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Importance

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.

Preliminary estimates show that approximately 47.8 million pounds of phosphorus reached the Bay during the 2011 water year (October 2010-September 2011). This is 29 million pounds more than the revised loads in 2010 and approximately 28 million pounds more than the 19 million pound average load from 1990-2011 and was the highest delivered yield of phosphorus to the Bay since 1990.

Goal

Scientists have estimated the threshold level of phosphorus that should not be exceeded for a healthy Bay. However, the phosphorus loads entering the Bay each year can not be directly related to the threshold. Work is ongoing to develop a total phosphorus load indicator that can be compared to the threshold and that can better track the progress of nutrient management programs within the watershed.

Trends

Phosphorus Load Trends

Note: Previously reported data for 2010 were considered provisional.  The 2010 data have since been revised using and the entire dataset has undergone revision due to updates in the watershed model.  Additionally, prior to 2010, the phosphorus indicator reflected loads from the watershed only and did not include loads to the Bay from atmospheric deposition of phosphorus to tidal waters.  The historic data have been revised in order to account for those additional loads.

Long-term Trend (since start of data collection)

1990 was the first year where all necessary loads data were available. Rigorous statistical analyses to determine trends in loads have not been developed at this time.

The amount of nutrients delivered to the Bay from the watershed changes dramatically from year-to-year complicating efforts to determine trends through time. Between 1990 and 2011, phosphorus loads averaged 19.4 million pounds per year and ranged from 6.7 to 47.8 million pounds per year.

Short-term Trend (10-year Trend)

The last 10 years have highly variable phosphorus loads, largely related to yearly fluctuations in streamflow.

Change from Previous Year (2010-2011)

Phosphorus loads increased from 19.0 to 47.8 million pounds.

River Flow Trends

Long-term Trend (since 1990)

Between 1990 and 2011, annual mean river flow to the Bay has averaged 54 BGD and has ranged from 29 to 77 BGD.

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

Short-term Trend (10-year Trend)

The last 10 years have had highly variable flow. Water years 2002 and 2009 were dry whereas 2003, 2004 and 2011 were wet.  Water years 2005 through 2008 and 2010 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 (2010-2011)

Annual mean river flow increased from 52 to 73 BGD.

Additional Information

To calculate the loads of phosphorus flowing to the Bay, scientists use a combination of:

• 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 9.0 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.

Contact

Katie Foreman
University of Maryland Center for Environmental Science (UMCES)
(410) 267-9837

Source of Data

Chesapeake Bay Program