ANTHROPOGENIC ALTERATION AND FLOODING IMPACTS ON THE UPPER MISSOURI RIVER

On the Upper Missouri River in North Dakota, the Garrison Dam, which was completed in 1953, regulates flow to the 70-mile section downstream. The reach is bounded downstream by Lake Oahe (just outside the city of Bismarck), the reservoir that was created by the Oahe Dam completed in 1959. This reach is managed for flood control, recreation, water supply, fisheries, and as habitat for threatened and endangered species. The largest flood since dam regulation occurred in early summer of 2011 following an abnormally high snow pack season and a week-long rain event in the headwaters. This flood has highlighted the critical need for quantifying the complex interaction between the river geomorphology and anthropogenic impacts. It is necessary to understand and quantify the human alterations of the river planform, morphology, and sediment dynamics in order to determine the impact of the 2011 controlled flood releases from the dam. We have created a conceptual model of the new governing fluvial and deltaic processes for the reach. Methods used include interpretation of historical repeat aerial photography, stream gage data, and cross-sectional surveys, ground-truthed with sediment cores, channel surveys, and dendrogeomorphic tools. Preliminary results indicate that prior to the 2011 flood the reach had achieved a dynamic equilibrium in response to dam closure and managed flows. The perturbation from the dam closure attenuated over approximately a 30-year period. Cross-sectional analysis indicates that the reach just below the Garrison Dam increased in bankfull capacity over this period and the lower end of the reach (just upstream of the delta) either maintained capacity or experienced a reduction. Analysis of channel planform through repeat aerial photography suggests channel adjustment of varying types and degrees. Loss of islands and erosion has occurred in the 20 miles just below the dam; the next 30 miles have generally remained meta-stable retaining pre-dam channel characteristics, and many islands in the remaining 20 miles of the river have become bank-attached. Loss of conveyance and increased frequency and growth of islands has occurred in the delta above the reservoir. The Upper Missouri River is still responding to the historic flood of 2011, and analysis of post-flood impacts to the river geomorphology is ongoing.