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.
Learn more about river flow.
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.
The indicator is not related to a goal at this time.
Long-term trend (since start of data collection)
Between 1938 and 2008, river flow to the Bay has averaged 47.2 BGD and has ranged from 21.9 to 78.2 BGD.
The Climate Change and the Chesapeake Bay State-of-the-Science Review and Recommendations: A Report from the Chesapeake Bay Program Science and Technical Advisory Committee (STAC) highlighted an examination of the 1957-2000 record of annual streamflow into the Chesapeake, which found substantial interannual and decadal variability characterized by dry conditions during the 1960s, wet conditions during the 1970s, and relatively normal conditions since then. There was no obvious long-term trend.
Short-term trend (10-year trend)
The last 10 years have had highly variable flow. Between 1999 and 2008, river flow increased from 21.9 to 37.5 BGD.
Change from previous year (2007-2008)
River flow decreased from 41 to 37.5 BGD.
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 concurrently monitoring the volume of fresh water flowing into the Bay from the three biggest rivers since 1937.
Katie Foreman at (800) 968-7229 ext. 837
Chesapeake Bay Program Office