Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Streamflow
This indicator describes trends in the amount of water carried by streams across the United States; as
well as the timing of runoff associated with snowmelt.
Background
Streamflow is a measure of the rate at which water is carried by rivers and streams, and it represents a
critical resource for people and the environment. Changes in streamflow can directly influence the
supply of drinking water and the amount of water available for irrigating crops, generating electricity,
and other needs. In addition, many plants and animals depend on streamflow for habitat and survival.
Streamflow naturally varies over the course of a year. For example, rivers and streams in many parts of
the country have their highest flows when snow melts in the spring and their lowest flows in late
summer. The amount of streamflow is important because very high flows can cause erosion, flooding,
and ecosystem disruption, while very low flows can diminish water quality, harm fish, and reduce the
amount of water available for people. The timing of high flow is important because it affects the ability
of reservoir managers to store water to meet needs later in the year. In addition, some plants and
animals (such as fish that migrate) depend on a particular pattern of streamflow as part of their life
cycles.
Climate change can affect streamflow in several ways. Changes in the amount of snowpack and earlier
spring melting (see the Snowpack indicator) can alter the size and timing of high streamflows. Because
of the relationship between precipitation and runoff, more precipitation will potentially cause higher
average streamflow in some places, while heavier storms (see the Heavy Precipitation indicator) could
lead to larger peak flows. However, more frequent or severe droughts could reduce streamflow in
certain areas.
About the Indicator
The U.S. Geological Survey measures streamflow in rivers and streams across the United States using
continuous monitoring devices called stream gauges. This indicator is based on 193 stream gauges
located in areas where trends will not be substantially influenced by dams, reservoir management,
wastewater treatment facilities, or land-use change. The indicator also excludes stream gauges with
substantially overlapping watershed areas.
This indicator examines four important measures of streamflow conditions that occur during the course
of a year. Figure 1 shows trends in low flow conditions, which are commonly calculated by averaging the
lowest seven consecutive days of streamflow in a year. In many locations, this method captures the
year's driest conditions. Figure 2 shows trends in high flow conditions, which are commonly calculated
by averaging the highest three consecutive days of streamflow in a year. Three days is an optimal length
of time to characterize runoff associated with large storms and peak snowmelt. Figure 3 shows changes
in the annual average streamflow, which is calculated by averaging daily flows over the entire year.
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Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 4 shows trends in the timing of winter and spring runoff. This measure is limited to 56 stream
gauges in areas where at least 30 percent of annual precipitation falls as snow. Scientists look at the
total volume of water that passes by a gauge between January 1 and June 30, then determine the date
when exactly half of that water has gone by. This date is called the winter-spring center of volume date.
A long-term trend toward an earlier date could be caused by earlier spring snowmelt, more precipitation
falling as rain instead of snow, or other changes in precipitation patterns.
Key Points
• Over the past 73 years, seven-day low flows have generally increased in the Northeast and
Midwest (in other words, on the days of lowest flows, streams in these areas are carrying more
water than before). In parts of the Southeast and the Pacific Northwest, low flows have
generally decreased (that is, streams are carrying less water than before). Overall, sites show
more increases than decreases (see Figure 1).
• Three-day high-flow trends vary from region to region across the country. For example, streams
in the Northeast have generally seen an increase or little change in high flows since 1940, while
some West Coast streams have seen a decrease and others have seen an increase. Overall, sites
show more increases than decreases (see Figure 2).
• The largest changes in annual average streamflow have taken place in the Northeast and
Midwest. Other regions saw few substantial changes. Overall, sites show more increases than
decreases (see Figure 3).
• Nearly half of the streams studied show winter-spring runoff happening more than five days
earlier than in the mid-20th century. The largest changes occurred in the Pacific Northwest and
Northeast (see Figure 4).
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d\ Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 1. Seven-Day Low Streamflows in the United States, 1940-2012
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d\ Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 2. Three-Day High Streamflows in the United States, 1940-2012
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This map shows percentage changes in the maximum annual rate of water carried by rivers and streams
across the countrybased on the long-term rate of change from 1940 to 2012. Maximum streamflow is
based on the consecutive three-day period with the highest average flow during a given year.
Data source: USGS, 20142
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d\ Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 3. Annual Average Streamflow in the United States, 1940-2012
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This map shows percentage changes in the annual average rate of water carried by rivers and streams
across the countrybased on the long-term rate of change from 1940 to 2012. This map is based on daily
streamflow measurements, averaged over the entire year.
Data source: USGS, 20143
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d\ Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Figure 4. Timing of Winter-Spring Runoff in the United States, 1940-2012
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This map shows changes in the timing of annual high spring flow carried by rivers and streams from 1940
to 2012. This analysis focuses on parts of the country where streamflow is strongly influenced by
snowmelt. Trends are based on the winter-spring center of volume, which is the date when half of the
streamflow between January 1 and June 30 of each year has passed a streamflo w gauge.
Data source: USGS, 20144
Indicator Notes
Streamflow measurements were used from gauges in areas where streamflow is not highly affected by
human influences such as dams, land development, or changes in land cover. However, changes in land
cover and land use over time could still influence streamflow trends at some streams. The gauges used
for this indicator are not evenly distributed across the country.
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Climate Change Indicators in the United States: Streamflow
www.epa.gov/climatechange/indicators - Updated May 2014
Data Sources
Streamflow data were collected by the U.S. Geological Survey. These data came from a set of gauges in
watersheds with minimal human impacts, which have been classified as reference gauges.5 Daily
average streamflow data are stored in the National Water Information System and are publicly available
at: http://waterdata.usgs.gov/nwis.
1 USGS (U.S. Geological Survey). 2014. Analysis of data from the National Water Information System. Accessed
January 2014.
2 USGS (U.S. Geological Survey). 2014. Analysis of data from the National Water Information System. Accessed
January 2014.
3 USGS (U.S. Geological Survey). 2014. Analysis of data from the National Water Information System. Accessed
January 2014.
4 USGS (U.S. Geological Survey). 2014. Analysis of data from the National Water Information System. Accessed
January 2014.
5 Lins, H.F. 2012. USGS Hydro-Climatic Data Network 2009 (HCDN-2009). U.S. Geological Survey Fact Sheet 2012-
3047.
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