Breeding Birds of the Platte River Valley
The Physical Setting
Climate
The climate of central Nebraska is continental, characterized by hot summers, cold winters, and irregular precipitation. Precipitation is seasonal, occurring mainly between May and September. At North Platte, the climate is semiarid, with annual precipitation averaging 450 mm (Stevens 1978). Near Grand Island, the climate is semihumid, with average precipitation of 570 mm (Stevens 1978).
Geology
The Platte River Valley is underlain with Quaternary alluvium made up of clay, gravel, sand, and silt (Bose 1977, Huss 1981). Interfingering beds of clay, gravel, sand, sandstone, and silt lie beneath the alluvium. The Ogallala Formation, highly porous and permeable Tertiary sediments, acts as a water reservoir known as the Ogallala Aquifer beneath a large portion of the Great Plains.
Near Grand Island, alluvial sediments of the Platte River are deposited in a series of troughs cut into the underlying Ogallala, Niobrara, and Pierre Formations (Hurr 1978). The Niobrara and Pierre Formations consist of chalk, limestone, and marine shales. Above this layer lies a shallow alluvial aquifer that occurs beneath the Platte River throughout central Nebraska (Hurr 1978).
Topography
Alluvial bottomlands, river terraces, and gently rolling bluffs along the river escarpment are the dominant physiographic features of the Platte River Valley (Fig. 2). The bottomlands are flat and extend for 24 km in some places. The rich prairie soils of the valley are the basis for the productive row-crop agriculture that dominates the landscape today. Bluffs along the river escarpment are overlain with loess deposits and dissected by natural ravines that drain uplands. There is a gradual rise in elevation to the west throughout the valley, from 540 m near Chapman to 1070 m near Lewellen. The mean rise in the river bed from east to west is about 1.2 m per kilometer.
Hydrology
The North Platte and South Platte rivers originate as snowmelt runoff streams in the Rocky Mountains of Colorado. The North Platte River flows northward from Colorado into east-central Wyoming and then flows southeast to its confluence with the South Platte River. The South Platte River flows northward through Denver to Greeley, Colorado, and then northeastward to its confluence with the North Platte River near North Platte Nebraska. From this junction, the Platte River flows east across Nebraska to the Missouri River at Plattsmouth, Nebraska. Precipitation in the Platte River basin contributes to the flow in the river, but most of the flow is derived from spring snowmelt in the Rocky Mountains (Eschner et al. 1981).
The pattern of streamflow in the Platte River has been substantially altered by the diversion and storage of water, primarily for use in irrigation and power generation (Fig. 3). Before settlement, the river was characterized by high spring flows and relatively low mid-summer flows (Eschner et al. 1981). Reservoirs in Wyoming, Colorado, and Nebraska have resulted in a reduction in peak flows and mean annual discharge in the North Platte and Platte rivers (Currier et al. 1985). Since 1915, peak flows have declined from a mean of 550 cubic meters per second to 150 cubic meters per second at Overton (Williams 1978). Corresponding to this decline in mean annual discharge has been a reduction in the width of the Platte River channel. Today, the river channel is 10 to 70 percent of its width at the time of settlement in the 1860's (Williams 1978). Lake McConaughy and Tri-county supply canal are the principal impoundment and diversion structures affecting instream flows in the Platte River.
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