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 sediment and nutrient 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.
Scientists have estimated the threshold level of sediment that should not be exceeded for a healthy Bay. However, the sediment loads entering the Bay each year can not be directly related to the threshold.
Sediment 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 sediment delivered to the Bay from the watershed changes dramatically from year to year, complicating efforts to determine trends through time. Between 1990 and 2008, sediment loads:
- Averaged 4.1 million tons per year, but ranged from 0.6 to 12.2 million tons per year.
- Increased from 2 to 3.3 million tons per year.
Short-term trend (10-year trend)
The last 10 years have had highly variable sediment loads, with 1999-2002 being very low load years followed by two much higher load years. Between 1999 and 2008, sediment loads increased from 0.7 to 3.3 million tons per year.
Change from previous year (2007-2008)
Sediment loads increased from 2.6 to 3.3 million tons 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 sediment flowing to the Bay, scientists take water samples near the head of tide (fall line) in the Bay's major rivers. The volume of water flowing past the monitoring gauges –- or river flow –- and sediment concentrations are measured.
The sediment load estimates do not account for sediment from the coastal plain areas of the watershed (areas below the fall line). Scientists are currently developing methods to quantify the total loads of sediment to the Bay.
Katie Foreman at 800-968-7229 ext. 749
Chesapeake Bay Program Office