SPATIAL AND ISOTOPIC ANALYSIS OF SOIL EROSION AND SEDIMENT FLUXES IN THREE RAPPAHANNOCK RIVER TRIBUTARIES, STAFFORD COUNTY, VIRGINIA
CLARK, Robert, Earth & Environmental Science, University of Mary Washington, 1301 College Ave, Fredericksburg, VA 22401 and ODHIAMBO, Ben K., Earth and Environmental Sciences, University of Mary Washington, 1301 College Avenue, Fredericksburg, VA 22401, rclark2@mail.umw.edu
The Rappahannock River is the leading source of sediments in tons/mi2 to the Chesapeake Bay which leads to increases in eutrophication and decreases in water clarity. This study evaluates erosion rates and sediment source areas in three sub-watersheds of the Rappahannock River; Claiborne Run, Little Falls Run and Horsepen Run. Claiborne Run watershed is predominantly human developed, Little Falls is of mixed land-use whereas Horsepen Run is primarily forested. The GIS based Revised Universal Soils Loss Equation (RULSE) model was used together with Sediment Delivery Ratios (SDR) to estimate watershed soil losses. Pb-210 signatures of different land uses and other potential source areas were compared with suspended sediments to identify the dominant sediment source areas in the three sub-watersheds. In addition, Pb-210 inventories in watershed soil cores were used to quantify erosion/depositional rates at different land uses for each watershed.
The RULSE/SDR estimates that 332, 130, and 212 tons/yr are currently being delivered to Claiborne Run, Little Falls Run and Horsepen Run respectively. These results directly correspond to the level of anthropogenic impact observed, where Claiborne Run, which has the most human development, has the highest soil loss and Horsepen which is mostly forested has the least. Preliminary isotopic fingerprinting results shows that stream bank erosion and re-suspension of internally stored sediments are the dominants sources of sediments in watersheds characterized by increased human development, i.e. Claiborne Run. Whereas the more pristine Horsepen Run has suspended sediments reflecting a variety of sources. The Pb-210 soil core inventories results shows that grassland areas have erosional rates ranging from 0.02 to 2.5 tons/hectare/ yr and farmland has depositional rates ranging from 0.32 to 0.36 tons/hectare/ yr. These results show that human development greatly increases erosional processes as well as suspended sediment loads currently being observed in the Rappahannock River and its tributaries.
