GEOMORPHIC AND HYDROLOGIC CHARACTERISTICS OF SMALL URBANIZED TRIBUTARIES TO A FALL ZONE STREAM

Many East Coast rivers contain major knickpoints, which define the Fall Zone. The Northwest Branch of the Anacostia River, an urbanized stream in Maryland, has a prominent knickpoint with 20-40 meters of channel incision. In this project I am studying tributaries that enter the channel above and below the knickpoint, as well as non-urbanized reference streams without knickpoints. Tributary spacing decreases by over 50% below the knickpoint, which suggests that mainstem incision initiated the formation of new tributary channels. Suburban development is concentrated on the flatter ridge tops of the tributary catchments, which often convey runoff via stormwater channels to outfalls near the drainage divides. Geophysical surveys indicate that development covers the thickest soil depths above the bedrock.

The goals of this project are to determine how mainstem channel incision has influenced tributary and catchment morphology, water storage dynamics, and hydrologic response. Geomorphic analysis indicates that tributary spacing, catchment size, and stream gradients are significantly different above and below the knickpoint. Tributaries above the knickpoint have shallower gradients and wider floodplains than downstream tributaries. Seismic refraction studies indicate that the regolith/bedrock boundary is significantly deeper under the hilltops and thin to absent near the incised channels. We hypothesize that there is more soil available for storage in catchments above the knickpoint, and the combination of lower gradients and higher soil thickness results in greater storage of urban runoff.

Methods include measurement of stream morphological characteristics (width, depth, gradient, velocity, grain size), discharge, and stream chemistry (temperature and conductivity). We established stream gages on seven tributaries, three above and four below the knickpoint to monitor differences in stream response to storm events. Preliminary data indicate that baseflow discharge increases with both urbanization and distance above the knickpoint. Unit bankfull discharge however, is greater in the tributaries below the knickpoint. Both unit baseflow and bankfull discharge are greater in the urbanized tributaries than the reference streams. Both urbanization and geomorphic adjustments to incision significantly affect stream discharge in this system.