Breeding Birds of the Platte River Valley
Summary of Habitat Changes
Settlement and agricultural development in the Platte River Valley over the past 120 years have impacted the natural communities of plants and animals. The major habitat changes, are as follows (Currier et al. 1985):
(1) 66% reduction in average mean discharge of the Platte River (gauged at Overton) as a result of diversion and storage of water for irrigation, power generation and other uses.
(2) 68% reduction in average peak discharge of the Platte River (gauged at Overton) as a result of diversion and storage of water for irrigation, power generation and other uses.
(3) 65% to 79% reduction in channel width on the Platte River and a corresponding increase in encroachment of the channel by trees and shrubs following regulation of water in the river (Table 3).
(4) 58% to 87% reduction in area in the Platte River channel as a result of encroachment of wooded vegetation.
(5) As much as 97% loss of optimal sandhill crane roosting habitat in some segments of the Platte River as a result of wooded vegetation encroachment.
(6) 73% loss of native grasslands and wetland meadows within 5.6 km of the Platte River.
(7) Extensive drainage of the wetlands adjacent to the Platte River channel following declines in river flows and the construction of open ditch drainage systems.
(8) A reduction in production and cover of native grasslands and a shift in composition to early-maturing (cool-season) grasses, as a result of intensive grazing and the suppression of natural fires.
Changes in the Platte River habitat since the time of settlement are quantified in (Table 3). These data were generated from the geographical information system of the Platte River Whooping Crane Trust, Grand Island, Nebraska and compared with the status of native habitat before settlement (1840's). Since settlement, 2 major habitat components have greatly diminished; (1) open, unvegetated river channel, and (2) native lowland grasslands and tallgrass prairie.
Within the Big Bend stretch of the Platte 10,500 ha of river channel have been altered or lost; on the North Platte River an additional 3,239 ha of river channel has been lost. This amounts to a loss of 230 to 260 ha of river channel. These estimates are conservative, because lands in agricultural production (e.g., cropland, alfalfa) that lie within the historic 1840's floodplain boundary have been excluded from the analysis of channel losses.
Losses of native lowland grasslands and sandhills prairie on the lands adjacent to the river channel have also been substantial. Within the database area, native prairie grassland losses have averaged 52%. In the Big Bend reach of the Platte, grassland losses have ranged from 56% near Odessa to about 80% near Grand Island and near Shelton. The average loss of grassland in the Big Bend reach has been 72%. This loss is greater than the 61% overall loss of grasslands in the counties adjacent to the Big Bend, and indicates that conversion of grassland to cropland in the Platte Valley has occurred at an accelerated rate in comparison with adjacent lands.
Losses of native grassland along the North Platte River have been considerably less than on the Platte, ranging from 25% near Lewellen to 37% near North Platte. On average, grassland loss along the North Platte River has been 31%. As along the Platte River, the conversion of grassland to cropland in the North Platte River valley has been greater than in the surrounding counties where the loss has been 21%.
Current management practices have also seriously degraded the quality of much of the remainingnative grassland. On the dissected uplands surrounding the Platte River Valley the native range is in relatively good condition, but the rangeland on the river bottom is generally in poor condition. Years of season-long grazing, high stocking rates, and suppression of natural fires have permanently altered the character of many native grasslands. Species composition has gradually shifted from native late-maturing (warm-season) prairie grasses to a dominance by early-maturing (cool-season) grasses. This change has resulted in lower forage production and a reduction in cover for nesting birds and other species (e.g., small mammals that provide food for some migratory birds).
Human disturbances and development account for a relatively small proportion of the total land area (7.4%) in the Platte River Valley. The location of these disturbances, however, is critical when assessing their impact on migratory birds. For instance, one of the most serious disturbances in the Platte River Valley is Interstate Highway 80, which parallels the Platte River for over 161 km in central Nebraska. The roadway was completed in the mid-1960's in the floodplain of the river, less than 0.1 km was from the channel in many areas. Along the north side of the Platte River the habitat for migratory birds has nearly been eliminated by the presence of I-80. The wet meadows and sloughs that existed where I-80 lies today provided some of the best nesting and feeding habitat for migratory birds along the Platte River.
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