HISTORICAL AND PREDICTED FUTURE SEDIMENT ACCUMULATION IN THE NIOBRARA RIVER DELTA AND LEWIS AND CLARK LAKE HEADWATERS

Sedimentation triggered by dams is a long-term management concern for many rivers and associated reservoirs. Gavins Point Dam on the Missouri River near Yankton, SD was closed in 1955 and resulted in the formation of a prograding delta into Lewis and Clark Lake beginning at the confluence of the Niobrara River. Periodic channel survey data collected by the U.S. Army Corps of Engineers (USACE) was used to evaluate sedimentation patterns and sedimentation rates over time. The Missouri River Institute also conducted vegetation and geomorphology surveys in the delta. These surveys, combined with predicted future delta growth data from the USACE, were used to create a series of visualizations showing how Lewis and Clark Lake may be transformed if sedimentation continues to accumulate at current rates. Data suggest that dramatic sedimentation began in the decade following the construction of the dam. The primary area of sedimentation has progressed downstream along with the progradation of the delta. High flow years such as 1997 and 2011 resulted in the most dramatic geomorphic changes in the delta and in the rate of sedimentation. Under the present sedimentation conditions, the delta will prograde an additional ~14 km by 2113.