Processes and rates of shoreline bluff recession at Lake Sharpe, South Dakota

Abstract

Lake Sharpe is the center of three reservoirs formed by dams built by the U.S. Army Corps of Engineers on the Missouri River in South Dakota in the early 1960's. The design service life of the reservoir is 400 to 700 years based on the expected rate of erosion of the shoreline to the boundary of the Federally purchased land. In only thirty years however, this buffer zone has already been eroded and the shoreline continues to retreat into private land in some locations. Processes of shoreline erosion at Lake Sharpe were observed in the field and correlated to bank geology. Active processes of erosion observed included: slump, block slide, debris flow, toppling, slaking, and gullying. Each process was unique to the physical properties of the bank material. Long term rates of erosion were measured at twenty stations from historical profile surveys conducted by the U.S. Army Corps of Engineers. Total shoreline retreat at each station was found to be not related to bank material or retreat process Total shoreline losses varied from 28 ft to 622 ft since the filling of the reservoir. This yields an average rate of retreat of 1 to 22 ft/yr. Predominant wind fetch and bank height are not related to total shoreline loss. Graphs of average amount of retreat over time at each site show that the trend of retreat rate does vary with geology and retreat process. Most sites had a generally constant retreat rate after an initial high rate except the gravel sites where retreat is decreasing in rate. The factor causing the extreme erosion is the ease of erosion of the bank materials and their tendency to stay in suspension until they reach deep water. Because the fine grained bank materials are carried away in suspension rather than be deposited to form a protective beach, the toe of the bluff is continuously subjected to wave action and retreat continues. Only at the shorelines where the there is a significant sand or gravel layer does a protective beach develop to slow the retreat rate.