Hydrodynamic and morphodynamic response to river engineering documented by fixed-discharge analysis, Lower Missouri River, USA

Abstract This research detects long-term trends in flow conveyance on the Lower Missouri River, and uses equal-discharge analysis of channel-gaging time series to assess the mechanisms driving these trends. Five long-term gaging stations along the Lower Missouri were examined using specific-gage analysis, which is a technique that holds discharge constant in order to observe trends in water-surface elevation (or stage) over time. This analysis reveals that for all flood conditions on the Lower Missouri River, stages have systematically risen for equal discharge volumes over the period of record. Flows that were fully contained within the Missouri channel in the early 20th century now create floods, and extreme high flows today are associated with stages as much as 3.7 m higher than at the start of the record. Equal-discharge analysis also can be used for analyzing time series of other parameters that co-vary strongly with discharge and that change systematically over time. On the Lower Missouri, long-term records of river gaging measurements, including cross-sectional area, flow velocity, and channel width, have been collected for the past ∼70 years. Equal-discharge analysis of these parameters illustrates the mechanisms of channel change driving flood magnification. At three stations, decreased flow velocity has been the dominant mechanism driving stage changes. At two other stations, constriction in channel cross-sectional area has increased flood stages. These changes in channel geometry and flow dynamics correlate with wing-dam construction and other engineering of the Lower Missouri River, but the changes occur progressively over the duration of record as a gradual and reach-scale re-equilibration of the fluvial system. Magnification of flood stages should be recognized on the Missouri River and incorporated into current estimates of flood hazard and into strategies for river management and flood mitigation in the future.

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