Large-scale bedforms in the Platte River downstream from Grand Island, Nebraska; structure, process, and relationship to channel narrowing

The Platte River channel in Nebraska, especially downstream from Grand Island, is characterized by large, periodic, and geometrically distinct bedforms called macroforms. Macroforms have dimensions commensurate with the width and depth of the channel and are emergent at all but the highest flow stages. The encroachment of vegetation on macroforms and their consequent stabilization since the large-scale development of irrigation in the Platte River basin is the major cause of the reduction in channel width upstream from Grand Island. From simple geometrical considerations of macroform shape, an equation is developed to predict the depth and duration of flow required to erode the stoss sides of the macroforms to remove new vegetal growth each year. The methods developed in this report to eliminate vegetal growth on macroforms may provide a useful management tool for controlling width of the Platte River channels. Although further testing is required to establish the validity of these methods at actual stream sites, a sample calculation for the Silver Creek reach shows good agreement between the flow conditions predicted by the methods developed in this report and actual flow conditions. (USGS)