SPATIAL AND PB-210 ANALYSIS OF SOIL EROSION AND FLUVIAL SEDIMENT DEPOSITION IN TWO SUB-WATERSHEDS OF THE RAPPAHANNOCK RIVER

Understanding processes that control the erosion and deposition of surficial sediments is critical for assessing the impacts certain anthropogenic factors have on these migrating sediments. Alteration of watersheds through increases in human development greatly diminishes vegetative land cover, and creates surfaces which accelerate runoff and erosion. A multi-approach analysis facilitates the isolation of specific factors that contribute to variance in erosion rates and sediment fluxes such as slope, land use, watershed area, and stream densities. Pb-210 geochronology and the Universal Soil Loss Equation (USLE) model coupled with Sediment Delivery Ratios (SDRs) were used to estimate watershed soil loss and accumulation rates in the Deep Run (68.7 km²) and the Claiborne Run (16.8 km²) basins; sub-watersheds of the Rappahannock River in Stafford County, Virginia.

Results show that the soil loss in the relatively pristine Deep Run basin (85% forest cover) ranged from 3.8 - 217.2 g/m²/yr with a total basin soil loss of 1.86 x 10¹⁰ g/yr. A core collected from a pond located at the bottom of the Deep Run watershed had Pb-210 based accumulation rates of 0.0782 g/cm/yr. Whereas the more human impacted Claiborne Run watershed (67% urban development) yielded soil loss ranges from 12-129.6 g/m²/year and a total basin soil loss value of 5.8 x 10⁹ g/yr. These estimates will be compared with continuous stream suspended sediment measurements and characterization. Both the Deep Run and Claiborne Run watershed basin results portray valuable information that illuminates the adverse affects of erosion, largely caused by human development. With continuing increases in Northern Virginia's population, urban development and erosive surfaces, we can expect the present soil loss in both watershed basins to increase substantially.