Platte River Channel Becomes a Focus for Platte River Cooperative Program Studies2001

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SUMMARY THE PLATTE RIVER CHANNEL: HISTORYAND RESTORATION" by Peter J. Murphy and Timothy J. Randle Hydraulic Engineers The Department of the Interior, as part of its efforts to develop a cooperative, basinwide, proposed recovery program for the whooping crane, piping plover, least tern, and pallid sturgeon, has prepared a technical report describing the factors that have caused the Platte River channel to narrow and habitat for the birds to be lost. It also presents options for offsetting those trends and restoring river habitat. This report is based upon a review and analysis of more than a century of field data from the river, going back to the earliest records of channel conditions, flow, and sediment measurement. The Causes of Channel Narrowing. Over the last 150 years, storage and diversion of waters upstream from the Central Platte have greatly reduced both springtime flows and the amount of sand carried by the river. Peak flows have been reduced from roughly 20,000 cubic feet per second (cfs) to about 6,700 cfs, and the sand load in the river has dropped by nearly 90 percent. This has allowed cottonwood and other trees and shrubs to take over up to 80 -90 percent of the historically open and unvegetated channel areas. Relatively few wide areas of the river and high sandbars remain for use by the threatened and endangered species. Four processes that have lead to narrowing are identified. 1. The reduction in annual peak flow: The large decrease in flow rates occurred over the period 1860 -1960. The higher areas of the channel were abandoned by the river flow and formed islands, and the total wetted channel width decreased significantly. Since the flow -rate reductions extended downstream for hundreds of miles, so too did this channel narrowing. The channel narrowing from this cause occurred during the same period that the peak flows were reducing. At present (1960- 2000), the mean annual flows and annual peak flows are no longer decreasing and channel narrowing from this cause is no longer occurring. Further reductions in annual peak flow rates, should they occur, would continue this channel narrowing effect into the future. 2. Colonization by vegetation: The exposed sandy areas of the channel abandoned over the period 1860 -1960 were ripe for the rapid colonization by vegetation (grass, shrubs, and trees). After a few decades, the banks of the narrow channels were covered with mature vegetation. The roots of that vegetation stabilized the banks and prevented erosion during subsequently higher flows. The large flood peaks of 1973 and 1983 did not produce significant, sustained widening of river channel. Because of the dense vegetation, high flows, when they occur, do not widen the channel, but merely deposit more fine sand on top of the wooded islands and banks.