QUANTIFYING THE EFFECTS OF STREAM CHANNELIZATION ON EVAPOTRANSPIRATION AND SEDIMENT TRANSPORT CAPACITY IN LARGE (~200 KM2) URBAN WATERSHEDS

Streams in urban areas are often channelized to improve flood conveyance; in addition, many stream bank erosion control measures may also reduce channel migration, overbank flooding and thus evapotranspiration and sediment storage on floodplains. The purpose of this study is to quantify evapotranspiration and sediment transport capacity in the Anacostia Watershed, a large, Coastal Plain urban watershed, and to compare these processes to those in similarly sized non-urban Coastal Plain watersheds. Time series data of hydrological and stream hydraulic changes in the Anacostia River as urbanization progressed, between 1939 and 2014, were also analyzed. These data indicate significantly lower values of warm season runoff in non-urban comparison streams compared to the Anacostia, suggesting a shift from evapotranspiration to runoff in urban streams. Channelization of the Anacostia increased flow velocities at all river stages and decreased flow width at high discharges in comparison with non-urban watersheds. Time series channel hydraulics data in the Anacostia indicate that these channel adjustments continued after the period of flood control channelization (1960-1972) and these continued adjustments are associated with the stream bank protection measures designed to inhibit channel migration and reduce sediment loads. The high stream velocities associated with these channel and floodplain changes and the downstream transport of coarse sediment to beyond the tidal limit in the Anacostia suggest that the increase in sediment transport capacity and reduction of sediment storage sites within the floodplain system maintains sediment loads as stream bank stabilization progresses.