U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3017

March 2005

Streamflow Trends in the United States

... from the National Streamflow Information Program

Printed on recycled paper

This Fact Sheet is one in a series that highlights information or recent research findings from the USGS 

National Streamflow Information Program (NSIP). The investigations and scientific results reported in this 

series require a nationally consistent streamgaging network with stable long-term monitoring sites and a 

rigorous program of data quality assurance, management, archiving, and synthesis. NSIP produces multipurpose, unbiased surface water information that is readily accessible to all.

Introduction

Recent studies have reported 

increases in precipitation across the 

United States during the 20th century 

(Karl and Knight, 1998; Groisman 

and others, 2004). These increases 

have been observed over a range of 

precipitation intensities, and particularly in categories characterized 

as heavy and extreme. This has led 

some researchers to suggest that 

extreme hydrological events, particularly floods, may be increasing 

in frequency and/or magnitude as 

well. The basis for this suggestion 

is related to climate change research, 

which hypothesizes that increasing temperatures will accelerate the 

hydrologic cycle and increase the 

occurrence of floods and droughts. 

Using long-term streamflow records 

from the U.S. Geological Survey 

(USGS) National Streamflow Information Program (NSIP), it is possible to evaluate whether floods and 

droughts have, indeed, increased in 

recent decades in response to climatic 

conditions.

Streamflow Data

The key to determining changes in 

floods and droughts is a stable, longterm network of streamgages, including streamgages on watercourses 

that are relatively free of confounding human influences such as dams, 

impoundments, and diversions. The 

USGS maintains a database representing approximately 22,700 

streamgages. Only about 7,000 of 

these are still active and more than 

one-half of these gages have less than 

Puerto Rico

Hawaii

AK

Virgin Islands

Alaska

Figure 1. Location of 1,659 Hydro-Climatic Data Network (HCDN) streamgaging stations.

30 years of continuous record to the 

present time. Streamgage records 

of at least 30 years, and preferably 

greater than 50 years, are essential 

for assessing trends because climate 

naturally varies from year-to-year 

and decade-to-decade. Such shortterm natural variations can obscure or 

falsely indicate a long-term trend.

In addition, the flow of water 

in most rivers in the United States 

reflects some level of human activity. 

Dams and diversions for irrigation 

may be the most cited activities, but 

ground-water pumping and land-use 

change are also important factors. 

Human activities can substantially 

influence the variability of streamflow in response to weather and 

climatic conditions. Indeed, dams 

are constructed in part to reduce 

flooding and to increase flows during 

drought. Thus, to evaluate streamflow variability and change in a 

climatic context, the USGS identified 

over 1,600 streamgages where the 

discharge was primarily influenced 

by climatic variations (fig. 1). These 

streamgages form the USGS HydroClimatic Data Network (HCDN), 

monitoring watersheds where the data 

are appropriate to the study of such 

Federal interest problems and issues 

as flood frequency, drought severity, 

and long-term climatic change (Slack 

and Landwehr, 1992). 
Although more than 1,600 streamgage records have been identified 

as being climate-sensitive, most of 

these have record lengths of less than 

30 years and, in many instances, 

less than 20 years. The number of 

HCDN streamgages in the United 

States with 60 years of record (1940-

1999) is 435, but this number drops 

to 211 with 70 years of record, 69 

with 80 years of record, and only 15 

streamgages with 90 years of record. 

As a result, it is not possible to get an 

accurate estimate of trends in United 

States streamflow for the entire 20th

century, although it is possible to get 

an accurate estimate for the later twothirds of the century. 

Changes in Streamflow

The USGS has studied trends 

in streamflow using a variety of 

approaches (Lins and Michaels, 

1994; Lins and Slack, 1999; McCabe 

and Wolock, 2002). Although each 

approach has yielded unique insights, 

all have arrived at one important conclusion: streamflow has been increasing in the United States since at least 

1940. In addition, these increases 

have not been uniform across the 

range of annual streamflows, from the 

minimum to the maximum, nor have 

they been uniform geographically or 

seasonally. 

For example, most of the increases 

were found to occur in low to moderate streamflows. In one study, trends 

were determined across the entire 

range of streamflows, from the annual 

minimum (the 0th percentile) to the 

annual maximum (the 100th percentile), using data from the 435 HCDN 

stations (fig. 2). Between 1940 and 

1999, 40 percent of the stations tested 

experienced a streamflow increase in 

the annual minimum flow, 43 percent 

in the annual median flow, but only 

10 percent in the annual maximum 

flow. In contrast, only 8 percent 

of the stations had decreases in the 

annual minimum flow, less than 1 

percent in the annual median, and 3 

percent in the annual maximum. In 

other words, low to moderate streamflow volumes have been increasing at 

many locations, but high streamflow 

volumes have increased at relatively 

few locations.

