development apparently do not 

have a general influence on annual low flows even as 

wet-season baseflow is low in urban streams (Konrad, 

2000).

TQmean and Qmax appear to be reliable indicators 

of hydrologic trends resulting from urban development. 

Although TQmean generally is less variable (annual 

coefficient of variation ranging from 0.09 to 0.21 for 

the streams in this analysis) than Qmax (coefficient of 

variation of 0.26 to 0.63), the relative change in TQmean

as a result of urban development is less than the relative 

change in Qmax. Thus, trends in TQmean resulting from 

urban development may be less significant than trends 

in Qmax despite the low annual variability of TQmean. 

TQmean is a broader measure of streamflow patterns 

than Qmax and offers a practical advantage when 

monitoring large numbers of streams that are likely to 

have brief and synchronous peak discharges. Whereas, 

Qmax requires accurate measurement or estimation of 

these peaks, TQmean is less sensitive to the accuracy or 

the lack of high flow measurements in a continuous 

record.

Land use or management activities that have a 

small incremental effect on runoff processes in a 

stream basin may be difficult to detect given other 

sources of hydrologic variability. Indeed, trends may be 

evident only for streams with extensive urban 

development where the magnitude of the trends are 

large relative to the variation caused by climatic 

conditions and where hydrologic data span a relatively 

long period of time.

Table 4. Error rates for comparisons of trends in streamflow statistics for 10-year periods of 

record with trends over the entire period of record for each of the 10 selected streams in the Puget 

Sound Basin, western Washington

[Hydrologic trends in streamflow statistics were evaluated with nonparametric and parametric tests. Kendall’s τ: 

Nonparametric test of correlation between streamflow statistic and time. Pearson ρ: Parametric test of correlation between 

streamflow statistic and population density. Type I error: Fraction of streams with a trend over a 10-year period but no 

trend over the period of record or a trend with the opposite direction. Type II error: Fraction of streams with no trends over 

10-year periods but a significant trend over the period of record]

Tests

Error rates for comparisons of trends in streamflow statistics

Fraction of year annual 

mean discharge was 

exceeded (TQmean)

7-day low flow (Qmin) Log maximum annual 

discharge (Log Qmax)

Type I error 

(false 

positive

Type II error 

(false 

negative)

Type I error 

(false 

positive

Type II 

error 

(false 

negative)

Type I error 

(false 

positive

Type II 

error 

(false 

negative)

Kendall's τ 0.30 1.00 0.40 0.50 0.10 0.33

Pearson ρ 0.90 0.33 0.90 0.25 0.90 0.25
Implications of Trend Analysis for Hydrologic Monitoring 37

IMPLICATIONS OF TREND ANALYSIS FOR 

HYDROLOGIC MONITORING

The analysis of streamflow trends in this 

investigation holds several implications for monitoring 

the hydrologic response of drainage basins to urban 

development and to activities intended to mitigate the 

hydrologic effects of urban development. 

Streamflow records spanning a decade generally 

are unreliable for identifying trends due to urban 

development because of background variability in 

streamflow statistics and the relatively incremental 

effect of urban development on streamflow patterns at 

the scale of a decade. Analysis of 10-year periods are 

likely to show trends in a streamflow statistic even if 

there is no significant trend over a long period of record 

for the stream (Type I error). Conversely, a 10-year 

period may be insufficient for identifying a trend for a 

longer period (Type II error). The dependency of trend 

analyses on record length suggests that there are 

sources of hydrologic variability unrelated to land use 

that operate over time scales of a year to a decade and 

that streamflow patterns may change incrementally in 

response to land use. As a result, any conclusions about 

hydrologic trends based on periods of 10 years or less 

should be tentative and may not apply to periods before 

or after the period analyzed.

Long periods of record are not available at many 

streamflow-gaging stations for estimating either 

previous or current values of TQmean. The feasibility of 

extending records using a reference station with a long 

period of record was evaluated by comparing values of 

TQmean for Newaukum and Issaquah Creeks. Neither 

basin has had major systematic land-use changes for 

the period of simultaneous gaging (1964-98) and 

neither stream had a trend in TQmean over time (table 

3). However, annual TQmean was not correlated 

between Newaukum and Issaquah Creeks (fig. 6). 

Thus, a short-time series of TQmean from one 

streamflow-gaging station cannot be extended based on 

a longer TQmean time series from another gaging 

station.

Figure 6. Fraction of year daily mean discharge exceeds annual mean 

discharge (TQmean) in two rural streams — Issaquah and Newaukum Creeks in 

the Puget Sound Basin, western Washington, water years 1964-2000.

0.25

0.30

0.35

0.40

0.45

0.50

0.25 0.30 0.35 0.40 0.45

FRACTION OF YEAR DAILY MEAN DISCHARGE EXCEEDS 

ANNUAL MEAN DISCHARGE FOR ISSAQUAH CREEK

FRACTION OF YEAR DAILY MEAN DISCHARGE EXCEEDS 

ANNUAL MEAN DISCHARGE FOR NEWAUKUM CREEK
38 Hydrologic Trends Associated with Urban Development for Selected Streams in the Puget Sound Basin, Western Washington

Nonparametric or parametric tests may be useful 

for analyzing hydrologic trends. The results of the 

analyses for the period of record (table 3) generally are 

similar for nonparametric and parametric tests, though 

the Pearson correlation coefficient appears to be 

slightly more sensitive than the Kendall correlation 

coefficient. The sensitivity of the Pearson correlation 

coefficient was evident in its consistently high Type I 

error rate when 10-year trends were compared to trends 

over the entire period of record.

SUMMARY

Annual streamflow statistics were analyzed for 

trends in 10 selected streams in western Washington. 

The statistics, which represented a variety of flow 

conditions, included mean discharge (Qmean), fraction 

of year Qmean was exceeded (TQmean), 7-day low flow 

(Qmin), and maximum discharge (Qmax). TQmean 

decreased over time in two urban streams. Qmax

increased over time in three urban streams and one 

rural stream. Trends in Qmin were mixed, increasing in 

one urban stream and one suburban stream and 

decreasing in one suburban and two rural streams. 

Qmean increased over time in two urban streams and 

decreased in one rural stream.

TQmean may offer practical advantages over 

Qmax for assessing the hydrologic effects of urban 

development or activities designed to mitigate those 

effects. The error in TQmean is likely less than that of 

Qmax, which results from measurement error at high 

flow. Qmax is often difficult to measure particularly for 

a group of small urban streams where flood peaks are 

brief and simultaneous. Moreover, TQmean provides a 

broader measure of the hydrological changes produced 

by urban development than Qmax. As a result, TQmean

may better indicate ecological consequences of landuse changes.

The analyses of trends in streamflow statistics 

over 10-year periods in many streams were not 

consistent with analyses for the entire period of record. 

Thus, trend tests of any hydrologic statistic using 10 

years or less of streamflow record are likely to identify 

trends not associated with land-use changes and may 

fail to identify the hydrologic trends produced by urban 

development that proceed incrementally over periods 

of more than a decade.

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P

rintedonrecycledpaper

HYDROLOGIC TRENDS ASSOCIATED WITH URBAN DEVELOPMENT FOR SELECTED STREAMS

IN THE PUGET SOUND BASIN, WESTERN WASHINGTON

HYDROLOGIC TRENDS ASSOCIATED WITH URBAN DEVELOPMENT FOR SELECTED STREAMS

IN THE PUGET SOUND BASIN, WESTERN WASHINGTON

C.P. KONRAD AND D.B. BOOTH USGS WRIR 02-4040