Erosion Below Main Stem Dams on the Missouri River

Abstract

Six dams were constructed on the main stem of the Missouri River between 1937 and 1963. The dams, built and maintained by the U.S. Army Corps of Engineers, include Fort Peck, Garrison, Oahe, Big Bend, Fort Randall, and Gavins Point. The “Big Muddy,” a nickname derived from the Missouri9s tremendous silt load, has been transformed into a chain of lakes through most of South Dakota and North Dakota, and much of Montana. Below Gavins Point, the lowermost dam, the Missouri River begins anew as a sediment-free stream, and the water immediately starts to derive a new load, primarily from lateral erosion of the river banks. Normal summer water releases from Gavins Point since 1953 have been about 32,000 cubic feet per second (900 cubic meters per second), but releases during the summer of 1975 were about 60,000 cfs (1,700 cms) because of high runoff from headwater areas. This increased discharge caused severe erosion and hundreds of acres of adjacent farm and forest land sloughed into the stream during 1975. While the Corps of Engineers assumes no responsibility to riparian landowners for this erosion, it has provided $94,000 to prevent the erosion of the Sacred Heart Hospital at Yankton, South Dakota. Inspection of aerial photographs and space imagery taken during 1953, 1973, and 1976 indicates the areas where erosion is active within a 55 mile (88 km) reach studied below Gavins Point dam. In general, the geomorphological pattern of erosion and deposition on alternate sides of a meander bend has been replaced by active erosion along all sides of the river, creating an ever-widening, shallow, braided stream.