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. . . 1930 the Arkansas River. The aquifer width varies from l to 14 mi with an average width of 3 to 5 mi. The thickness of the alluvium varies from 0 to about 250 ft. The alluvium consists of fairly well-soned sand and gravel with minor amounts of clay. Depth to water varies from 0 ft in wetland areas to about 40 ft in eastern Colorado, and the saru- rated thickness varies from less than lOft to about 21O ft. Ground-water flow in the alluvial aquifer is generally from west to east. Snowmelt from the mountainous upper basin is the primary source of discharge in the Arkansas River. Snowmelt runoff typically begins in late April or May and peaks in June. In addition to native snowmelt runoff. discharge in lhe Arkansas River is supplemented by the transmountain diversion of water from the Colorado River Basin. Transmountain waler is divened into the basin during the summer months and is stored in several off-channel reservoirs in the upper basin or in Pueblo Reservoir until being released to meel downstream munici- pal or irrigation water-supply demands. Rainfall runoff, ground-water inflow, and irrigation-rerum flow also con- tribute to flow in the Arkansas River. A substantial amount of the Arkansas River is divened and consumptively used for irrigation in the srudy area. During 1955-94, the median annual discharge in the Arkansas River decreased dO\\'Ilstream by about 88 percent from Pueblo (448,000 acre-ftIyr) to Lamar (53,700 acre-ft/yr), largely because of irrigation diversions. Irrigarion rerum flow from tributary streams, drainage ditches, and the alluvial aquifer supplements flow in the Arkansas River. Much of the discharge in the river downstream from La Junta is comprised of irrigation rerum flow during pans of many years (Cain, 1987). RY REPOff? \oR.EU'M\N~ ;, "c. - . _.,- _...... 0,- . r-'r-.. ',,' ....1 'r'; l_~. ;. "--- . .:":. ..-'" 10