THE SIGNIFICANCE OF FLUVIAL SEDIMENT ON FISH HABITAT AND AQUATIC RESOURCES, BROAD RIVER BASIN, SC

The Broad River and its tributaries make up the largest watershed within the Santee River Basin located on the southeastern U.S. coast. The Broad River Basin, which terminates at the confluence with the Congaree River in Columbia, South Carolina, is almost entirely situated within the Piedmont physiographic ecoregion. Piedmont streams are often characterized by transport-limited conditions, high turbidity, sandy mobile bottoms and shoals, and unstable banks. Conditions such as these typically cannot support specific designated uses of the river owing to impaired biotic conditions from high sediment loads. Unfortunately, the data needed to quantify complex relationships between fluvial sediment and in-stream habitat are scarce in SC Piedmont rivers and a discernible data gap exists in the Broad River Basin. Our objectives were to (1) calculate annual sediment transport rates (2) determine substrate character and (3) assess relationships between sediment flux, grain size, and the ecological community. To carry out our objectives, over 800 suspended-sediment samples, 800 turbidity measurements, 700 bed material samples and 52 flood deposit samples were collected over a 24 month period from June 2012 to June 2014. In-stream habitat was assessed at each field site and 55 fish sampling surveys were carried out. Statistical analyses relating fish type and quantity with sediment load and bed material character were also performed. Sediment rating curves were developed for eleven major tributaries and individual annual yields ranged from over 200 t/km² to 2.5 t/km². Typically, coarse sediment (>63 µm) was transported during high flow events only and constituted the majority of measured flows approximately 10% of the time. Observed abundances of fish species representing sensitive groups such as benthic and water column-dwelling members of the families Catostomidae (suckers), Cyprinidae (minnows) and Percidae (darters) showed clear negative responses to higher suspended sediment levels and positively responded to increasing bed particle size (d50 and d90). Finally, low-resolution sediment source tracing was carried out and results indicate that while sources of excess sediment depend on varying land use within the basin, bank erosion is a prominent contributor of sediment in several assessed streams.