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Platte River Ecology Study


The Study Area


Geography

The Platte River originates at the convergence of the North Platte and South Platte Rivers near the city of North Platte, Nebraska (Fig. 1). From its source, the Platte flows eastward along an S-shaped course and empties into the Missouri River near Omaha, Nebraska. Along this 500 km (310 mile) route approximately 77,210 km2 (29,800 mi2) are drained by the Platte and its major tributaries.

The South Platte River originates as snowmelt in central Colorado at about 3,310 m (12,500 feet) above sea level (Fig. 1). From its source, the River flows southeastward, then north-northeastward, and, after crossing the Colorado-Nebraska border, flows almost due east to join the North Platte. The South Platte is about 730 km (450 miles) long and drains approximately 62,960 km2 (24,300 mi2).

Also beginning as snowmelt, the North Platte River flows northward from north central Colorado into central Wyoming where it gradually curls to the southeast before joining the South Platte River (Fig. 1). From its source at about 3350 m (11,000 feet) above sea level to its confluence with the South Platte, the North Platte River traverses approximately 1070 km (665 miles) and drains an area of 90,430 km2 (34,900 mi2).

The study area and its location in Nebraska is shown in Fig. 5. The primary focus of attention was from the town of Chapman westward to the city of North Platte, and along the North Platte River Valley from North Platte westward to the Kingsley Dam at the east end of Lake McConaughy (Fig. 5). This included 327 km (203 miles) of river valley in the Nebraska counties of Hamilton, Merrick, Adams, Hall, Kearney, Buffalo, Phelps, Gosper, Dawson, Lincoln, and Keith. The largest population centers in the study area are Grand Island, North Platte and Kearney (1980 preliminary population estimates of 33,159, 24,475, and 21,149, respectively). U.S. Interstate Highway 80 parallels the Platte River from Grand Island to North Platte.


Climate

The climate of the Platte River basin is typical of the interior of large, mid-latitude continents. The precipitation, two-thirds of which falls during the growing season, and humidity are low, summers are hot, and winters severe. Temperature and precipitation vary widely among years. Short-term weather changes are influenced by large masses of warm, moist air from the Gulf of Mexico; cold, dry air from central Canada; cool, dry air from the northern Pacific Ocean; and hot, dry air from the southwestern United States.

Mean monthly temperatures and total monthly precipitation for the duration of the study are summarized in Table 1. Mean values are comparable to those of the period 1941-70. During 1978 and 1979, however, January and February temperatures were lower than normal, and the heaviest precipitation fell earlier in the year than usual.


Geology

The Platte River Valley, like most of western and central Nebraska, is underlain by Ogallala sedimentary rock of the Tertiary period (Bose 1977). Prior to Pleistocene glaciation (1,000,000+ years ago), easterly flowing rivers cut deep valleys across what is now Nebraska. The advance of the Nebraskan ice sheet dammed many of the rivers, causing the valleys to fill with clay, sand, and gravel some were partially reopened by erosion during the Aftonian Interglacial period (900,000 years ago). Valleys were filled again, and river drainage patterns further altered, during and after the Kansan glacial advance (600,000-700,000 years ago). Only minor changes occurred during the ensuing glacial advances and retreats. Alluvial deposits now cover the Platte River Valley and overlie thick layers of sand and gravel that contain a major aquifer.


Vegetation

In central and western Nebraska, the Platte River Valley is characterized by forest, shrub, and sandbar vegetation on the river floodplain lowland prairie and cultivated fields on the river terraces and upland prairies on the loess bluffs along the ancient river escarpment. The floodplain forest, shrub, and sandbar communities have developed on coarse-textured alluvial soils adjacent to the river channel. The forest communities have open canopies and are dominated by cottonwoods with an understory of red cedar and rough-leaf dogwood. Green ash, hackberry, American elm, red mulberry, and slippery elm also occur in the floodplain forest. Adjacent to the major river channel and in areas where the forests are limited to a narrow strip along the river bank (e.g., Fort Farm Island and Mormon Island), low shrub islands and vegetated sandbars predominate. Peach-leaf willow, sandbar willow, and indigo bush are the dominant shrub species, whereas lovegrass, nutsedge, barnyard grass, cocklebur, and scattered willow and cottonwood seedlings characterize the vegetation on the low shrub islands and recently exposed sandbars.

Some lowland grasslands have been maintained for grazing and hay production but much of the native lowland prairie on the river terraces adjacent to the floodplain has been converted to cropland. The river terraces are derived from a mixture of loess and alluvial parent soil material, and frequently extend for a mile or more between the floodplain vegetation and the ancient river escarpment. Mid- to tall-grasses such as big bluestem, side-oats grama, western wheatgrass, sand dropseed, indian grass, switchgrass, and bluegrass characterize the vegetation on the lowland prairie (Hopkins 1951). Minor grasses and sedges include needle-leaf sedge, plains muhly, Canada wild rye, and small panic grass. Western snowberry, poison ivy, wild rose, black-eyed susan, American germander, fringed loosestrife, and blazing star are conspicuous forbs in the lowland prairie vegetation.

