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SELECTED HYDROGRAPHS AND STATISTICAL ANALYSES CHARACTERIZING THE WATER RESOURCES OF THE ARKANSAS RIVER BASIN, COLORADO By Alan W. Burns ABSTRACT Hydrographs of annual precipitation from 30 stations, April 1 snowpack measurements from 18 snow-survey courses, annual discharge from 46 streamflow- gaging stations, and instantaneous water levels from 37 wells are presented to illustrate the temporal and spatial variability of the water resources of the Arkansas River basin in Colorado. Statistical analyses indicate no apparent time trends in annual precipitation or April 1 snowpack, but they do indicate declines in annual discharge for locations in the eastern part of the basin. A composite hydrograph indicates a negligible change in ground-water levels between 1930 and 1980 in the alluvial aquifer downstream from Pueblo. Gener- ally poor correlation occurs between precipitation data and snowpack data (less than 0.40 for monthly data and less than 0.61 for annual data). In addition, precipitation data did not correlate very well with discharge (less than 0.57 for monthly data), leading to the conclusion that the typical streamflow conditions are affected little by direct precipitation. Main-stem discharge correlates quite well with snowpack (as much as 0.85 for annual data), indicating its dependence on snowmelt runoff. INTRODUCTION The Arkansas River drains about 25,400 mi2 in Colorado (fig. 1). From its headwaters in the Sawatch and Mosquito Ranges, it flows southward through its own alpine valley. The flow increases many times as numerOllS rivulets and tributaries merge along its alpine course. Upon reaching the Sangre de Cristo Mountains, the river trends eastward toward the Great Plains. Leaving the high-mountain valleys, the river cuts through the foothills of the Rocky Mountains through the deeply incised Royal Gorge (not in fig. 1) near Canon City. Upon reaching the prairie of eastern Colorado, the river then meanders steadily toward Kansas and, ultimately, the Mississippi River. Streamflow in the river is derived mainly from the melting of snow that accumulates in the mountains during October through May each year. Although devastating floods from intense thunderstorms can occur along the foothills and on the prairie, rainfall contributes little to the normal flow of the river. Ground water is an additional source of water to the river; its contribution is particularly significant along the eastern plains where irrigation-return flow as ground water is the main source of streamflow. 1