Fine sediment sources in Conservation Effects Assessment Project watersheds

Abstract

Two naturally occurring radionuclides, Beryllium-7 (⁷Be) and Lead-210 (²¹⁰Pb_xs) were used as tracers to discriminate eroded surface soils from channel-derived sediments in the fine suspended sediment loads of eight Conservation Effects Assessment Project (CEAP) benchmark watersheds. Soil, bank, and suspended sediment samples, as well as rainfall samples, were collected in the watersheds from single storm events and analyzed for the radionuclide activities, which were then applied to a two end-member mixing model to determine the relative proportions from the two source areas. In larger watersheds where the transport length of the sediment was longer, the suspended sediment load contained lower proportions of eroded upland soils compared to smaller systems. The longer transport paths contained more depositional areas in which fine sediment could settle, and hence, less eroded surface soils reached the stream channel, resulting in higher proportions of sediment from the channels. This study showed that more than 50% of the fine sediment in six of the eight watersheds originated from channel sources that included stream banks, the riverine bed, and gullies. These results underscore the need to consider channel and gully processes when management practices are designed to reduce sediment yield in agricultural watersheds.