Hydrologic and morphologic changes in channels of the Platte River basin: A historical perspective
The channels of the Platte River and its major tributaries, the South Platte and North Platte Rivers in Colorado, Wyoming, and Nebraska, have undergone major changes in hydrologic regime and morphology since 1860. These changes are attributed here to agricultural, municipal, and industrial water use. Although water-resource development varied temporally throughout the basin, the history of development along the Platte River and tributaries followed four stages: (1) Construction of small, crude ditches to irrigate flood plains; (2) construction of larger canals to irrigate bench lands; (3) construction of reservoirs to store snowmelt runoff; and (4) accelerated development of ground-water resources. Despite differences in rates of development, diversion and storage of water for irrigation, municipal, and industrial use have changed streamflow patterns throughout the basin. At some stations, significant changes in flood peaks, annual mean discharges, and shapes of flow-duration curves have been recorded. Changes in streamflow patterns are manifested by changes in appearance of channels of the Platte River. Prior to water development in the 19th century, the Platte was a wide (-2 kilometers), shallow (1.8 to 2.4 meters) river characterized by bankfull spring flows and low summer flows. Although timber generally was scarce in the valley, the Platte channels contained hundreds of small, timbered islands. Since development, the channels have changed radically. Comparing surveyor's maps (General Land Office), drawn during the 1860's, with six sets of aerial photographs, taken between 1938 and 1979, for six 5-kilometer reaches of the river shows that the channels have narrowed considerably above the confluence with the Loup River. The width of the channels in 1979 ranged from 8 to 50 percent of the channel width in 1860. Below the confluence with the Loup River, the width of the river in 1979 was about 92 percent of the channel width in 1860. Above the confluence with the Loup River, width reduction has occurred by progressive encroachment of vegetation and consequent vertical and horizontal accretion on sand bars in the channel. Vegetative encroachment on sand bars has occurred because (1) the present hydrologic regime provides more favorable conditions for germination and growth on sand bars, and (2) since development of the basin, flood peaks are no longer capable of scouring vegetation from the sand bars. Overbank flows evidently have become more common, probably because channel narrowing and vegetative encroachment have increased the hydraulic roughness of the channels. Moreover, the magnitude of low flows has increased and the days of no flows has decreased giving the channels a more perennial character.