INCREASE IN SUSPENDED LOAD CONTRIBUTIONS FROM TRIBUTARIES TO THE MISSOURI RIVER, SOUTHEAST SOUTH DAKOTA

Suspended sediment plays a critical role in riverine ecosystem health and river processes. Dams and land use practices have resulted in changes in the suspended load of the Missouri River and its tributaries. Gavins Point Dam near Yankton, SD has substantially reduced the downstream sediment load. The James, Vermillion, and Big Sioux rivers downstream of the dam contribute sediment to the Missouri River. The purpose of this study is 1) to estimate the percentage of the Missouri River’s suspended load derived from these three tributaries during May-July, and 2) to examine the temporal changes in the suspended load of the James and Big Sioux Rivers. We used a depth-integrated sampler to collect suspended load samples of the tributaries. A sediment rating curve was used to establish the seasonal suspended load transported by the tributaries into the Missouri River. For the James, Vermillion, and Big Sioux rivers respectively, the seasonal suspended loads are 892,000 tons/year, 391,000 tons/year, and 1,420,000 tons/year. The tributaries contribute ~38% of the suspended load of the Missouri River during the summer. Bank erosion was estimated to account for an additional ~5% of the Missouri’s suspended load. Temporal changes were examined by comparing our recent measurements with historical data obtained from USGS gauging stations from which suspended load and discharge data are available from 1981-1983 on the James River and 1971-1980 on the Big Sioux River. Disparity existed between present and past data with the rivers carrying nearly an order of magnitude higher concentrations of suspended sediment in recent years. Increases in discharge have been documented in both river basins, and the greater South Dakota and Midwest regions have experienced increased precipitation, cropland expansion, and more artificial drainage systems resulting in discharge rise. Higher discharge could lead to increased erosion and suspended load concentrations; however, further research is needed to verify the cause of temporal changes in suspended load in the James and Big Sioux Rivers. Given the impact of suspended sediment on aquatic health and the potential for climate and land use changes to modify discharge, this research is relevant to remediation efforts and river management.