-20

-10

0

10

20

30

40

50

Annual Minimim Annual Median Annual Maximum

Streamflow Category

Percent of Stations (out of 435)

Increasing Trend

Decreasing Trend

Figure 4. Annual departures from the maximum (A), median (B), and minimum (C) daily 

streamflow for 400 streamgaging stations in the conterminous United States 1941-1999.

Figure 2. Percent of streamgages, out of 435 nationwide, where significant trends were 

observed in the annual minimum, median, and maximum streamflow during the 60-year 

period, 1940-1999.

1940 1950 1960 1970 1980 1990 2000

1

0

-1

1

0

-1

1

0

-1

Standardized Departure

Year

1940 1950 1960 1970 1980 1990 2000

1940 1950 1960 1970 1980 1990 2000

A. Maximum

B. Median

C. Minimum

Geographically, the pattern of 

trends is quite varied (fig. 3). Regions 

that experienced the most widespread 

increases were the Upper Mississippi, Ohio Valley, Texas-Gulf, and 

the Mid-Atlantic. Fewer trends were 

observed in the South Atlantic-Gulf, 

Missouri, and regions of the far West. 

The Pacific Northwest and the South 

Atlantic-Gulf actually had a number 

of streamflow decreases, particularly 

in the lowest percentiles.

The seasonal distribution of 

observed trends also is not uniform. 

The pattern of trends is dominated 

by increases in the late summer and 

autumn months of September through 

December. This is particularly true in 

the case of the Upper Mississippi and 

Ohio Valley regions. It is consistent 

with observed increases in streamflow 

at the low to moderate percentiles, 

which generally occur during the late 

summer and autumn period. This 
Figure 3. Trends (p<0.05) in (a) annual maximum daily, (b) annual median daily, and (c) 

annual minimum daily discharge in relation to U.S. water resource regions for the period 

1940-1999. Upward-pointing triangles indicate increasing discharge, downward-pointing decreasing, and open circles (○) denote no trend.

result also is consistent with reported 

precipitation increases in the United 

States, which have been greatest during the autumn season.

Another aspect of the observed 

trends is that they appear to have 

occurred around 1970 as an abrupt 

rather than gradual change (fig. 

4). An assessment of trends at 400 

HCDN stations between 1941 and 

1999 found that annual departures 

from long-term average conditions 

for the annual maximum, median, 

and minimum flows were primarily 

negative prior to 1970 and primarily 

positive thereafter. The pattern was 

less striking in the annual maximum, 

but apparent nonetheless. Both sudden and gradual changes can be found 

in historical climatic and hydrologic 

records, and each type of change has 

distinct implications. A slow, gradual 

trend implies a pattern that is likely to 

continue into the future. A rapid step 

change typically indicates a regime 

shift from one set of conditions to 

another, with the new conditions 

likely to persist until the next sudden 

shift occurs. What this may mean for 

future variations and changes in U.S. 

streamflow will only be revealed with 

time but, based on nearly a century of 

observations, we should expect our 

rivers and streams to continue to be 

characterized by both short- and longterm variations.

For additional information, contact:

Harry Lins

U.S. Geological Survey

415 National Center

Reston, VA 20192

(703)648-5712 or 

hlins@usgs.gov

References

Groisman, P.Ya., Knight, R.W., Karl, 

T.R., Easterling, D.R., Sun, B. and 

Lawrimore, J.H., 2004. Contemporary 

changes of the hydrological cycle over 

the cotiguous United States: trends 

derived from in situ observations: J. 

Hydrometeor., 5, 64-85.

Karl, T.R. and Knight, R.W., 1998. 

Secular trends of percipitation amount, 

frequency, and intensity in the United 

States: Bull. Amer. Meteor. Soc., 79, 

231-241.

Lins, H.F. and Michaels, P.J., 1994. 

Increasing U.S. streamflow linked to 

greenhouse forcing: Eos, 75, 281,284-

285.

Lins, H.F. and Slack, J.R., 1999. Streamflow trends in the United States: Geophys. Res. Lett., 26, 227-230.

McCabe, G.J. and Wolock, D.M., 2002. 

A step increase in streamflow in the 

conterminous United States: Geophys. 

Res. Lett., 29, 2185-2188.

Slack, J.R. and Landwehr, J.M., 1992. 

Hydro-climatic data network: a U.S. 

Geological Survey streamflow data set 

for the United States for the study of 

climate variations, 1874-1988: U.S. 

Geol. Surv. Open-File Rept. 92-129, 

193 pp.