|Shields Jr, Fletcher|
Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 9/20/1999
Publication Date: N/A
Interpretive Summary: The closure of Fort Peck Dam in the mid-1930's caused changes in flow patterns and channel size and shape along the Missouri River between the dam and the North Dakota border. In particular, landowners have been concerned about the loss of farmland due to bank caving, and the burial of pump sites by deposition of sandy bed sediments. Average bed elevations have decreased as much as 8 feet since dam closure. However, future bed erosion is expected to be minimal. Still, changes in the height, timing and duration of flows have contributed to creating saturated and unstable streambanks at certain times of the year. This is particularly critical in areas where the base of the bank is composed of sand. To assess these conditions at unstudied sites, a method to evaluate relative bank stability is included. In addition, migration of meander bends has accelerated since closure of the dam. New soils being formed by the river are distinctly different (sandier) than the soils being lost from the flood plain by bank failure. The effects of river ice are also very important in determining locations of erosion and deposition.
Technical Abstract: The closure of Fort Peck Dam in the 1930's significantly altered the flow regime of the Missouri River. Peck-flows of the spring-summer snowmelt season are now stored behind the dam and replaced with a more constant, annual flow series. Discharges over the remainder of the discharge range have increased. The trapping of sediment behind Fort Peck Dam resulted in channel-bed erosion downstream of the dam. Average bed elevations decreased as much as 8 feet while thalweg (deepest part of the channel) elevations decreased by as much as 15 feet. This effect generally decreases downstream with increasing distance from the dam and with time such that at channel-bed degradation as a result of Fort Peck Dam has virtually ceased. Given the average flow conditions, projected future amounts of bed-level lowering due to the dam are less than one foot. The average rate of meander migration in the study reach is six-times less than it was before the closure of Fort Peck Dam. Bank failures are, however, common. The most unstable banks are those with the lowest shear strength, contain sand at the bank toe, and are subjected to prolonged periods of wetting by maintenance of high flows in the channel. A bank-stability index is developed for concerned agencies and citizens to evaluate erosion conditions and to prioritize bank-erosion control measures. The increased magnitudes of ice-covered flow, increased ice movement up and down banks, bank freezing at a higher level and more frequent freezing and thawing of bank materials can severaly aggravate bank erosion channel-bed shifting and, therefore, the silting of pump sites along the river.