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1781 during the elevated flow of the snowmelt.nmoffperiod because of the lower pH ofsnow.xnelt runoff. Instantaneous _ values for main-stem sites were generally within the acceptable range set by the S~te of Colorado for stream water-quality standards (Colorado Department of Health, 1994). . ',~.:t;'-':.';.'~:~';':":':-".,..'r::-_>,".~'";. ..r"';'o.,:....::~.: "'.'.'. :".'. ' . '..... . ~ . . '-' _ ~~;i~!~:~~~~~~ Dissolved Solids and Major Ions Figure 4 near here. Variations in pH in the Arkansas River, Aprli:,tb!i~~a~ch ~hicn :~..:.:.~ I\iU. \.:.l.....VJ~: Vl< K6:!.C:J;"~~C .~, ~t A . I . . ~...':~~'~ n ... n . .,.,." .. . ........m~ P?ov.,i oy D,rc;c;.{.r, 1''::'- , . I" '-~ ~ ~~Ol"",,,,,,,,,,,,,,,, -":.J.'r.~v .. -'-'::;.--'lwI1.-i. Water samples were collected and analyzed for total-dissolved-solids concentrations at all sites in the Arkan- sas River Basin. Main-stem sites were sampled 24 to 28 times during the stUdy period. Tributary sites were generally sampled less frequently. Tweniy-four to 28 water samples were collected and analyzed for major-ion concentrations at most main-stem sites in the Arkansas River Basin. The major ions analyz.ed we!e.ca!ciw:n. sodium, magnesium, bicarbonate, sulfate, and chloride. Potassium and fluoride were not analyzed because historical data indicated that . concentrations were not significant in the hasin. Most tributary sites were not sampled for major ions. Dissolved-solids concentrations in the Arkansas River are influenced by many factors, including nmofffrom snowmelt or rainfall, geology, land and water use, mine drainage, and groundwater. These factors resulted in distinct spatial variations of dissolved-solids concentrations in the river (fig. 5). Dissolved solids were slightly elevated at Leadville and Malta because of mineralized drainage from mines and mine tailings in the Leadville mining district. Downstream of Malta, dissolved-solids concentrations decreased as flow from Lake Creek diluted the dissolved sol- ids in the river. As such, the lowest median main-stem concentration in the basin (62.5 mgIL) occurred just down- stream at Granite. A gradual increase in concentration occurred from Granite downstream to Cafton City. Dissolved solids generally doubled along this 103 mile reach. The increase was due to decreased precipitation and runoff, changes in geology and chemical composition of rocks, and increased water use. Measured tributaries along this reach contributed less than 50 percent of the observed dissolved solids. Changes in geology from igneous and meta- morphic rock to sedimentary rock and increased return flows resulted in dissolved-solids concentrations nearly dou- .g again along a 16 mile reach from Canon City to Portland. Only 35 percent of the dissolved solids along this 11