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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, Nebraska. 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, Nebraska, reach shows good agreement between the flow conditions predicted by the methods developed in this report and actual flow conditions. INTRODUCTION The channels of the Platte River in Nebraska are characterized by large, spatially-periodic deformations of the channel bed, which, considered collectively, constitute a series of periodic and geometrically distinct bedforms. These bedforms, here termed macroforms, have dimensions commensurate . with the width and depth of the channel and are emergent at all but the highest flow stages. Macroforms are quasi-stable features, adjusted to prevailing flow regimes, that move at relatively slow rates through the channel during high flows; individual macroforms can be identified on aerial photographs taken during several decades. Since the advent of large-scale irrigation in the Platte River basin (about 1880), hydrology of the Platte River has undergone several changes. In general, peak discharges, mean discharges, and ranges in discharges have decreased with time through most of the basin (Kircher and Karlinger, 1981). Hence a new flow regime has been imposed on the river channels; channel features adjusted to previous regimes, such as macroforms, are no longer in equilibrium with prevailing flows. The most immediate effect of these hydrologic changes has been to decrease the rate of movement of macroforms through the channel by decreasing the time during which the forms are submerged each year. As a result, vegetation has become established on the macroforms and several of the macroforms have been transformed into islands. Stabilization of macroforms by vegetation, caused by changes in peak-flow regime, appears to be the major mechanism that causes channel narrowing. Encroachment of vegetation in the channels of the Platte River has created several management problems; the growth of vegetation in the channels increases hydraulic roughness, reducing channel capacities and increasing flooding hazards. Additionally, recent studies (U.S. Fish and Wildlife Service, 1980) indicate that encroachment of vegetation on the channels in central Nebraska has degraded the riverine habitat for several species of waterfowl, including sandhill cranes and whooping cranes, that use the river during the late winter and early spring each year. Stabilization of channel width, with respect to increasing demands for water within the basin, presents a major challenge to land-use and wildlife managers. The purpose of this report, one of a series of reports resulting from a study of the Platte River basin by the U.S. Geological Survey, is to describe the structure of the macroforms and some of the basic relationships between macroform geometry and flow regime; these relationships can be used to develop useful criteria for maintaining riverine habitats. Several U.S. Geological Survey personnel made important contributions to the report: L. E. Dobson, T. R. Eschner, E. M. Held, J. E. Kircher, D. J. Reynolds, and H. H. Stevens, Jr., ably assisted with one or more aspects of onsite laboratory, or office work. D. A. Baldwin, W. F. Curtis, G. E. Ghering, D. W. Hubbell, and C. F. Nordin, Jr., provided equipment and advice on its use and maintenance. Personnel of the Nebraska and Colorado Districts, provided the data shown in figures 2, 3, and 9. ENVIRONMENTAL SETTING The Platte River basin, a major component in the Missouri River basin, drains an area of approximately 222,000 km2 (square kilometers) on the High Plains of Colorado, Wyoming, and Nebraska (fig. 1). The Platte River basin comprises three major subbasins: The North Platte and South Platte rivers with headwaters on the east side of the Continental Divide in Colorado, and the Platte River, which is formed by the junction of the North Platte and South Platte rivers in western Nebraska. In the western reaches of the Platte River basin, streams flow through valleys eroded into Precambrian crystalline rocks and Paleozoic and Mesozoic sedimentary rocks; on the High Plains, streams flow on reworked Quaternary sediments in shallow valleys of varying width eroded into Cretaceous and Tertiary sedimentary rocks.

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