AGRICULTURE RATES OF EROSION IN THE CHESAPEAKE BAY WATERSHED

Fine-grained sediment is a major pollutant in the Chesapeake Bay and its receiving waters. In order to reduce sediment inputs to the Bay it is necessary to identify the important sources of sediment and quantify erosion rates for these different sources. Sediment sources in the Chesapeake Bay watershed have been identified using modeling (SPARROW, HSPF), sediment fingerprinting, and sediment budget (Cesium-137) approaches. Modeling and sediment-fingerprinting results indicate that agriculture and streambanks are important sources of sediment. The Cesium-137 technique was used to quantify soil redistribution (erosion and deposition) rates for agricultural lands in the Linganore Creek (146 km²) and Little Conestoga Creek (109 km²) watersheds that drain the Piedmont Physiographic Province of the Chesapeake Bay. The Cesium-137 technique quantifies soil redistribution rates over a 50-year period. Agricultural erosion rates measured in 2009 for Linganore Creek using Cesium-137 were 1,550 t/km²/yr and erosion rates measured in 2004 for Little Conestoga Creek were 1,940 t/km²/yr. With 25% of the Linganore Creek watershed in agriculture, over 56,000 t/yr may be eroded off of agriculture lands which is more than 7 times the average annual suspended-sediment load (8,050 t/yr) leaving the watershed. In Little Conestoga Creek, with 13% of the basin in cropland, over 27,000 t/yr may be eroded off of agricultural lands which is more than 3 times the average annual suspended-sediment load (7,100 t/yr) leaving the watershed. The higher agricultural erosion rates compared to the transported suspended-sediment loads out of each watershed indicate that large amounts of sediment from agricultural sources are being deposited on hillslopes and in stream channels. Further work is needed to determine the location of these sediment storage areas and at what flows this sediment is remobilized. Following post-European land clearing in the late 18th to 20th centuries, agricultural was a major source of sediment in the Chesapeake Bay watershed. Recent measurements on agricultural lands indicate that cropland still remains an important source of sediment. Streambank erosion as a source of sediment in the Chesapeake Bay watershed has increased since the 20^th Century, as streams incised into legacy sediment deposits.