GIS ANALYSIS OF SOIL EROSION AND FLUVIAL SEDIMENT FLUCTUATIONS IN TWO SUB-WATERSHEDS OF THE RAPPAHANNOCK RIVER, STAFFORD COUNTY, VIRGINIA

The effect of changing land-use and land cover characteristics has a profound influence on fluvial systems. Urban development is known to remove stable vegetative cover and increase impermeable surfaces thereby enhancing the threat to sustainable use of water resources posed by soil erosion and associated off-site sedimentation in fluvial systems. The traditional Universal Soil Loss Equation (USLE) was used in conjunction with sediment delivery ratios (SDRs) to estimate soil loss from various land plots along the longitudinal profile of two watersheds, Little Falls Run and Horsepen Run, in Stafford County, Virginia. The study area is one of the fastest developing regions in the Untied States, which makes it an ideal location for soil loss analysis where rapidly changing land-use practices exist.

Estimates for theoretical pristine conditions, which assume the watershed land cover to be uniformly forested, were used as the base datum to evaluate the impacts of development within the watersheds. Little Falls Run has experienced considerable amounts of urban development, primarily in the form of residential subdivisions. Horsepen Run has experienced minimal development, mainly in the form of deforestation for herbaceous agriculture and rural residential use. Only 54% of pristine lands remain in the Little Falls Run basin, while 63% of pristine lands have been retained in the Horsepen Run watershed. The sediment flux data for current land-use conditions shows a soil loss increase from pristine conditions of 22% for Horsepen Run and 62% for Little Falls Run. The sizable sediment influx for Little Falls Run can be attributed to increased residential development within the watershed.

Further analysis in the form of bed load and suspended sediment sampling in both watersheds will be carried out to assess the accuracy of the USLE and SDR models. The results can then be used to evaluate the possible adverse effects of current watershed developmental patterns on Stafford County fluvial systems.