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River Flow Into Chesapeake Bay

Annual average river flow to the Bay during the 2013 water year (October 2012-September 2013) was 49 billion gallons per day. This is 3 billion gallons per day less than 2012 and close to the 51 billion gallon per day average flow from 1937-2012.

September 09, 2011

When rainfall runs across roads, lawns and golf courses, it can pick up pollutants before it enters local waterways. Mike Fritz from the Chesapeake Bay Program explains why so-called “stormwater runoff” is a major source of pollution to the Chesapeake Bay and what we can do to prevent it.

September 09, 2011

Fritz Schroeder, Director of LIVE Green Lancaster (a program of the Lancaster County Conservancy), explains how the city is using green infrastructure to capture stormwater runoff before it makes its way to the Chesapeake Bay.
Close Captions: http://www.youtube.com/watch?v=vIMb7ldNWx4


Each day, billions of gallons of fresh water flow through thousands of streams and rivers that eventually empty into the Bay. That water also carries polluted runoff from throughout the watershed.

The amount of water flowing into the Bay from its tributaries has a direct impact on how much pollution is in the estuary:

  • Generally, as river flow increases, it brings more nutrient and sediment pollution to the Bay.
  • Runoff from winter and spring rains delivers pollution loads that drive summer water quality conditions in the Bay.
  • Years with low or high amounts of precipitation can result in changes to pollution levels in the Bay, but not mean the health of the watershed is improving or declining.

Not all rain water runs off the land. Some water seeps into the soil, carrying nutrients into groundwater. The travel time of nutrients through the watershed ranges from weeks to centuries. This can result in a lag time between implementing management actions and improvements in water quality.

Long-term trend (1937-2013)

U.S. Geological Survey analyses indicate annual average river flows mostly in the normal range prior to 1960, a dry period during the 1960s followed by wetter conditions in the 1970s. Since 1990, river flow has been extremely variable.

Between 1938 and 2013, annual mean river flow to the Bay has averaged 50.9 billion gallons per day (BGD) and has ranged from 29.3 to 78.2 BGD. The normal range of flow during this time period is between 43.6 and 57.7 BGD.

Short-term trend (10-year trend)

The last 10 years have had highly variable flow. Water year 2009 was dry whereas 2003, 2004 and 2011 were wet.  Water years 2005 through 2008 and 2010, 2012 and 2013 were relatively normal.  2011 was one of the five wettest years on record due to a very wet spring followed by a hurricane and a tropical storm that hit the region in the late summer/early fall of 2011.

Change from previous year (2012-2013)

Annual mean river flow decreased from 52 to 49 billion gallons per day.

Additional Information

About half the water in the Bay comes from its rivers; the other half from the Atlantic Ocean. In an average year, three rivers deliver most (about 81 percent) of the river flow: the Susquehanna (48 percent), Potomac (19 percent) and James (14 percent).

The U.S. Geological Survey, one of the Bay Program’s federal partners, has been using monitoring from the three largest rivers entering the Bay to provide an estimate of total river flow to the Bay.  The USGS updates flow to the Bay each month and also provides an annual summary.

U.S. Geological Survey analyses of trends in river flow are available in USGS Circular 1316, Chapter 5 and in a more recent report assessing trends between 1985 and 2010.

Source of Data

US Geological Survey

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