SEASONAL VARIATION IN SURFACE-GROUNDWATER EXCHANGES IN AN URBAN FLOODPLAIN WITH ACTIVE GRAVEL-BAR FORMATION

Many urban stream channels are incised or channelized, which reduces overbank flooding and constrains transport of water, sediment, and solutes within channels. Changes in sediment regimes can result in gravel bar deposition that aggrades stream beds and reattaches channels to floodplains. Similar features are built into some stream restoration designs. These gravel bars, lowered stream banks, and other features may increase surface and subsurface flow from the channels into the floodplains. In the U.S. mid-Atlantic region, discharge exhibits seasonal cycles driven by warm season evapotranspiration. We examined the effects of this seasonal cycle on groundwater and surfacewater interactions along a major tributary of the Anacostia River. The site is a floodplain fragment along the Northeast Branch of the Anacostia River with actively accreting gravels bars, chute channels, and a wide fine-grained floodplain. We measured topography, grain size, hydraulic conductivity, groundwater heads, and documented seasonal variations in groundwater flow directions between the channel and the floodplain. Water flowed from the stream into the adjacent gravel bars for most of the year. Groundwater flowed from topographically higher, fine-grained floodplains during cool seasons, but evapotranspiration generated seasonal reversals in flow from the stream and gravel bar into the adjacent floodplain.