Case Study: Channel Stability of the Missouri River, Eastern Montana

Abstract

The construction of Fort Peck Dam in the 1930s on the Missouri River, eastern Montana, initiated a series of changes in hydrologic conditions and channel morphology downstream from the dam that impacted channel stability. Impacts included streambed degradation of up to 3.6 m and substantially altered magnitude, frequency, and temporal distribution of flows. To investigate the effects of the altered flow regime and bed degradation on bank stability, two independent bank-stability analyses (one for planar failures, the other for rotational failures) were performed on 17 outside meanders. Both included the effects of matric suction and positive pore-water pressures, confining pressures, and layering. Instability occurred from the loss of matric suction and the generation of positive pore-water pressures. In this semiarid region, such hydrologic conditions are most likely to occur from the maintenance of moderate and high flows (greater than 425–566 m