Sediment Loads and River Flow to the Bay
Approximately 2.35 million tons of sediment reached the Bay during the 2012 water year, which is below the 1990-2012 average load of 5.4 million tons. The 2012 sediment load is 26.55 million tons less than the 2011 load. The 2011 load of 28.9 million tons was the highest load during the 1990-2012 period.
Annual average river flow to the Bay during the 2012 water year was 51.7 billion gallons per day (BGD), which is below the 1990-2012 mean flow of 53.5 BGD. The 2012 flow is 21.3 BGD less than the 2011 flow. The 2011 flow of 73 BGD was the fourth highest flow during the 1990-2012 period.
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.
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.
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.
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 sediment loads from non-tidal rivers 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 sediment 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.
Another difference between these two indicators is, as mentioned above, this indicator does not account for sediment from areas below the fall line. The other indicator does account for sediment from areas below the fall line since the model is able to simulate those loads.
Because of these differences, the two indicators can report different pollutant load amounts in a particular year. For example, in this indicator, the annual sediment load to the Bay from non-tidal rivers in 2009 was 1.5 million tons. This represents the best estimate of how much sediment from non-tidal rivers reached the Bay in 2009. In the other indicator, the simulation of 2009 loads was 8,675 million pounds (4.3 million tons). This value cannot be compared to the former since it also accounts for sediment from areas below the fall line (since the model is able to simulate those loads). Furthermore, this simulation does not represent how much sediment 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