Bedform Distributions and Dynamics In A Large, Channelized River: Implications For Benthic Ecological Processes

Sand bedforms are fundamental habitat elements for benthic fish in large, sand-bed rivers. We explored bedform distributions and dynamics in the Lower Missouri River, Missouri, with the objective of understanding the implications of these features for benthic fish habitat, including how bedforms might act as flow refugia, mediators of food transport, and instigators of ecological disturbance. We mapped portions of an 8-kilometer reach of the highly engineered Lower Missouri River 24 times over a four-year period from 2018-2021 using a multibeam echosounder. Surveys included precise water surface and bed elevations over discharges ranging from 1,360-8,550 cubic meters per second (m3*s-1). This included weekly surveys during a large flood event with a peak of 9,290 m3*s-1 in the spring and summer of 2019. Velocity was mapped with an acoustic Doppler current profiler during 14 of the 24 multibeam surveys. Repeat transects on most survey dates in the thalweg and behind (i.e., downstream and landward from) wing dikes were used to quantify rates of bedform movement. The dataset illustrates how bedforms are distributed in a typical Missouri River reach and how they evolve with changes in discharge. We measured a variety of bedform characteristics, including height, length, lee and stoss angles, and crest orientation, and examined their relationship to habitat properties; this information helped to guide sampling efforts for larval endangered pallid sturgeon (Scaphirhynchus albus). Bedforms ranged in size from less than a meter in wavelength and amplitude to dunes greater than 4 meters high and 150 meters long. Small dunes were located in lower velocity regions on the inside of a bend and behind wing-dikes, as well as superimposed on larger dunes. Larger dunes were generally located in the channel thalweg and were associated with higher flow velocities. However, bedform size did not necessarily scale with discharge over the course of the 2019 flood, possibly due to sediment supply limitations and hysteresis effects. Changes in bedform size over the course of the flood event were most pronounced in the thalweg; less change in bedform size occurred behind wing dikes on the inside of channel bends, indicating some degree of habitat stability. Bedforms moved at every measured discharge, with higher rates occurring at higher discharges. Bedform orientations were affected by flow expansion around wing dikes, indicative of their role in influencing exchange of material between the thalweg and channel margins. Increased understanding of bedform distributions and dynamics will inform future sampling designs for larval pallid sturgeon and contribute to increased understanding of their influence on benthic ecological processes.