SEDIMENT YIELDS AND SOURCES IN SELECTED PARTS OF THE CHESAPEAKE BAY WATERSHED

Excess sediment is having an adverse effect on the living resources and habitat of the Chesapeake Bay and its watershed. The U.S. Geological Survey (USGS) is involved in a 5-year study to examine sources of sediment to the Chesapeake Bay. As part of this study, historical data on suspended sediment were compiled for selected watersheds in the Chesapeake Bay. Another aspect of the study is to determine the major sources of suspended sediment in selected subbasins of the Chesapeake Bay. Three subbasins were selected for sediment-source studies between 2001 and 2004: the Pocomoke River subbasin in, Maryland and Delaware (157 km²), the Little Conestoga Creek subbasin in Pennsylvania (110 km²), and Mattawoman Creek, subbasin in Maryland (144 km²). Only data from the Pocomoke River have been analyzed.

Analysis of 35 USGS daily-load sediment stations with at least 3 years of data collected between 1985 and 2001, indicates that 4 of the 5 locations with the highest average-annual sediment yields are for stations draining to the Susquehanna River in Pennsylvania. These include the Little Conestoga Creek near Churchtown, Pa. (368 tons/ km²/yr); Little Conestoga Creek site 3A, near Morgantown, Pa. (116 tons/ km²/yr); Mill Creek at Eshelman Mill Road near Lyndon, Pa. (113 tons/ km²/yr); and Raystown Branch Juniata River at Saxton, Pa. (91 tons/ km²/yr). Of these four stations, three are in the Conestoga River Basin. Meteoric ¹⁰Be erosion indices match the spatial patterns compiled from the daily-load sediment stations and are consistent with the high erosion rates in the Conestoga River subbasin. The Conestoga River Basin subbasin drains primarily agriculture. This may be influencing erosion rates and sediment yields.

The Pocomoke River subbasin in Maryland was chosen to test multiple fingerprinting techniques for identifying sediment sources in the watersheds. Fingerprints include: ⁷Be, ¹³⁷Cs, ²¹⁰ Pb, total carbon, total nitrogen, total phosphorus, and stable isotopes (N and C). The subbasin sampling strategy is focused to distinguish upland sediment sources (land use) versus origin in the stream corridor (stream bed and banks) Preliminary results indicate that the bed of the Pocomoke River and its tributaries are a major source of sediment.