GEOMORPHIC CONTROLS ON THE TRANSPORT AND STORAGE OF WATER, SOLUTES, AND SEDIMENT IN AN URBANIZED, ATLANTIC FALL ZONE STREAM

Rivers along U.S. mid-Atlantic Coast contain major knickpoints, many of which became sites of major cities. We evaluated geomorphic influences on water, solute and sediment fluxes in the NWB Anacostia River, examining longitudinal stream profiles, tributary profiles, valley and channel width, sediment storage bars, and suspended and bedload transport data. Distinct geomophic zones are defined by floodplain width, channel incision, and gradient. Upper valleys are wide with channel/valley width ratios < 0.1. Tributaries connect to the floodplain, but not always to the main channel. Further downstream, valleys narrow (C/V width ratio 0.15- 0.35) and tributaries connect to the main stem, supplying the reach with sand and gravel sediment. Major sediment bars store sediment, some supplied by road construction. Downstream, the stream valley narrows above the main knickpoint (C/V width ratios > 0.5). Tributaries have concave profiles that transport sand to the channel. Below the main knickpoint, the main channel is incised into bedrock. Tributaries are steep, eroded to bedrock, and supply cobbles and boulders to the mainstem. Finer material from upstream is transported to storage reaches in the Coastal Plain. Channelization, however, reduces the supply of sediment to the floodplain, and the flood-control channels convey gravel-size material into the upper Anacostia estuary.

Urban encroachment on stream channels is limited by floodplain width, hillslope steepness, and bedrock outcrops. In the upper watershed, suburban development occupies hilltops and gentle hillslopes along the main and tributary channels, but there is little development on the floodplains. Stormwater is contained in stormwater management ponds/wetland in the newer upper basin developments whereas it is conveyed to tributaries heads in older developments in the lower basin. In the incised, reach downstream of the knick-point, urban development is confined to ridgetops. Stormwater runoff from these developments is is discharged to tributary headwaters, where some is stored or transpired by hillslope forests. Water quality characteristics are similar through much of the watershed, but a significant decrease in water quality is observed as urbanization and channelization have modified the floodplain of the Coastal Plain reaches.