USING REPEAT PHOTOGRAPHY TO DOCUMENT BANK EROSION RATES AND LOCATIONS IN THE MISSOURI RIVER NATIONAL RECREATIONAL RIVER

The 59-mile segment of the Missouri National Recreational River (MNRR) downstream of Gavins Point Dam and upstream of Sioux City, Iowa is one of the most natural sections of the Missouri River existing today. Although substantially affected by flow management, it is minimally affected by channelization and not subject to reservoir inundation. This reach has numerous sandbars, variably vegetated islands, and laterally eroding banks characteristic of the historic braided river. Land adjacent to the Missouri River straddles a patchwork of public and private of land in Nebraska and South Dakota. Whereas designation of this segment as a National Recreational River in 1978 charged the National Park Service with preserving its free flowing condition, the river is also subject to management actions intended to stabilize banks, maintain habitats for threatened and endangered species, and improve recreational opportunities. In the past few decades an estimated 30 % of the shoreline in this reach has been stabilized by rock revetment, mostly to protect private land. Improved understanding of the timing and spatial distribution of bank erosion and the fundamental controls on fluvial processes in the segment is needed to inform management and future bank stabilization decisions. High resolution digital orthophotography dating from 1993 to 2003 was used to document bank erosion rates for the Missouri River in the 59-mile segment. This time period is of particular interest because the1997 runoff was the highest on record for the Missouri River downstream of Gavins Point Dam. Results indicate that lateral bank erosion is spatially variable in the MNRR ranging from undetectable to more than 450 meters. Although the reasons for highly variable erosion rates are not yet clear, mapping of discrete areas characterized by contrasting erosion rates serves to identify reaches with similar suites of geomorphic processes, and therefore inherently similar management challenges.