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Where does sediment come from?

Sediment forms when rocks and soil weather and erode. More than 18.7 billion pounds of sediment are believed to enter the Chesapeake Bay each year.

There are two major sources of sediment: eroding land and stream banks—called watershed sources of sediment—and eroding shorelines and coasts—called tidal sources of sediment.

Watershed sources of sediment

Eroding land and stream banks are called watershed sources of sediment. Watershed erosion increases when land is cleared of vegetation to make way for agriculture and development. Scientists estimate that most of the sediment that flows into the Chesapeake Bay comes from watershed sources.

In the Bay watershed, river basins with the highest percentage of agricultural lands yield the highest amount of sediment each year. Basins with the highest percentage of forest cover, on the other hand, yield the lowest amount of sediment. On a per-acre basis, construction sites can produce the most sediment of all land uses—as much as 10 to 20 times that of agricultural lands.

Since the seventeenth century, watershed-wide changes in land use and land cover have disrupted the natural processes of erosion:

• During the eighteenth and nineteenth centuries, 70 to 80 percent of the watershed’s forest cover was removed to supply people with firewood and farmland. Cleared, exposed land is prone to erosion.
• Deforestation peaked in the late nineteenth century, and while reforestation did take place during the twentieth century, increased urbanization has continued to contribute to high erosion rates.
• Studies of sediment cores in the Bay and its tributaries show a four- to five-fold increase in sediment accumulation rates in some parts of the Bay since the 1800s.

Tidal sources of sediment

Eroding shorelines and nearshore areas, as well as the resuspension of already-eroded sediments, are known as tidal sources of sediment. Tidal erosion increases when shoreline vegetation is removed and there are not enough bay grasses growing in the offshore shallows to lessen the force of waves against the shoreline.

The Bay’s shorelines have been slowly eroding for thousands of years, due to wave action and natural sea-level rise. Tidal erosion can help maintain the Bay’s complex ecosystem; beaches and tidal wetlands, which are created and replenished by erosion, provide habitat for countless critters, including blue crabs, herons and terrapins.

However, human development along the shoreline can have negative impacts on the natural processes of tidal erosion:

• Man-made, hardened shorelines—those lined with rocks, wood or concrete—can interfere with natural tidal erosion, blocking the formation of wetland habitat and starving beaches of new sediment.
• Hardened shorelines can also lead to “nearshore erosion,” during which waves erode the shallow area immediately in front of the man-made shore. This increases the amount of sediment that is suspended in the water.
• Between 1988 and 2000, more than 19 miles of Virginia tidal shoreline was hardened on average each year. In Maryland, more than 300 miles of tidal shoreline were hardened between 1978 and 1997.

Accelerated sea-level rise caused by climate change can also increase tidal erosion.

How does sediment harm the Chesapeake Bay?

Excess sediment is a leading factor in the Chesapeake Bay’s poor health.

Because of their small size, the particles of sand, silt and clay that we call “sediment” often float through the water rather than settling to the bottom, and can be carried long distances during rainstorms. When there are too many sediment particles suspended in the water, the water becomes cloudy and muddy-looking. Cloudy water does not allow sunlight to reach the plants that grow on the bottom of the Bay’s shallows. Without sunlight, these plants—including underwater grasses—die, which affects the young fish and shellfish that depend on them for shelter.

Excess sediment can also have harmful effects on the wider Bay and the people who use it:

• Nutrients and chemical contaminants can bind with sediment, spreading through the Bay and its waterways with particles of sand, silt and clay. Fish and shellfish that live and feed on or near contaminated sediment can become contaminated themselves, triggering fish consumption advisories in various portions of the watershed.
• Excess sediment can smother oysters and other bottom-dwelling species.
• Accumulating sediment can clog ports and channels, affecting commercial shipping and recreational boating.

A case study in sediment: the Conowingo Dam and Chesapeake Bay water quality

Each year, the Susquehanna River—which flows through New York, Pennsylvania and Maryland—provides the Bay with one-quarter of its sediment loads. Working to curb these loads are three large reservoirs located along the lower portion of the river. Each of these reservoirs sits behind a dam; together, they hold back much of the sediment that would otherwise flow downstream into the Bay.

These sediment-trapping reservoirs have served as effective “pollution gates” for three-quarters of a century. However, recent studies have drawn attention to their changing efficiency, with special attention paid to those behind the Conowingo Hydroelectric Generating Station (or Conowingo Dam) in Havre de Grace, Md.

According to the U.S. Geological Survey, the reservoirs behind the Conowingo Dam are beginning to lose their ability to trap upriver sediment and the phosphorous that is often attached. Strong storms, severe flooding and faster-moving water can also alter the reservoirs’ efficiency, causing sediment to “scour” from behind the reservoirs and flow downstream.

In September 2011, the Lower Susquehanna River Watershed Assessment (LSRWA) team was formed to evaluate the sediment management options for all of the dams on the Lower Susquehanna River. The team has entered into a three-year, $1.4 million study led by the Army Corps of Engineers, and includes representatives from the Maryland Department of Natural Resources (DNR) and Department of the Environment (MDE), as well as the Susquehanna River Basin Commission (SRBC), the Nature Conservancy and Chesapeake Bay Program. The first year of the study has focused on information gathering, data collection and model development.

The team expects to recommend a draft list of sediment management strategies for dams along the Susquehanna River in 2013. Options being discussed include, but are not limited to, the following:

• Reducing sediment yield from the watershed
• Minimizing sediment deposition
• Increasing or recovering sediment-trapping volume
• Dredging, or enlarging the reservoirs’ storage capacity
• Innovative reuse, or using the sediment to develop light-weight aggregate or restore eroded islands
• Replenishment, or using the sediment as cover for landfills or abandoned mines or as material for agricultural fields

For more information, visit this Bay Program backgrounder on the Conowingo Dam and water quality.

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From Around the Web

• An Introduction to Sedimentsheds - Scientific and Technical Advisory Committee
• Chesapeake Bay Sediment Distribution - Maryland Geological Survey
• Where has all the top soil gone? - Bay Link
• Shore Erosion Control - Maryland Department of Natural Resources
• Coastal Erosion Near the Mouth of the Rappahannock River - University of Mary Washington