SEDIMENT SOURCES AND TRANSPORT IN THE CHESAPEAKE BAY WATERSHED

Fine-grained sediment is having an adverse effect on the living resources and habitat of the Chesapeake Bay and its watershed. In order to reduce sediment inputs to the Bay, it is necessary to identify the important sources of fine-grained sediment and to quantify erosion rates and sediment yields. Sediment sources in the Chesapeake Bay watershed were identified using different methods at several different scales. At the Chesapeake Bay scale (103,000 km²), U.S. Geological Survey suspended-sediment data collected at 35 stations from 1985 through 2001 showed that 4 of the 6 highest sediment yields were in the Conestoga River Basin, Pennsylvania, a tributary of the Susquehanna River. At 92 river outlets in the Susquehanna River Basin (43,600 km²), erosion rates were determined using atmospheric ¹⁰Be and also showed that the highest rates of erosion were in the Conestoga River Basin.

In three small watersheds draining to the Chesapeake Bay -- the Pocomoke River (157 km²), Mattawoman Creek (92.8 km²), and Little Conestoga Creek (68.1 km²) -- sediment sources were identified using a sediment-fingerprinting approach. Suspended sediment (< 0.062 mm) collected during storm runoff was compared to upland sediment sources (cropland, construction sites, and forest) and channel corridor sources (channel banks and bed) using radionuclides (²¹⁰Pb, ¹³⁷Cs), stable isotopes (¹³C, ¹⁵N), and total C, N, and P. Preliminary results for the Pocomoke River watershed indicate that cropland was a significant source for the two highest sampled streamflows (peak discharges of 15.7 and 20 m³/s). Ditch beds were significant sources during low to moderate flows (peak discharges ranging from 0.2 to 6.3 m³/s). For Mattawoman Creek, construction sites and cropland were the primary sources for all streamflows. For Little Conestoga Creek, sediment sources for a range of streamflows were distributed equally among construction sites, channel banks, and cropland. Erosion rates for nine cropland sites in the Little Conestoga Creek watershed were determined with ¹³⁷Cs inventories and indicated an average rate of erosion of 16.4 tons/hectare/year, which is about 25 times the sediment yield of the Little Conestoga Creek (0.65 tons/hectare/year). This finding indicates substantial sediment storage in the watershed.