Longitudinal dam interactions control channel morphology: The impacts of the Garrison and Oahe Dams on the Upper Missouri River

Abstract:

Although the downstream impacts of dams on channel morphology and the upstream effects of dams on reservoir sedimentation are well documented, these effects were generally studied in isolation, with relatively little attention paid to their potential interaction along a river corridor. We examine the morphological and sedimentological changes in the Upper Missouri River, between the Garrison Dam in ND (operational in 1953) and Oahe Dam in SD (operational in 1959) (the Garrison Dam segment). Through historical aerial photography, stream gage data, and cross sectional surveys, we demonstrate that the influence of the upstream dam is still a major control of river dynamics when the backwater effects of the downstream reservoir begin. We propose a conceptual model of how interacting dams could affect river morphology, resulting in distinct and recognizable geomorphic sequences that we term “Inter-Dam Sequence”. Based on previous research on the geomorphic adjustment and recovery of sediment loads downstream from large dams, we define a distance criterion for dam interaction. We use this criterion to demonstrate that there are more than 400 reaches on large rivers in the United States that could be geomorphically controlled by dam interactions, similar to the Garrison Dam segment.