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.
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 new flow-adjusted 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.
Phosphorus Load Trends
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 2008, phosphorus loads:
- Averaged 21.3 million pounds per year and ranged from 7.1 to 46.4 million pounds per year.
- Decreased from 17.2 to 13.8 million pounds per year.
Short-term trend (10-year trend)
The last 10 years have highly variable phosphorus loads, with 1999-2002 being very low load years followed by two much higher load years. Between 1999 and 2008, phosphorus loads increased from 7.1 to 13.8 million pounds per year.
Change from previous year (2007-2008)
There was no significant change in phosphorus loads (14.1 to 13.8 million pounds per year).
River Flow Trends
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.
To calculate the loads of phosphorus flowing to the Bay, scientists use a combination of water samples and computer modeling.
- Whenever possible and practical, samples from rivers and wastewater pipes are used to measure pollution levels. Using this technique, pollution loads can be calculated for almost 80 percent of the watershed.
- For the remaining area, computer modeling is used to calculate pollution loads.
Katie Foreman at 800-968-7229 ext. 837
Chesapeake Bay Program Office