RECENT AND HISTORIC FLOODPLAIN AND CHANNEL DYNAMICS ALONG DIFFICULT RUN, VIRGINIA

High suspended-sediment concentrations occur on Mid-Atlantic Piedmont streams, including Difficult Run in Virginia, an urbanized tributary of the Potomac River and Chesapeake Bay. Accurate information on catchment level sediment budgets is rare and difficult to determine. Further, the sediment trapping portion of a sediment budget represents an important ecosystem service that profoundly affects downstream water quality. Our objectives, with special reference to human alterations to the landscape, include the documentation and estimation of floodplain sediment trapping (present and historic) and bank erosion along an urbanized Piedmont stream and the construction of a preliminary sediment budget. We used feldspar markers to measure floodplain sedimentation rates and steel pins to measure erosion rates on floodplains and banks, respectively. Additional data were collected for/from legacy sediment thickness and characteristics, mill pond impacts, stream gaging station records, topographic surveying and LiDAR, sediment density, texture and, organic content. Results are interpreted relative to two human alterations: post-colonial bottomland sedimentation (legacy) and modern watershed hardening associated with urbanization. Six floodplain/channel sites, from high to low in the watershed, were selected for intensive study. Bank erosion ranges from 0 to 470 kg/m/yr and floodplain sedimentation ranges from 18 to 1369 kg/m/yr. Upstream reaches are net erosional, while downstream reaches are distinctly net depositional with a budget of 2184 kg/m/yr trapped within the system. The amounts of both deposition and erosion are large and suggest non-equilibrium channel conditions. Both peak discharge and number of peaks above base have substantially increased since the mid 1960’s when urbanization of the watershed began. Deposition patterns are most closely associated with channel gradient, sinuosity, and channel width/floodplain width for both recent and probably historic periods. The amounts and location of sediment deposited on the floodplain, from both recent and from legacy episodes do not appear to be related to historic mill pond presence or location. The floodplain continues to provide the critical ecosystem service of sediment trapping in the face of multiple human alterations.