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USGS Scientists Document Widespread Increases in Streamflow and Changes in the Timing of Snowmelt Over the Past 50 Years
Released: 5/5/2005

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U.S. Department of the Interior, U.S. Geological Survey
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Reston, VA 20192
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Reston, VA – U.S. Geological Survey (USGS) scientists have identified nationwide trends toward increasing streamflow in many areas of the nation since 1940 based on data collected from long-term USGS streamgages. This conclusion and several more interesting trends in our nation’s streamflows can be found in four new fact sheets recently issued by the agency.

"Understanding streamflow trends is essential to effective management of the nation’s water supply and is critical to developing strategies that mitigate the potential negative impacts of floods and droughts," said USGS Associate Director for Water Robert Hirsch.

In the first study, USGS scientists identified a nationwide trend that streamflow has been increasing in the United States since at least 1940. Most of the increases were during low-and-moderate streamflows. This means that, during typically dry periods, more water is now available in the stream.

In the second study, scientists discovered that over the last 30 years, winter/spring streamflows occurred one to two weeks earlier than in previous decades in northern or mountainous areas of New England. Similarly, in the third study, scientists found that streamflows in most western rivers occur almost one to three weeks earlier now than they did in the middle of the 20 th century.

The fourth study shows that the streamflow of the Mississippi River was influenced by both climate and human activities such as construction of water reservoirs, agricultural irrigation and groundwater pumping. Streamflow of the Mississippi River increased at a rate of 4.5 percent per decade largely because of an increase in precipitation.

The USGS has been measuring and recording streamflow in the United States since the late 1800’s. Today, the USGS monitors streamflow at 7,400 locations nationwide. The USGS streamflow information is used for many purposes such as water resource appraisal and allocation, design of the nation’s infrastructure such as bridges and water treatment plants, flood hazard planning, National Weather Service flood forecasting, reservoir operations, water-quality management, habitat assessment and protection, recreational enjoyment and safety, and understanding changes in streamflow due to land-use and climate changes. USGS streamflow data are available at http://water.usgs.gov/waterwatch/.

Summary of the Fact Sheets:

Streamflow Trends in the United States (http://pubs.water.usgs.gov/fs2005-3017/)

Streamflow has been increasing in the United States since at least 1940. Regions that experienced the most widespread increases were the Upper Mississippi, Ohio Valley, Texas-Gulf, and the Mid Atlantic.

  • Of the nearly 22,700 streamgages for which the USGS has records, 435 monitor natural basins and have records of sufficient length to analyze climatic trends.
  • Streamflow increased across most of the United States during the 20th century at 40-45 percent of these 435 stations.
  • Increases were most prevalent in low to moderate streamflows (seen at 40 percent of the stations), with relatively few decreases (seen at 8 percent of stations).
  • Comparatively few stations (10 percent) had increases in annual maximum streamflow.
  • Streamflow increases occurred as a sudden rather than gradual change around 1970, suggesting the climate shifted to a new regime.

Changes in Streamflow Timing in New England During the 20th Century (http://pubs.water.usgs.gov/fs2005-3019/)

During the last 30 years, the timing of winter/spring streamflow has shifted earlier by one to two weeks in northern and mountainous New England streams.

  • The date when half of the total volume of streamflow for winter/spring (January 1 to May 31) now arrives earlier than it did in the first half of the 20th century at 14 of 27 streamgages in New England.
  • This shift to earlier streamflow was evident at all of the gages in the northern and mountainous areas of Maine and New Hampshire where snowmelt has the greatest effect on streamflow (11 of the 27 streamgages).
  • Only 4 of the 27 streamgages exhibited shifts in the timing of fall/winter streamflow (October 1 to December 31), and all of these tended toward earlier streamflow.

Changes in Streamflow Timing in the Western United States in Recent Decades (http://pubs.water.usgs.gov/fs2005-3018)

As much as three-quarters of water supplies in the western United States are derived from snowmelt. Trends toward earlier snowmelt and streamflow need to be considered in the water-resource and flood-management systems and procedures in many western settings.

  • The average streamflow center-of-volume date (the date on which one-half of the total annual flow volume passes a streamgage) in the western United States is about nine days earlier now than in the 1950s.
  • These shifts in timing result both from late winter and early spring temperature increases, and from changes in the form of precipitation (increasing liquid precipitation, smaller percentage of snow) in late winter and early spring.

Trends in the Water Budget of the Mississippi River Basin, 1949-1997 (http://pubs.water.usgs.gov/fs2005-3020/)

This study involved analysis of trends in precipitation, streamflow, evapotranspiration, depletion of ground water, and the filling of reservoirs. This study describes the influences of both climate trends and human alterations on streamflow from 1949 to 1997.

  • Streamflow in the Mississippi River basin increased at a rate of 4.5 percent per decade during the second half of the 20th century.
  • This increase resulted primarily from an increase in precipitation offset by increases in evaporation from reservoirs and irrigated cropland in the basin.

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