WATERSHED EROSION ASSESSMENT IN THE RAPPAHANNOCK USING ISOTOPIC AND SPATIAL ANALYSIS

The Chesapeake Bay, located on the Atlantic Coast of Maryland and Virginia, is the largest estuary in the United States and home to ecologically important wetlands and aquatic ecosystems. Increased soil erosion within the bay’s watershed has led to excess sediments and associated nutrients leading to continued decline in the bay’s water quality. The bay's mean annual freshwater inflow is 1,953 m³ s^-1, 90% of which is contributed by eight major tributaries. Of the major tributaries the Rappahannock River generates the highest sediment yield per unit area, 329 tons mi^-2 yr^-1.

Comprehensive watershed management requires robust biogeochemical data critical for proper evaluation of accurate environmental conditions and implementation of BMPs. Of the needed data, sediment fluxes (a significant conveyer of most contaminants) and associated watershed erosion data for the bay’s basin is still significantly nominal. This study used isotopic (²¹⁰Pb and ¹³⁷Cs) sediment core analyses and empirical models (RUSLE and WEPP) to evaluate sediment accumulation rates and watershed soil erosion rates in three sub-watersheds of the Rappahannock River Basin. Our results show that accumulation rates within the three sub-watersheds range from 0.05 to 0.10 g cm^-2 yr ^-1, with modern increasing trends. These results points to the role of topography and anthropogenic land-use and land cover changes as the dominant erosion and sediment flux factors. However, other watershed characteristics such as sizes and various soil types also play significant roles. These results will be compared with our ongoing suspended sediment monitoring data for a comprehensive assessment of soil erosion and sediment fluxes to identify the dominant sediment contributing areas within the three sub- watersheds and the entire Rappahannock basin.