Bank Instabilities along the Missouri River: The Role of Positive Pore-Water Pressures and Flow Releases

The closure of Fort Peck Dam in the 1930's on the Missouri River in eastern Montana initiated a series of changes in channel morphology downstream from the dam that impacted bank stability. These included streambed degradation and the consequent increase in bank heights, and a substantial alteration to the magnitude, frequency, and temporal distribution of flows. A new bank-stability model, developed by the ARS was performed on 17 sites and provided consistent results: The model includes the effects of both matric suction and positive pore-water pressures, confining pressures, and layering. Instability occurs from the loss of matric suction and the generation of positive pore-water pressures. In this semi-arid region, these hydrologic conditions are most likely to occur from the maintenance of moderate and high flows for extended periods, thereby providing a mechanism for saturation of streambanks.