Annual average river flow to the Bay during the 2012 water year (October 2011-September 2012) was 52 billion gallons per day. This is 21 billion gallons per day less than 2011 and close to the 51 billion gallon per day average flow from 1937-2012.
The amount of water flowing into the Bay from its tributaries has a direct impact on how much pollution is in the estuary:
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 (1937-2011)
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 2012, 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 42.1 and 57.8 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 and 2012 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 (2011-2012)
Annual mean river flow decreased from 73 to 52 billion gallons per day.
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.
US Geological Survey