Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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April and increased during the high -flow months, May and June. This trend was caused by the mixing of seasonally low- specific- conductance water and seasonally high - specific- conductance water in Pueblo Reservoir, thus narrowing the annual range in specific conductance in the reservoir outflow. Few trends, except for increased specific conductance in June, August, and December, were detected in specific conduc- tance at the station located 24 miles downstream from Pueblo Reservoir. The increase in specific conductance probably was caused by the combined effects of water storage and mixing in Pueblo Reservoir and the increased inflow of relatively high- specific- conductance water from Fountain Creek. At Las Animas, located 120 miles downstream from Pueblo Reservoir, streamflow increased significantly during all months after 1974. Specific conductance tended to decrease during all months, but the decreases generally were not statistically significant at the 95- percent confidence level. At the two stations located downstream from John Martin Reservoir, specific conductance was affected by changes in John Martin Reservoir operations, increases in the reservoir inflow, and decreases in the specific conductance of the reservoir inflow. Specific conductance decreased during September through April and did not change substantially during May through August. These trends were very similar to trends observed immediately downstream from Pueblo Reservoir and were attributed largely to increased storage and increased mixing of seasonally low - and seasonally high- specific- conductance water in John Martin Reservoir. These factors tended to increase the minimum specific conductance and decrease the maximum specific conductance in the reservoir outflow. INTRODUCTION The lower Arkansas River in southeastern Colorado extends about 200 mi downstream from Pueblo Reservoir to the Colorado- Kansas State line (fig. 1). The Arkansas River is the primary municipal water supply for most of the 165,580 people who live in the five counties that compose the lower Arkansas River Valley and the primary agricultural irrigation supply for about 300,000 acres of irrigated land in the lower basin. Because of this dependence on the Arkansas River as a municipal and an agricultural water supply, the quality of water in the Arkansas River is very important. The quality of water in the Arkansas River is markedly different throughout the study area. Specific conductance, which is directly related to dissolved - solids concentration, increases downstream from a median of about 500 µS /cm near Pueblo to about 3,900 µS /cm at Lamar (fig. 1); this range in specific conductance corresponds to a range in dissolved - solids concentration of about 340 to 3,600 mg/L (Cain, 1987). The downstream increase in specific conductance and, hence, in dissolved solids, largely is due to the consumptive use of surface water and ground water for agricultural irrigation (Miles, 1977). High specific conductance is indicative of high dissolved -solids concentration; dissolved solids can affect the suitability of water for domestic, industrial, and agricultural uses. The secondary maximum contaminant level (SMCL) for dissolved solids in drinking water is 500 mg/L (U.S. Environmental Protection Agency, 1986). In the lower Arkansas River, 500 mg/L of dissolved solids is equivalent to a specific conductance of about 700 to 800 µS /cm (Cain, 1987). At higher levels, drinking water may have an unpleasant taste or odor or even cause gastrointestinal distress. Additionally, high dissolved - solids concentrations can cause increased deterioration of plumbing fixtures and appliances. Relatively expensive, advanced water - treatment processes, such as reverse osmosis, are needed to remove excessive dissolved solids from water. Agriculture also can be adversely affected by high- specific- conductance water. Depending on the crop, agricultural losses might occur when dissolved - solids concentrations reach 700 to 850 mg /L (U.S. Department of the Interior, 1994), which is equivalent to a specific conductance of about 950 to 1,200 µS /cm in the Arkansas River (Cain, 1987). With increasing specific conductance, special agricultural management practices may be needed and crops having a substantial salinity toler- ance may need to be grown. Generally, crops with a higher salinity tolerance have a lower market value than salt- sensitive crops (Miles, 1977). 2 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado