Platte River Ecology Study
Introduction
Fresh water is among the most critical factors limiting urban growth and agricultural development in the western United States. Water development projects now impound for consumptive use much of the annual precipitation that falls in the West. Development of this resource has affected fish and wildlife resources in many areas but seldom have the potential impacts to migratory bird populations been as significant as in the Platte River Basin. With a watershed extending westward to the Continental Divide in Colorado and Wyoming, the waters of the Platte must traverse hundreds of miles of arid plains before reaching south central Nebraska. During the past century, man has developed numerous impoundments and diversion systems to tap the annual flows for irrigation and other purposes at numerous sites in the Upper Platte River Basin (Fig. 1). As a consequence, approximately 70% of the annual flows of the Platte are now withdrawn before reaching south central Nebraska (Kroonemeyer 1978). With much of its waters diverted upstream, the North Platte River downstream from Kingsley Dam, and the Platte River have undergone massive physical changes during recent decades. River channel width, for example, in some areas has diminished by 90% in the past 100 years (Williams 1978). Concurrent with channel shrinkage, extensive woodlands have become established on much of the former channel area (Fig. 2), and islands with woody vegetation now dot the river.
Changes in the character of the Platte River caused by shrinkage of the river channel and associated woody vegetation encroachment, plus the destruction of adjoining native grasslands concomitant with the decline of the water table and conversion of these lands to cropland (Fig. 3), have greatly altered habitat conditions for migratory birds. Recent habitat loss has caused concern for the welfare of the millions of migratory birds that stop along the Platte for several weeks during spring while enroute to breeding grounds on the northern plains, taiga, and arctic. Attention has been focused primarily on the potential impact of changing riverine conditions on the populations of sandhill cranes and whooping cranes. The plight of the cranes and concern about water development in the Platte River Basin became a major national resource issue in the early 1970's, when plans were in progress to develop the Mid-State Irrigation project along the Platte in south central Nebraska. This project, which would have irrigated 566.8 km2 (218.8 mi2) and withdrawn 363,000,000 m3 (295,000 acre-ft) from the river in areas used by cranes, was later withdrawn following its defeat in a local referendum. Abandonment of this project, however, did not defuse the issue because numerous other plans remained which sought to divert most of the Platte's remaining waters.
In 1973, the U.S. Fish and Wildlife Service, recognizing the need to maintain habitat in the Platte River Valley to meet the needs of migratory bird populations, attempted to preserve a key area encompassing approximately 60.7 km2 (23.4 mi2). This site, centering on Shoemaker Island, approximately 13 km southwest of Grand Island, was to be developed as a national wildlife refuge. When unveiled, however, the plan met with strong opposition from local landowners fearing condemnation of their lands (Wallenstrom 1976) and was withdrawn. The Fish and Wildlife Service began seeking alternative approaches to preserve habitat for cranes and other migratory birds because of the continuing need to resolve issues concerning habitat preservation. As part of that process, the Service agreed to conduct a study following criteria set by the Water Resources Council to evaluate whether a refuge was needed to meet bird requirements.
Preliminary steps to develop an information package on migratory bird requirements led to recognition of the need for detailed studies to evaluate the impact of changing habitat conditions on migratory bird populations using the Valley. Although millions of migratory birds use the Platte River Valley and nearby Rainwater Basin Area annually, habitat requirements of most, including several key species such as the sandhill crane, white-fronted goose, and the endangered whooping crane and bald eagle, remained largely undetermined. The problem was made more acute by a general lack of information on the role of staging areas in the life history of sandhill cranes and waterfowl. Of particular concern was recent scientific evidence that nutrient reserves acquired before arrival on the breeding site are vital to reproduction in certain species of waterfowl (Ankney and MacInnes 1978; Raveling 1979; Drobney 1980; Krapu 1981), which suggested that staging areas such as the Platte may serve to prepare birds physiologically for reproduction.
A multidiscipline team was formed at the Northern Prairie Wildlife Research Center to develop the research program following approval of funding in October 1977. The team was given 3 years (1978-80) to complete its investigations (Fig. 4). Research has dealt principally with two major issues: (1) habitat needs of migratory bird populations, and (2) the role of Platte River flows in maintenance of habitat. The latter issue was jointly investigated with the U.S. Geological Survey conducting studies to determine water requirements necessary to sustain the habitat base identified by the U.S. Fish and Wildlife Service. The Bureau of Reclamation also participated in the joint investigations by evaluating present water use and future water needs within the Upper Platte River Basin.
Personnel from the Migratory Bird and Habitat Research Laboratory, the Migratory Bird Management Office, the Ecological Services Office at Grand Island, Nebraska, and several universities joined researchers from the Northern Prairie Wildlife Research Center in undertaking the project. Because numerous important species of migratory birds utilize the Platte River Valley, a research effort developed that was broad in scope yet thorough in its treatment of species that appeared most vulnerable to the changing habitat conditions on riparian lands. Concurrent studies described existing habitat conditions and determined factors controlling plant succession on riparian lands. These studies were followed by experiments to determine how water regimes can be manipulated to inhibit germination and prevent establishment of woody vegetation, a step needed to maintain suitable habitat for species that utilize broad open reaches of river channel, especially the sandhill and whooping cranes.
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