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. 0217 and irrigation needs in the basin. Lake Creek contrib- utes the largest tributary flow to the Arkansas River. A number of mine-drainage tunnels, most notably the Yak Tunnel and the Leadville Mine Drainage Tunnel, historically have supplied poor-quality water to the basin (Moran and Wentz, 1974). East of Canon City, the Great Plains Province ranges in elevation from 3,500 to 5,000 ft. Mean annual precipitation ranges from 10 to 14 inches (Bums, 1985). Sedimentary rock and alluvial fill underlie the region. Land use primarily is agricultural along the river corridor; the remaining areas mostly are rangeland. More than 411,000 acres in the Arkansas River Basin are irrigated, including about 56,000 irrigated acres in the upper basin (Dash and Ortiz, 1996). Most of the irrigated areas are east of Pueblo and are concentrated along the river corridor. Primary crops are wheat, com, sorghum, and alfalfa. Fruit crops and commercial vegetables also are produced. Over 600,000 head of cattle per year have ranged in the Arkansas River Basin (Colorado Depart- ment of Agriculture, 1981), and most of the cattle and feedlots are in the lower basin, from Rocky Ford to Lamar (fig. I). The two largest population centers are Pueblo and Colorado Springs (fig. I). These two cities account for more than 70 percent of the 641,720 people in the Arkansas River Basin (U.S. Bureau of the Census, 1991). Pueblo is located on the main stem of the Arkansas River downstream from Pueblo Reser- voir at the confluence with Fountain Creek. Colorado Springs is located on Fountain Creek 40 mi upstream from the confluence. On the west edge of the Great Plains Province, east ofCaiion City, the gradient of the streambed decreases to less than 9 ft/mi as the river flows out of the mountains and onto the eastern plains. Just upstream from Pueblo, the river flows into the 357,678 acre-ft Pueblo Reservoir (fig. I). The reser- voir is a multipurpose facility completed in 1975 as part of the Fryingpan-Arkansas project. Fountain Creek flows into the Arkansas River within the city limits of Pueblo. Downstream from Fountain Creek, the stream channel changes from a rock andcobhle hottom to a shifting sand channel that meanders along the alluvial flood plain. Numerous diversion structures between Avondale and La Junta (fig. I) remove water from the river for irrigation purposes. In tenus of streamflow contribution to the Arkansas River, some of the larger tributaries include Fountain Creek, Timpas Creek, and the Purgatoire River (pI. I). The Purgatoire River flows into the Arkansas River just upstream from the 6l5,500-acre-ft John Martin Reser- voir (fig. I), which was completed in 1943 by the U.S. Army Corps of Engineers as a multipurpose project. Generally, recreational activities on the Arkansas River east of Canon City are concentrated at the two main-stem reservoirs. Acknowledgments The authors would like to thank the many indi- viduals who assisted in the collection of these data. Thanks to hydrographers Frank Kipple and Anthony Gutierrez of the State of Colorado for their sample- collection activities. Additionally, thanks are extended to Paul Martin and George Woods of the Pueblo Board of Water Works for their excellent work collecting numerous water-quality samples in the upper basin. Finally, thanks to Jeffery West, Charles M. Moore, and Matthew Kurchinski of the USGS for their tireless efforts under harsh field conditions. STREAMFLOW Precipitation and transmountain diversions are the principal sources of water in the upper Arkansas River Basin. Snowfall during winter produces a deep snowpack in the mountains that subsequently melts to supply most of the total annual streamflow in the basin. Transmountain diversions from the Colorado River Basin are supplied from several sources high in the basin. Much of this water is routed through Turquoise Lake to Twin Lakes Reservoir (fig. I) through closed conduit for the purpose of generating power and for storage to meet downstream irrigation and municipal-supply demands (Abbott, 1985). Several open channels also transport water from the Colorado River Basin to the Arkansas River Basin. During the study period, April 199Q-March 1993, the monthly mean streamflow of the Arkansas River in the u~per basin ranged from 70 ft3/s at Leadville to 688 ft Is at Parkdale (Ugland and others, 1991, 1992, and 1993). Water released from Twin Lakes Reservoir by way of Lake Creek is the largest source of tributary flow to the upper Arkansas River. The monthly mean reservoir release from Twin Lakes Reservoir during the study period was 284 ft3/s (Ugland and others, 1991,1992, and 1993). These reservoir releases repre- sented about 45 percent of the annual flow at Granite, STREAMFLOW 7