U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3018

March 2005

Changes in Streamflow Timing in the 

Western United States in Recent Decades

... 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

Mountain snow fields act as natural 

reservoirs for many western water-supply systems, storing precipitation from 

the cool season, when most precipitation falls and forms snowpacks, until the 

warm season when most or all snowpacks melt and release water into rivers. 

As much as 75 percent of water supplies 

in the western United States are derived 

from snowmelt. Thus, water-resource 

management of western rivers commonly is planned around the knowledge 

that much of the runoff to reservoirs and 

lowlands occurs during the early parts of 

the warm season, when water demands 

for irrigation and other uses are at their 

greatest. During the cool season, water 

demands are low and, in West Coast 

states, the potential is high for winter 

storms to cause disastrous floods. Separation in time between the cool-season 

risks of flooding and the warm-season 

benefits of snowmelt runoff is a fundamental assumption of water-resource 

management strategies in the West.

 Trends toward diminished snowpack and earlier snowmelt in western 

states may be related to global warming 

(Knowles and Cayan, 2002; Mote, 2003; 

Stewart and others, 2004) or to naturally 

occurring variability in winter and spring 

temperatures and in precipitation form 

or timing (Dettinger and Cayan, 1995; 

Cayan and others, 2001). These changes 

also affect streamflow timing (Dettinger 

and Cayan, 1995; Cayan and others, 

2001). Trends toward earlier snowmelt 

and streamflow, whatever the causes, 

threaten finely tuned water-resource and 

flood-management systems and proceTumultuous springtime snowmelt-fed streamflow in the eastern Sierra Nevada, 

East Fork of the Carson River above Grovers Hot Springs, Markleeville, California, May 2000.

dures in many western settings. Therefore 

recently observed trends toward earlier 

snowmelt and streamflow in western 

rivers measured by the U.S. Geological 

Survey (USGS) are a source of considerable interest and concern to resource 

managers.

Streamflow Data

Benjamin Franklin observed that 

“When the well is dry, we learn the worth 

of water.” A similar sentiment applies to 

the availability of information on water, 

particularly information on streamflow 
1900 1920 1940 1960 1980 2000

0.2

0.3

0.4

0.5

0.6

0.7

Spring Fraction

April-July Flows as Fraction of Water-Year Total

180 150 120 90 60 

70 

60 

50 

40 

30 

20 

10 

March - May

Temperature ( C) Trends

1950-1997

6

4

2

0

-2

-4

-6

2

2

2 2

2

1

1

1

1

0

0

0

0

0

0

0

-1

-1

-2

-2

o

o o o o o

o

o

o

o

o

o

o

magnitude and timing. Streamflow 

information is required for numerous 

water-facility design and operational 

needs, as well as for research bearing 

on the long-term implications of climate 

variability and human activities. The 

USGS has been measuring and recording 

streamflows in the western United States 

since the late 19th century. Because 

USGS streamflow data (http://waterdata.

usgs.gov/usa/nwis/sw) are collected to 

consistent national standards, span long 

periods of time, cover the full range of 

streamflow from floods to droughts, and 

include observations for unregulated 

natural streams, they can be analyzed to 

determine when and where snowmelt and 

streamflow-timing vary. 

Streamflow timing can be described 

by many different measures, depending 

on data availability and on the aspects 

of streamflow timing that are of most 

concern. Roos (1991) and Dettinger 

and Cayan (1995) analyzed the fractions of annual streamflow that occur in 

spring and summer seasons, because this 

fraction of total flow is, in many waterresource systems, the most readily stored 

and distributed for warm-season uses. 

Cayan and others (2001) characterized 

streamflow timing by the day of year 

when wintertime low-flow conditions 

rapidly transition to springtime high-flow 

conditions with the onset of warm-season 

snowmelt. These “spring-pulse dates” are 

important because they indicate the timing of snowmelt and the divide between 

winter and spring conditions. Stewart and 

others (2004) characterized streamflow 

timing by the “center of volume” of each 

year’s hydrograph. The center-of-volume 

date is the date by which roughly half 

of the streamflow for a year has passed. 

