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Organizations managing aging dams and those designing new dams must consider the long term sediment sustainability of these projects. Flushing, a common sustainable management alternative, entrains and transports reservoir delta deposits by drawing the reservoir down to run of river flows. Agencies often apply one-dimensional sediment transport models to design these flush releases and predict their impact. A flushing event at Spencer Dam on the Niobrara River was monitored, measured, and modeled with HEC-RAS 5.0, an unsteady, one-dimensional (1D), mobile bed sediment model. This paper documents qualitative observations from the flush and compares model results to prototype measurements to evaluate the viability of 1D assumptions for flushing analyses. Reservoir stratigraphy, including cohesive layers deposited during large flows, decreased flushing efficiency. The model performed well in upper half of the reservoir where the model computed scour within 4% of measured erosion, with almost no parameter adjustment. The model also predicted deposition downstream of the dam well, within 5% of observed values. The model under-predicted erosion in by 43% in lower half of the reservoir, missing sediment eroded by lateral mechanics (e.g. toe scour, undercutting, and bank failure). Reservoir stratigraphy (e.g. clay lenses deposited during large events) affected the pattern of channel formation and total sediment removal.