SEASONAL DYNAMICS OF SANDBARS OF THE LOWER PLATTE RIVER, NEBRASKA

The processes associated with deposition and erosion of macroform sandbars are fundamental to the dynamics of sediment storage exchanges and channel adjustment in sand-bed rivers of the Great Plains region of North America. These sandbars also provide important nesting and foraging habitats for migratory birds, including federal and state-listed species. Previous studies have described the geologic and ecologic significance of macroform sandbars in this region, but relatively little is known about the effects of flow variability on their formation, stability, and persistence. This study examined the geometries and spatial distributions of emergent sandbars along a 35-kilometer study segment of the lower Platte River over four ice-free seasons in 2011 and 2012. Locations and emergent areas of all sandbars were surveyed in the study segment in the spring, summer, and fall of 2011, and the spring of 2012. Measurements of the heights of the high-platform were also made on sandbars larger than 0.8 hectares, and were indexed to a common stage. Sandbars were classified into three primary types, mid-channel, island-attached, and bank-attached. Results indicate that large, rare floods transfer substantial amounts of sand into storage in emergent sandbars, much of which was accounted for by very large, bank-attached sandbars. Average-magnitude floods, which occurred between the spring and summer of 2011, lowered median sandbar height by 0.5 m, and reduced total emergent sandbar area, but the erosion was focused on bank-attached sandbars, whereas mid-channel emergent sandbar area tended to increase. Low-flows, which occurred between the summer and fall of 2011, and the fall of 2011 and spring of 2012, tended to reduce total emergent sandbar area, most of which was accounted for by mid-channel sandbars. Despite measuring substantial changes in emergent sandbar numbers and geometries from season to season, the longitudinal distribution of emergent sand area remained generally stable. These observations suggest that reach-scale channel morphology may be useful for predicting emergent sandbar deposition potential, but prediction of the stability and fate of sandbars over varying hydrologic conditions, and subsequent feedbacks with reach-scale morphology, is dependent on the specific size and type of sandbar.