Correction of Tailwater Erosion at Prairie du Sac Dam

The hydroelectric power plant at Prairie du Sac, Wis., on the Wisconsin River was placed in operation in 1914. A general lowering of the tailwater and of the river bottom downstream from the plant became obvious a few years later. Because of the presence of a large island immediately below the east half of the spillway section of the dam when constructed, the recession of the tailwater level was not appreciable until 1920 when the island had been completely removed. From then until 1932 the recession of tailwater was more rapid. The average rate of recession of minimum monthly tail water levels over a 17-year period, 1915-1931, inclusive, was 0.43 ft per yr. Such river controls as an old bridge foundation, two highway bridges, and a railroad bridge and gravel shoals over certain river stretches caused the rate of recession to decrease to 0.10 ft per yrfrom 1932 to 1936, inclusive, and to practically a stable condition from 1937 to 1944, inclusive. During 1929 hydraulic model tests were made to determine a method of reducing excessive erosion of the river bed immediately downstream from the dam.