Fluvial sediment budgets in the North Carolina Piedmont

Abstract

Sediment budget for four large (> 1000 km 2) drainage basins in the Piedmont physiographic province of North Carolina were estimated from data compiled during erosion and sedimentation surveys. Budgets for the Upper Tar, Upper Neuse, Haw and Deep River basins showed broadly similar trends in allocation of eroded sediment among yield and storage. Sediment yield as a percentage of mean annual gross erosion within the basins averaged 10%. This was less than the rate of alluvial storage, which averaged 14% of annual gross erosion. About 76% of the mean annual erosion was stored as colluvium on hillslopes. There are relatively small differences in sediment delivery and storage ratios among the study basins. This suggests that the relative order of magnitude of the allocation of eroded sediment for southern Piedmont rivers is colluvial storage, alluvial storage and yield. Of the sediment that does reach the streams, more than half is stored as alluvium on an average-annual basis. Aggrading channels and floodplains, siltation of benthic habitats and sediment pollution problems are thus likely to persist unless dramatic reductions in upland erosion or in sediment delivery to streams are realized. With respect to longer-term basin evolution, results point to the need for an improved understanding of the temporal scales of colluvial and alluvial sediment storage.