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Phosphorus Loads and River Flow to the Bay

Approximately 14.4 million pounds of phosphorus reached the Bay during the 2013 water year, which is below the 1990-2013 average load of 21.1 million pounds. The 2013 load is 0.1 million pounds higher than the 2012 load.

Annual average river flow to the Bay during the 2013 water year was 49.1 billion gallons per day (BGD), which is below the 1990-2013 mean flow of 53.4 BGD. The 2013 flow is 2.6 BGD less than the 2012 flow.



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.


Decrease loads to levels that will result in the achievement of water quality standards in the Bay for dissolved oxygen, water clarity/submerged aquatic vegetation and chlorophyll a.

Additional Information

Scientists use a combination of the following to estimate the loads to the Bay:

  • Water samples collected at river input monitoring (RIM) sites to estimate loads from the majority of the watershed.
  • Water samples collected at wastewater treatment facilities downstream of the RIM sites.
  • Computer modeling to estimate loads from nonpoint sources downstream of the RIM sites.

Pollutant loads to the Bay in any given year are influenced by changes in land use activities and management practices, as well as the amount of water flowing to the Bay (hydrology). As mentioned above, annual rain and snowfall influence the amount of water in rivers flowing to the Bay.

  • This indicator tracks annual changes in river flow and Phosphorus loads to the Bay.  It is important to calculate the amount of river flow and pollution loads to the Bay in any particular year in order to understand and explain trends in Bay water quality conditions. 
  • Another indicator, featured in the “Restoration” section of this website, reports computer-simulated phosphorus loads to the Bay (using the CBP phase 5.3 watershed model). The simulations use long-term average hydrology in order to remove annual variability in hydrology.  This allows managers to understand trends in efforts to implement pollution reduction actions.  The simulations are also important for developing “what-if” scenarios managers can use to project future impacts of management actions on Bay water quality.
  • Because of these differences, the two indicators can report different pollutant load amounts in a particular year.  For example, in this indicator, the annual phosphorus load to the Bay in 2009 was 9.1 million pounds.  This represents the best estimate of how much phosphorus reached the Bay in 2009.  In the other indicator, the simulation of 2009 loads was 19.23 million pounds.  This simulation does not represent how much phosphorus reached the Bay in 2009 since it is based on long-term average hydrology rather than the actual amount of water flowing to the Bay in 2009.

Source of Data

Chesapeake Bay Program

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