LEGACY SEDIMENT – CULPRIT OR SCAPEGOAT?

Thick accumulations of alluvial sediment are an increasing focus of watershed management actions in the Chesapeake Bay watershed and other watersheds with a history of extensive soil erosion and floodplain deposition. Large public expenditures for stream restoration and bank stabilization programs are advocated in order to reduce the supply of sediment and nutrients to the Chesapeake Bay. It is increasingly important to seek a more certain answer to the questions: Where is the sediment coming from and what is the most effective suite of management actions to reduce sediment yield to the Bay? A debate has proceeded for decades about the origin and extent of ‘legacy' deposits and whether they are, in fact, the primary source of sediment to the Bay under present watershed conditions. The sources of the sediment and the rates of erosion and deposition along higher order streams must be identified so that management funds can be accurately allocated. Accordingly, we report on field observations in the Patuxent River watershed with the goal of evaluating the relative supplies of sediments from uplands, enlargement of first order gully channels, and erosion of alluvial deposits. We combine storm sampling of flow and transport with surveys of channel enlargement and sedimentation rate in ponds in first-order watersheds. A new survey of a large water supply reservoir provides a measure of sediment yield from the 90 km² drainage area. Comparison of sediment inputs and outputs provides an opportunity to evaluate the relative magnitude of different sediment sources and pathways in the Mid-Atlantic Piedmont.