CHANGES IN SOURCES AND SUPPLY OF FINE AND COARSE SEDIMENT, ANACOSTIA WATERSHED

The Anacostia River Watershed is heavily urbanized and channelized in its downstream reaches; it contains rural unchannelized reaches in its headwater watersheds. The sources and supply of sediment from different parts of the watershed has been documented by long-term studies of channel changes, augmented by studies of hillslope erosion and channel sediment transport in various part of the watershed. Sediment is supplied to the stream primarily from surface runoff (in both urban and non-urban areas) and by bank erosion. Sources, transport rates, and depositional sites for coarse and fine sediment in the watershed have changed as a result of land use changes and channel erosion associated with high magnitude floods and urban runoff. Prior to urban and suburban development, much of the area was agricultural and experienced wide-spread soil erosion of fine sediment from hillslope erosion and stream network expansion. Much of this sediment was re-deposited in channel and floodplain areas and one of the major sources of fine sediment now is from bank erosion of these sites. Fine sediment and associated contaminants are also conveyed directly from urban areas through the channelized stream reaches in the downstream portions of the watershed. Significant channel incision in the area was initiated as a result of upstream suburban development, and increased dramatically with large storms in 1972. In the past 15 years, a series of significant storms, including storms in June and July of 2006, have caused wide-spread bank erosion and significant downstream migration of coarse (sand and gravel) sediment.