These center-of-volume dates provide 

direct measures of overall streamflow 

timing based on runoff conditions 

throughout the year.

Changes in Streamflow Timing

Annual streamflow in most western 

rivers has come progressively earlier during the past several decades. The longterm tendency of springtime streamflow 

(that fraction of overall flow that occurs 

from April to July) to decline during the 

20th century in the central and northern 

Sierra Nevada is shown as a fraction 

of overall flow in figure 1. Regressions 

of data prior to the 1945 indicate no 

statistically significant trend, whereas 

regression of data after 1945 show a clear 

statistically significant trend toward earlier streamflow. As the springtime fractions of yearly flows have declined, the 

fraction in winter, and especially flows 

in March, have increased. These results 

broadly reflect a regional trend toward 

warmer winters and springs during the 

same period (fig. 2). 

Changes in daily streamflow variations 

and center-of-volume dates of western 

rivers can be illustrated by comparing 

Figure 1. April - July streamflow in eight major 

rivers of the western Sierra Nevada, California, as a fraction of water-year (October 

through September) total streamflow. Dots 

indicate yearly values, blue curve is 9-year 

moving averages, and dashed line is linear 

trend prior to 1945 and solid line is trend after 

1945.

Figure 2. Springtime temperature trends in 

North America, 1950-1997, using gridded 

temperature anomalies from Peterson and 

Vose (1997).
mean measured flows in the Clark Fork 

Yellowstone River, Wyoming, during the 

1950s with those during the 1990s (fig. 

3a). The overall river discharges in these 

two decades were quite similar, with 

average flow rates of 27.8 cubic meters 

per second in the 1950s and 27.9 in the 

1990s. In the 1990s, however, springtime 

flows were larger and late summer flows 

were smaller than in the 1950s. Thus, 

flow generally arrived earlier in the 1990s 

than in 1950s, with an average centerof-volume date that was about 4 days 

earlier in the 1990s than in the 1950s. 

The year-by-year history of center-of-volume dates for the Clark Fork Yellowstone 

River is shown in figure 3b, illustrating 

the general trend towards earlier flows 

in the river before, and especially after 

about 1950.

Geographic Extent

The geographic extent of the trends 

toward earlier streamflow in snow-fed 

1920 1940 1960 1980 2000

Water Year (October-September)

April 30

May 10

May 21

May 30

June 9

June 19

200

150

100

50

0

Average Daily Discharge

(Cubic meter per second)

1950 - 1959

1990 - 1999

Water Years

a)

b)

Feb Mar Apr May Jun Jul Aug Sep

Figure 3. (a) Comparison of mean daily streamflows in the Clark Fork Yellowstone River, Wyoming, 

during the 1950s and 1990s, and (b) center-of-volume dates for each year from 1922 to 2003, with 

the red curve showing the 9-year moving average and straight lines showing the trend before and 

after 1950.

streams is shown in figure 4, with timing measured by the dates on which the 

snowmelt runoff adds a large initial surge 

to the rivers (fig. 4a) across the western 

conterminous states and by the center-ofvolume dates in rivers throughout western 

North America (fig. 4b). Both measures 

indicate that flows in many western 

streams arrive a week to almost 3 weeks 

earlier now than they did in the middle 

of the 20th century. The largest changes 

have been identified in the Pacific Northwest, but the trends also are present in the 

Sierra Nevada of California, in the Rocky 

Mountains, and through parts of British 

Columbia and southern Alaska. 

The regional trend in streamflow timing developed in the midst of large yearto-year and basin-to-basin variations in 

both streamflow amounts and timing. The 

variations are due to contrasts in topographies, precipitation patterns, and snow 

conditions among the river basins. Trends 

toward later streamflow were detected in 

a few western rivers. Despite the basinto-basin and year-to-year differences, 

however, over 90 percent of the stations 

with statistically significant trends in 

figure 4 have trended toward earlier runoff, signaling the breadth and strength of 

the regional trend in western states. The 

average center-of-volume date for rivers 

across the western United States is about 

9 days earlier now than in the 1950s.

