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<br />0216
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<br />Tunnel (LMDT) and California Gulch (pI. I), the two
<br />mine-drainage sites, were added to the network in
<br />1991. In addition, streamflow and water-quality data
<br />collected at the Arkansas River near Coolidge, Kans.,
<br />were used in the report; although the site is outside the
<br />study area boundary, these data were used to describe
<br />water-quality conditions of the downstream part of the
<br />study area.
<br />
<br />Methods of Investigation
<br />
<br />Data-collection activities were conducted for
<br />3 years and included onsite measurements of stream-
<br />flow, dissolved oxygen, pH, and specific conductance
<br />at all sites as well as collection of water samples for
<br />laboratory analyses. Water samples were analyzed for
<br />concentrations of dissolved solids (including major
<br />ions), trace elements, nutrients, radiochemical constit-
<br />uents, pesticides, suspended sediment, and bacteria. A
<br />detailed description of the water-quality constituents
<br />listed above can be found in Dash and Ortiz (1996).
<br />Because of the. diverse physiography, geology, land
<br />use, and water use in the basin, not all the sites were
<br />sampled for the same constituents. The water-quality
<br />constituents measured or analyzed at each site and the
<br />site name that will be used in this report are listed in
<br />table I. The major changes in physiography, geology,
<br />land use, and water use generally occur between
<br />Cailon City and Pueblo. Therefore, for the purpose of
<br />this report, the Arkansas River Basin was divided into
<br />an upper and lower basin. The upper basin drains
<br />about 4,700 mi 2 of the Arkansas River headwaters
<br />upstream from Pueblo Reservoir. The lower basin
<br />drains almost 20,700 mi2 downstream from Pueblo
<br />Reservoir to the Colorado-Kansas State line.
<br />Data collected for this study are presented in
<br />Dash and Ortiz (1996). Their report presents instanta-
<br />neous streamflow and water-quality data for all the
<br />sampled sites as well as data collected as part of the
<br />quality-assurance program. A description of the
<br />sampling procedures, quality-assurance sampling
<br />protocols, and analytical techniques used in the study
<br />are presented in the data report.
<br />This study was designed to provide data that
<br />represent water-quality conditions for different
<br />streamflow regimes in the Arkansas River Basin. The
<br />timing of sampling events relative to different stream-
<br />flow regimes is shown in figure 2. Rainfall-runoff
<br />samples were collected when possible. Sites on the
<br />
<br />Arkansas River were sampled eight to nine times per
<br />year. Tributaries that contributed at least 5 percent of
<br />the annual streamflow to the Arkansas River were
<br />sampled four to eight times. The two mine-drainage
<br />sites were sampled six to eight times per year. During
<br />each sampling event, water-quality samples generally
<br />were. collected in a downstream order throughout the.
<br />basin.
<br />
<br />Description of the Study Area
<br />
<br />The Arkansas River Basin is located in the
<br />central and southeastern parts of Colorado. Fenneman
<br />(1931) divided the basin into two physiographic prov-
<br />inces at approximately the 1050 parallel, which is just
<br />east ofCaiion City. To the west of that parallel is the
<br />Southern Rocky Mountains Province; to the east is the
<br />Great Plains Province (fig. 1).
<br />The Southern Rocky Mountains Province is
<br />mostly mountainous and has elevations ranging from
<br />5,000 to more than 14,000 ft. Mean annual precipita-
<br />tion ranges from less than 10 inches on the valley floor
<br />to more than 40 inches at the crest of the mountains
<br />(Abbott, 1985). The area is heavily forested and
<br />underlain by igneous and metamorphic rocks. East of
<br />Salida, there is a transition from igneous and meta-
<br />morphic rocks to metamorphic and sedimentary rocks.
<br />Much of the upper basin is within the Colorado
<br />mineral belt and historically has been mined. Mining
<br />operations continue in the region but on a much
<br />smaller scale than in the past.
<br />Most of the population and related activities are
<br />concentrated along the river corridor; the major towns
<br />are Leadville, Buena Vista, Salida, and Caiion City
<br />(fig. 1). In 1990, the estimated population in the area
<br />was about 60,000 (U.S. Bureau of the Census, 1991).
<br />The area is used for many recreational activities
<br />including rafting, kayaking, and fishing. It was esti-
<br />mated that nearly 270,000 people rafted the Arkansas
<br />River in 1992 (Steve Reese, Colorado State Parks, oral
<br />commun., 1992). The Arkansas River, from the head-
<br />waters to Caiion City, is characterized by steep-
<br />gradient, high-velocity flows that are confined to a
<br />relatively narrow rock and cobble stream channel. The
<br />river gradient averages 40 ftlmi along this stream
<br />reach. Reservoirs on Lake Fork (Turquoise Lake),
<br />Lake Creek (Twin Lakes Reservoir), and Clear Creek
<br />(Clear Creek Reservoir) (fig. I) store native stream-
<br />flow or transmountain diversion water for municipal
<br />
<br />INTRODUCTION 5
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