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0336 <br />Purpose and Scope <br />This report lists the types of water - quality data <br />collected, describes methods of data- collection, analy- <br />sis, and quality- assurance procedures and presents a <br />compilation of selected water - quality data. Data were <br />collected from April 1990 through March 1993 at <br />59 surface -water stations in the Arkansas River Basin <br />of Colorado (pl. 1) that included 19 Arkansas River <br />stations, 31 tributary stations, 2 mine - drainage stations, <br />and 7 transmountain diversion stations. Water - quality <br />data presented in this report include onsite measure- <br />ments of instantaneous discharge, specific conduc- <br />tance, pH, water temperature, and dissolved oxygen, <br />bacteriological analyses; chemical analyses of selected <br />inorganic constituents, pesticides, and radiochemicals; <br />analyses of suspended - sediment concentration; and <br />analytical quality- assurance data for selected water - <br />quality constituents. <br />Acknowledgments <br />The authors gratefully acknowledge the assis- <br />tance of many government and local agencies that <br />cooperated with the U.S. Geological Survey to develop <br />a basinwide water - quality study. Appreciation is <br />extended to the many landowners along the Arkansas <br />River corridor who permitted access to their property <br />to measure discharge and to collect water samples. The <br />authors thank State hydrographers Frank Kipple and <br />Tony Gutierrez, Colorado Department of Natural <br />Resources, Division II of Water Resources, Office <br />of the State Engineer, for their timely assistance <br />with streamflow determination. Special thanks are <br />extended to Jeffrey West, Charles (Chuck) Moore, and <br />Matthew Kurchinski of the U.S. Geological Survey for <br />the collection and compilation of water - quality data. <br />DESCRIPTION OF THE STUDY AREA <br />The study area includes the entire Arkansas <br />River Basin of Colorado and consists of approximately <br />the southeast one - quarter of the State (fig. 1). Eleva- <br />tions in the basin range from 3,350 ft above sea level at <br />the Colorado -Kansas State line to 14,433 ft at the high- <br />est mountain peak in Colorado. Mean annual precipi- <br />tation in the basin ranges from more than 40 in. in the <br />mountains to less than 10 in. in areas of the eastern <br />plains (Colorado Climate Center, 1984). <br />Streamflow in the Arkansas River primarily is <br />from melting of snow that accumulates in the moun- <br />tains from October through May each year. At lower <br />elevations, runoff from summer thunderstorms can <br />contribute substantial quantities of streamflow for short <br />periods of time. Much of the streamflow that occurs <br />east of La Junta (fig. 1) can be irrigation -return flow <br />during parts of most years (Cain, 1985). Mean annual <br />runoff decreases from more than 30 in. in the moun- <br />tains to less than 0.1 in. downstream from Pueblo <br />(Abbott, 1985). For the interested reader, a more com- <br />prehensive description of the historical water resources <br />of the Arkansas River Basin of Colorado is reported in <br />Crouch and others (1984), Abbott (1985), Burns <br />(1985), Cain (1985), and Kuzmiak and Strickland <br />(1994). <br />Land use in the study area predominantly is <br />agricultural, consisting of rangeland and cropland <br />areas throughout the drainage basin. National forests <br />are located upstream from Canon City and cover about <br />one -third of the upper Arkansas River Basin. The <br />upper basin extends downstream to about Pueblo. <br />Historically, substantial mining of precious metals <br />occurred in the basin upstream from Canon City, <br />but most of the mines are abandoned and ranching <br />presently (1994) is a principal land use in the upper <br />basin. Irrigation water use constitutes the largest <br />withdrawals of surface water in the Arkansas River <br />Basin. It was estimated that irrigation used about <br />1,730 Mgal/d of water during 1985, with 88 percent <br />of the total irrigation withdrawal from surface - <br />water sources (Litke and Appel, 1989). More than <br />411,000 acres of alluvial lands in the basin are irri- <br />gated, including about 56,000 irrigated acres located <br />in the upper basin. Mainly alfalfa, hay, and pasture <br />grass are irrigated in the upper basin, although fruits <br />and some grain crops are grown on irrigated land in <br />the foothills located around Canon City. Truck - crops, <br />alfalfa, and grain -crops are grown on irrigated land in <br />the lower basin downstream from Pueblo. Most of the <br />nonirrigated agricultural lands in the basin are used for <br />rangeland or dryland wheat production. <br />Population in the study area was determined by <br />the 1990 census (U.S. Bureau of the Census, 1991) to <br />be 641,700, about 19 percent of the total population of <br />Colorado. Most of the population is located near the <br />cities of Colorado Springs and Pueblo or concentrated <br />in small towns and rural areas along the Arkansas River <br />corridor. <br />DESCRIPTION OF THE STUDY AREA <br />