Stream Bank Erosion and the Chesapeake Bay TMDL – Loading Rates and Bay Model Assumptions
Abstract:Sediment and nutrient loading to the Chesapeake Bay from its tributaries is a serious problem that has impaired the Bay and caused dead zones. It is assumed that pollution from agricultural lands is the dominant source of sediments and nutrients. However, the presence of legacy sediments along streams behind historic mill dams is a potentially significant source of sediments and nutrients as well. This study examined five streams in Lancaster County, Pennsylvania and measured erosion rates on streams with and without legacy sediments. Erosion rates during the four–month monitoring period were greatest on streams with legacy sediments, averaging 1.5 feet lateral erosion over a 4.4 month period on outer stream bends. Erosion rates on straight channel segments of streams with legacy sediments averaged 0.6 feet over the monitoring period. The control stream without legacy sediments averaged 0.02 ft of lateral erosion along its channel. Erosion rates were greatest when stream banks were higher and with greater bank angles, although these factors explained only about 40 percent of the variability in erosion rates. The mass of sediment, nitrogen, and phosphorus eroded (lbs/ft/yr) was much greater than assumed by the load reduction assumptions in the Chesapeake Bay watershed model. Stream bank erosion caused by the numerous historic mill dams in Lancaster County and elsewhere in the Bay watershed needs to be addressed as a major source of sediment and nutrient loading to the Chesapeake Bay and targeted for load reduction restoration practices.
Document Type: Research Article
Publication date: January 1, 2011
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