Cause of Trends

These trends in timing are most readily attributed to winter and spring warming, as depicted in figure 2, but that interpretation is complicated by recent trends 

toward more (or less) precipitation in 

some areas and by a broad trend towards 

slightly later precipitation. The causes of 

these long-term climatic trends remain 

uncertain. The observed streamflow 

timing and winter-spring warming trends 

are consistent with current projections of 

how greenhouse effects may influence 

western climates and hydrology; thus 

streamflow timing and trends may be 

attributed, in part, to global warming. The 

climate of the North Pacific Ocean basin, 

however, underwent a seemingly natural 

change toward warmer conditions in the 

eastern Pacific and western Americas 

around 1977 (Mantua, and others, 1997). 

This multidecadal change was part of a 

long-term cycle of climate fluctuations 

in the region and to an uncertain extent, 

has contributed to long-term climatic and 

hydrologic changes in the western states 

during the past 50 years. Continued and 

even enhanced streamflow monitoring 

and analysis of western snow-fed rivers 

will be needed to determine the precise 

natural and human-induced causes, 

and the likely future, of these western 

streamflow-timing trends. If the trends 

have natural origins, then they may well 

reverse themselves, but if they are driven 

by man-made influences on the climate 

system, the streamflow timing may 

continue to change. If the trends continue 

their present course, then the “natural reservoirs” provided by western snowfields 

will become progressively less useful for 

water-resources management, flood risks 

may change in unpredictable ways, and 

many mountain landscapes will endure 

increasingly severe summer-drought 

conditions.
Figure 4. Trends in (a) yearly dates of spring snowmelt onset and (b) centers of volume of yearly 

streamflow hydrographs in rivers throughout western North America, based on U.S. Geological 

Survey streamgages in the United States and an equivalent Canadian streamflow network. Large 

circles indicate sites with trends that differ significantly from zero at a 90-percent confidence 

level; small circles are not confidently identified.

For additional information:

Michael Dettinger

U.S. Geological Survey

Scripps Institution of Oceanography

9500 Gilman Drive, Dept. 0224

La Jolla, CA 92093-0224

(858)822-1507 or

mddettin@usgs.gov

References

Cayan, D.R., Kammerdiener, S., Dettinger, M.D., Caprio, J.M. and Peterson, D.H., 2001, Changes in the onset 

of spring in the western United States: 

Bull. Amer. Meteor. Soc. 82, 399-415.

Dettinger, M.D. and Cayan, D.R.,1995, 

Large-scale atmospheric forcing of 

recent trends toward early snowmelt in 

California: J. Clim. 8, 606-623.

Knowles, N. and Cayan, D., 2002, Potential impacts of climate change on the 

Sacramento/San Joaquin watershed 

and the San Francisco estuary: Geophysical Research Letters 29, No. 18, 

1891, doi:10.1029/2001GL014339.

Mantua, N.J., Hare, S.R., Zhand, Y., Wallace, J.M., and Francis, R.C., 1997, A 

Pacific interdecadal climate oscillation 

with impacts on salmon production: 

Bull. Amer. Meteor. Soc., 78, p. 1069-

1079.

Mote, P.W., 2003, Trends in snow water 

equivalent in the Pacific Northwest 

and their climatic causes. Geophysical Research Letters, 30, DOI 

10.1029/2003GL0172588.

Peterson, T.C., and Vose, R.S., 1997, 

An overview of the Global Historical 

Climatology Network temperature data 

base: Bull. Amer. Meteor. Soc., 78, 

2837-2849.

Roos, M., 1991, A trend of decreasing 

snowmelt runoff in northern California: Proc., 59th Western Snow Conference, Juneau, AK, 29-36.

Stewart, I., Cayan, D.R., and Dettinger, 

M.D., 2004, Changes in snowmelt 

runoff timing in western North 

America under a ‘Business as 

Usual’ climate change scenario: 

Climatic Change 62, 217-232.