COMBINED GEOMORPHIC AND LAND-USE CONTROLS ON TRIBUTARY DISCHARGE IN AN URBAN LANDSCAPE

To better understand competing geomorphological and urbanization factors on urban stream discharge, we investigated relationships among hydrological processes, channel morphology, and water quality in first-order tributaries of the NW Branch of the Anacostia River, MD. These tributaries contribute to the main channel, but convey water from storm sewer outflows below suburbanized ridge tops, through forested hillsides where recharge, discharge, and evapotranspiration can occur. We examined tributaries above and below a prominent knickpoint, selected because tributary spacing doubles below the knickpoint, leading to higher drainage densities. Tributaries above the knickpoint are longer, more sinuous, with greater basin area, gentler slopes, and wider floodplains than the downstream tributaries. Land use patterns are similar above and below the knickpoint. We also selected forested, non-urban reference streams located nearby in the Maryland Piedmont that are not affected by knickpoints.

We measured stream morphological characteristics (width, depth, gradient, velocity, grain size) discharge, precipitation, and stream chemistry (temperature, pH, and conductivity). The three reference streams are gauged by the USGS. We established stream gauges on six tributaries, three above and three below the knickpoint to monitor differences in stream response to storm events. Preliminary data indicates that baseflow discharge in these first order tributaries increases with both urbanization and distance above the knickpoint. Bankful discharge, however, is higher in the tributaries below the knickpoint. Both baseflow and bankful discharge are greater in the NWB urbanized tributaries than the non-urbanized reference streams. Water quality data for the warm season indicates that both water column and basal stream temperatures increase systematically downstream as stream elevation decreases. Summer baseflow specific conductance is high ~500 uS/cm, and the non-urban streams have similar values as the urban streams. Summer baseflow dissolved oxygen concentrations also decrease downstream with elevation and are below saturated values in the tributaries below the knickpoint. Both urbanization and position relative to the knickpoint significantly affect urban stream discharge.