BANK EROSION ON THE LOWER OSAGE RIVER DOWNSTREAM FROM BAGNELL DAM

This paper presents results from a project led by Dusty F. Ritter to investigate the processes of riverbank erosion, the rates of erosion, and the effects of dam operation on bank erosion in the Lower Osage River below Bagnell Dam in central Missouri. Rates and processes of bank erosion were studied over a 9-month period (Oct. 1982–July 1983) through direct field measurement at seven discrete monitoring stations, and over the long term by measurements on sequential aerial photographs and through dendrochronology. The Osage River flows through a substantial alluvial valley-fill inset in carbonate uplands of the Ozark Plateau. Groundwater percolates through fractures and solution cavities in the carbonate uplands and discharges through the alluvial valley-fill to the Osage River. Strong groundwater seepage was observed at every monitoring station during the study except station ES-2, where no discernable seepage was observed during any visit to the river. Bank erosion occurred by three main processes: (1) abrasion, (2) cantilever slab failure, and (3) block gliding. During the period of study, block-gliding, was the most significant mechanism of bank erosion. Block-gliding was only significant following heavy precipitation and high discharge followed by rapid drawdown that removed the confining hydrostatic pressure exerted by river water. Under these conditions, high pore pressure from groundwater seepage above a relatively low permeability clay unit in the lower portion of the bank stratigraphy served as the failure plane for block-gliding. During the period of direct measurement, the average erosion rates measured at seven study sites was 3.24 m/yr. This erosion rate was apparently much greater than the average long-term rate by a factor of approximately 3x based on a comparison with the long-term rates measured from aerial photos and dendrochronology. The abnormally high erosion rates measured during 1982-83 appear to result from several unusually intense, flood producing precipitation events. Furthermore, most of the erosion stations were consciously chosen to represent sites where maximum erosion was expected to gain data representing the worst-case scenario, and therefore should not be considered as representative of the entire Lower Osage River.