Geomorphic Dynamics and Sediment Transport in the Lower Missouri River

We examined geomorphic dynamics and sediment transport in four reaches of the heavily managed Lower Missouri River. Measures included changes in channel morphology and bed material, and indicators of sediment transport in 6-km long reaches located 8, 105, 272, and 884 kilometers downstream of Gavins Point Dam, SD. In each of the four reaches, a subset of 30 transects were randomly selected and resurveyed 7-10 times in 2006 and 2007 over a wide range of discharges. Hydroacoustic and terrestrial mapping used a Real Time Kinematic Global Positioning System to evaluate changes in the cross-section shape and area, sedimentary bedforms, and substrate characteristics. Acoustic Doppler current profiler measurements were used to evaluate bed-sediment velocity. Cross-section resurveys indicate substantial changes in bed morphology in response to sediment transport associated with varying magnitudes of discharge over the 2-year time period. The longitudinal arrangement of reaches also allows for cross-sectional change to be compared among geomorphically distinct segments of the 1300 kilometer long Lower Missouri River. The upstream-most reach near Yankton, South Dakota, was the least variable, due in part to coarse, relatively immobile bed material in the incised channel. The reaches near Ponca, Nebraska and Little Sioux, Iowa showed the most intra- and interannual variability in bed morphology. These data indicate that Missouri River habitats of potential importance to native fishes can be substantially altered by sediment transport associated with relatively small discharges.