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. ',",e",-- " . 1782 reach could be attributed to the two measured tributaries in the reach. Groundwater inflow and unmeasured return flow may account for the remainder. In the Pueblo area, median concentrations in the river increased by 15 percent due to tributary inflow from Fountain Creek. Downstream of Pueblo, concentrations increased steadily to Catlin Dam (about 5 mg/L per mile). A 71 percent increase in median concentration occurred between Catlin Dam and La Junta (16 mg/L per mile) as irrigation-return flows comprised a larger portion of the streamflow. Between La Junta and Las Animas, the dissolved-solids concentration in the river increased slightly. Median dissolved-solids concentrations increased 183 percent or at a rate of about 30 mg/L per mile from Las Animas to Coolidge; the high- est median concentration in the basin was 3,825 mgIL at Coolidge. Evapo-concentration of dissolved salts resulting from the use and re-use of irrigation water throughout the lower basin resulted in a more than lO-fold increase in dissolved solids in the lower basin. Overall, median dissolved-solids concentrations increased from a minimum of 62.5 mg/L at Granite to 3,825 mgIL at Coolidge. Seasonally, dissolved solids in the upper basin were smallest during snowmelt runoffand highest during low flow. As such, the relation between total-dissolved solids and streamflow in the upper basin was inversely propor- tional. Dissolved-solids concentrations were generally as much as 2.5 times larger during the low-flow period than during snowmelt runoff. In natural waters, specific conductance ia a good estimator oftolal-dissolved-solids concentrations (Hem. 1985). Regression models of specific conductance and IOtal dissolved solids were developed utilizing least-squares techniques for 24 sites in the Arkansas River Basin where IS or more paired measurements were available. A list- ing of the sites, regression coefficients, and various statistical qualifiers can be found in table 2. All regressions were significant at the 95th percent confidence level. Table 2 near bere. Regression equations for total dissolved solids and specific conductance for selected sites in the Arkansas River Basin, April 1990-March 1993 :~..'. ~f.\t;~l.~ ~~!~.~~:~~1;~~. i\~~<~ l. ;:~~~ !~; ~;:~~. ~ ..~......;.. ,I 12 S:Jbiect tc ~{svis~cn 50 NOT QUOTE OR RELEASE l'0nding Approval by Dircc~cr, U.S. Geolopkcl Survey