The case of the shrinking channels; the North Platte and Platte rivers in Nebraska

This reconnaissance investigation was undertaken to determine whether the channels of the North Platte and Platte Rivers in western and central Nebraska have been changing in character since the latter part of the 19th century and, if so, the general nature and extent of such changes. The 480-kilometer study reach extended from Minatare on the North Platte River to Grand Island on the Platte River. The channels have indeed changed considerably. Changes in the 365-km reach from Minatare to Overton differ in magnitude and sometimes in character from the downstream stretch from Overton to Grand Island (115 km). Within the former reach, the channel by 1969 was only about oneto two-tenths as wide as in 1865. The 1969 channel for this reach was less braided and slightly more sinuous than the 1938 channel. (No data are available for braiding and sinuosity prior to 1938.) From Overton to Grand Island, the 1969 channel was about sixto seven-tenths as wide as in 1865, and various changes in braiding and sinuosity took place between 1938 and 1969. The decreases in channel width are related to decreases in water discharge. Such flow reductions have resulted primarily from the regulating effects of major upstream dams and the greater use of river water by man. Much of the former river channel is now overgrown with vegetation. INTRODUCTION The North Platte and Platte Rivers in Nebraska (fig. 1) are among the most important rivers of the Great Plains. Much of the vast westward migration of the 19th century followed their routes, and travelers cursed and praised the rivers simultaneously for their benefits and disadvantages. Settlers depended on these rivers for water and built a very complex system of diversions and controls. Wildlife, too, plays a large role. Sandhill cranes, for example, use the North Platte and Platte channels as a major migration stopover. In mid-March each year, an estimated 70-80 percent of the world's lesser sandhill cranes move into a 260-km stretch of the river between Sutherland and Grand Island (fig. 1) (Frith, 1974; Farrar, 1975). Is man's extensive manipulation of the water flow changing the riverine habitat? This reconnaissance investigation examines the character of the Platte's channel to determine whether any significant changes have taken place since the latter part of the 19th century. Local historians Emil Roeser, Paul Henderson, and Ted Genoways took me on interesting excursions to old bridge sites, most of which today are unrecognizable as the important crossings they once were. Many people generously provided information and assistance during the study. Among these are Frank Dragoun of the Central Nebraska Public Power and Irrigation District, Glenn Tabor of the Union Pacific Railroad, Jim Potter and Paul Riley of the Nebraska State Historical Society, Jim Holmes of the Nebraska Department of Roads, Harold Nagel and Steele Becker of Kearney State College, Robert Petersen of the Nebraska Public Power District, Emery Coonrod and Ernie Peterson of the Burlington Northern Railroad, Warren Corbet and Fred Otradovsky of the U.S. Bureau of Reclamation, and Ray Bentall of the University of Nebraska-Lincoln. Ray Bentall, Richard Hadley, and Stanley Schumm improved the manuscript considerably with their many constructive comments. GEOGRAPHIC SETTING The North Platte River originates in the Rocky Mountains of Colorado, flows northward into central Wyoming, then generally southeastward into Nebraska. In west-central Nebraska it joins the South FIGURE I. Platte River study area. Platte River to form the Platte River (fig. 1). The Platte flows eastward to join the Missouri River at the eastern edge of Nebraska. The reach studied in this report extends from Minatare on the North Platte River, about 50 km east of the Wyoming-Nebraska boundary, to Grand Island on the Platte River in central Nebraska, a river distance of about 480 km (fig. 1). Throughout most of this stretch, the river is several hundred meters wide and only 0-2 m deep. The drop in elevation is from 1,162 m above mean sea level at Minatare to 558 m above mean sea level at Grand Island or about 604 m an average slope of about 0.00126 m/m. Many islands of various sizes, some bare sand and others completely vegetated, dot the channels. This gives the rivers a braided pattern. Few tributaries of any size enter the study reach from the south, but several enter from the north. Alluvial sand, gravel, and loam make up most of the Platte valley. The adjacent plains consist mainly of loess. The climate ranges from semiarid in the west to subhumid in the east. Mean annual rainfall near Lake McConaughy is about 460 mm and this gradually increases eastward to about 625 mm at Grand Island. About 80 percent of the annual precipitation falls during April-September. A few violent thunderstorms usually occur in late spring and early summer. HYDROLOGY GENERAL Analysis of U.S. Weather Bureau precipitative data for North Platte, Kearney, and Grand Island reveals that there have been no significant long-term trends or changes in total annual precipitation for these cities during the period 1900-74. Therefore, any observed long-term changes in streamflow probably are not attributable to changes in climate. Flow in the North Platte and Platte Rivers depends not only on climatic and geologic conditions but also on many influences of man. Transmountain diversions occur in the headwaters of the North Platte. Several dams on the North Platte (table 1) detain and store the flow and regulate the downstream discharge. Within the study reach, many structures especially Keystone Dam (and Sutherland Canal) since 1937 and the Tri-County Diversion Dam (and Tri-County Canal) since 1941 divert significant amounts of river water via canals to offstream reservoirs, irrigated croplands, or powerplants (fig. 2). A considerable quantity of ground water is pumped from wells for irrigation of lands bordering the Platte River. Excess irrigation water and hydropower releases reenter the Platte at various locations. Increasing population over the years has demanded more and more water from the river. 10 1° E X P L A N A T IO N