The upland prairie of the river escarpment has developed on loess-derived soils. Grasses on the upland prairies tend to be much shorter than those on the lowland prairies due to a reduction in soil moisture. Although species such as big bluestem are present in both upland and lowland prairie, their stature is much reduced on the upland sites. Blue grama and buffalo grass are the dominant short-grass species on the upland prairie, whereas little bluestem, junegrass, western wheatgrass, sedges, downy brome, and six-week fescue are less prominent grasses (Hopkins 1951). Skeleton weed, cone flower, lead plant, scarlet gaura, and wild alfalfa are characteristic forbs of the upland prairies.

Mixed short- and mid- to tall-grass prairies develop along ravines and on slopes between the upland and lowland prairie vegetation types. These mixed prairies are composed of species found in both upland and lowland prairies and have a stature intermediate between the two types.


History

The Platte River served as a major landmark during the development of western America. The broad, flat valley provided explorers and settlers with a fertile highway through what had become known as the "Great American Desert". The river furnished drinking water; fish dwelling in the river and game that drank from it provided food; livestock grazed on the surrounding grasslands; and at eastern locations trees and shrubs supplied the wood necessary to make repairs, build shelter, and generate heat. Pioneers found that following the Platte, North Platte, and Sweetwater River Valleys took them to South Pass, a natural gateway through the Rocky Mountains. This Platte-South Pass Trail, later a part of the Oregon Trail, became a main thoroughfare to the West, supporting about 90% of all westward travel in the early and mid 1800's. The first overland mail service, the transcontinental telegraph line, the Pony Express, and the Union Pacific railroad all followed the Platte River Valley.

Government passage of the Homestead Act in 1862 encouraged the settlement of lands surrounding the Platte River Valley. The completion of railroads further fostered the development of previously uninhabited areas and provided access to eastern markets. The development of irrigation techniques in the late 1800's prompted many to see great potential in the vast expanses of land that had been too dry to be effectively cultivated.

With the 20th century came the advent of major water development programs in the Platte River Basin. In April of 1909, Pathfinder Dam in south central Wyoming was completed for the purpose of retaining over 1.2 billion m3 (1.0 million acre-feet) of flood waters for later release to three major irrigation projects. Within the next 50 years, five additional major reservoirs were created by damming the North Platte River: Guernsey Reservoir, 1927; Alcova Reservoir, 1938; Seminoe Reservoir, 1939; Lake McConaughy, 1941; and Glendo Reservoir, 1957. By 1960 almost 8.0 billion m3 (6.5 million acre-feet) of North Platte waters and 1.2 billion m3 (1.0 million acre-feet) of South Platte waters were stored in reservoirs (Bentall 1975). As of 1975 nearly 1.0 million ha (2.5 million acres) of land were irrigated by waters diverted from the North Platte, South Platte, and Platte Rivers. These waters are used also to generate electricity by powering hydroelectric generators and cooling steam powered plants. Part of the water diverted for power is subsequently returned to the river.


Economy

Approximately 95% of the land in the Upper Platte sub-basin, which includes the study area, is in private ownership (Missouri River Basin Commission 1975). Agriculture accounts for 97.0% of all sub-basin lands. Of this, 57.7% is used for pasture and range. Nonirrigated and irrigated croplands represent 24.8% and 14.4%, respectively. Nearly two-thirds of the non-agricultural lands are urban developed areas. Remaining lands include privately owned irrigation and power structures, state and federal lands not cropped, canals, and any other non-agricultural lands. Less than 0.3% of all lands in the Upper Platte sub-basin are publicly-owned fish and wildlife areas.

According to the U.S. Census of Population Reports and the Nebraska Department of Labor, the number of persons employed in agriculture in counties of the study area declined by about 54% between 1940 and 1979. This decline was primarily due to technological advances which have allowed an increase of livestock and crop production although the number employed has been reduced. Employment in service-related jobs, and transportation and communication increased by 68% and 43%, respectively. Mining and construction employment increased by 148% and the number of persons working in the trades climbed 164%. The largest increases were in manufacturing (613%) and government, military, and other (721%). The overall work force in the region increased 83% during this period. The total population of the area has increased 21% from 1940-80. From 195078, total personal income for the region increased 123% (Source: U.S. Department of Commerce, obtained from Bureau of Business Research, University of Nebraska at Lincoln. Income data adjusted to reflect purchasing power in 1978).


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