Endangered and Threatened Species of the Platte River

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�1 INTRODUCTION Experience teaches that it is not wise to depend upon rainfall where the amount is less than 20 inches annually. The isohyetal or mean rainfall line of 20 inches ... in a general way... it may be represented by the one hundredth meridian. [In this region] agriculturalists will early resort to irrigation. John Wesley Powell, 1878 The 100th meridian is a defining geographic feature in the United States that marks important transitions for land, water, and life. In the Great Plains, this north -south line (immediately east of Cozad, Nebraska) transects the corridor of the central Platte River Valley of Nebraska, the subject of this report. The river is emblematic of the human and ecological complexities of policy, science, and management for a dryland river (Figure 1 -1). The river was named the Platte River, French for "flat river ", on June 2, 1739, by two French explorers who were searching for a route from Illinois to Santa Fe, New Mexico (Sheldon 1913). . The Platte River's two great branches, the North Platte and the South Platte, rise in the Rocky Mountains in the West and join in western Nebraska to the west of the 100th meridian (Figure 1 -2). The river flows 310 mi (about 500 km) through Nebraska across the meridian to a confluence with the Missouri River. The Platte River delivers the runoff of its 86,000 -mi2 (223,000 -km2) drainage area, largely the result of precipitation in the high western mountains, to an extensive system of water control for the highly productive agricultural area in the plains. The river is more than a conduit for water, however; along its course, it creates an aquatic and riparian ecosystem that provides wildlife habitat unlike habitats found outside the river valley. The river corridor, within the Central Flyway of North America, provides habitat for migratory and breeding birds, including three endangered or threatened species: the whooping crane (Gnus americans), the piping plover (Charadrius melodus), and the interior least tern (Sterna antillarum athalassos). The broad shallow waters of the Platte near its confluence with the Missouri River constitute an important habitat for at least one endangered species of fish: the pallid sturgeon (Scaphirhynchus albus). The infrastructure investments that made irrigated agriculture and urban water supply possible in the Platte River Basin have substantially altered the hydrologic regime of the Platte River. During the 20th century in the Platte River Basin, construction of storage reservoirs and diversion dams and installation of wells to tap groundwater supported the economic vitality of the region. The structures continue to provide flood reduction, water supply, hydroelectricity, and recreational benefits. By controlling and diverting water flows, however, the dams altered. the stream flows, and that caused widespread environmental changes. A major habitat change involved the expansion of woodland and the narrowing of river channels. Whooping cranes, piping plovers, and interior least terns, whose populations were already declining because of • other factors, prefer more sparsely vegetated, open, sandy areas near shallow water. 15