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the next downstream main-stem site, and about 35 percent of the total annual flow at Portland, the site farthest downstream in the upper basin. Streamflow in the lower Arkansas River Basin is highly regulated by storage and release operations at Pueblo Reservoir and John Martin Reservoir. Surface- water diversions and return flows downstream from Pueblo Reservoir have a pronounced effect on the streamflow of the Arkansas River. Irrigation diver- sions downstream from Pueblo Reservoir substantially decrease streamflow in the river. Irrigation return flows can make up a majority of the streamflow down- stream from Manzanola during certain times of the year (Cain, 1985). During the study period, the monthly mean streamflow of the Arkansas River between Pueblo Reservoir and John Martin Reservoir ranged from 784 ft3 Is at Avondale to 20 I ft3 Is at Las Animas (Ugland and others, 1991, 1992, and 1993). The monthly mean streamflow near the Colorado- Kansas State line was about 130 ft3/s. In the lower basin, the largest tributary to the river was Fountain Creek, which contributed about 15 percent of the annual streamflow at Avondale. Streamflow in the Arkansas River exhibits considerable temporal variability. During October into April, streamflow is at an annual minimum and is maintained by natural base flow and reservoir releases. In April, however, streamflow begins to increase in the upper basin as lower elevation snow begins to melt. The importance of this early snowmelt runoff, or "flushing" regime, on the water quality of the Arkansas River is discussed in the "Trace Elements" section of this report. Streamflow typically is at its annual maximum during snowmelt runoff in May and June. Peak runoff generally occurs during the second week in June. During July through September, stream- flow includes decreasing amounts of snowmelt and is augmented with releases of stored water. During this time, streamflow can increase substantially over short periods of time following intense rainstonus and subsequent runoff from tributaries. During July through September, most streamflow in the lower basin generally is diverted for irrigation. WATER QUALITY Water-quality data in this report are presented by streamflow regime. In general, the data were divided into three streamflow regimes that are defined as low flow (October-April), snowmelt runoff (May- June), and post-snowmelt runoff(July-September). However, the temporal analysis of trace elements in the upper basin was defined somewhat differently because of unique transport mechanisms. For trace elements in the upper basin, data are further divided into a pre-snowmelt runoff period in April; therefore, the low-flow regime for trace elements in the upper basin is defined as October through March. The trans- port characteristics for trace elements are discussed in greater detail in the "Trace Elements" section of this report. Statistical comparisons of selected water-quality properties and concentrations of constituents were made between sites in the Arkansas River Basin. Most statistical comparisons were made using Tukey's studentized range test (SAS Institute, Inc., 1985) with an alpha level of 0.05 to detenuine whether the varia- tions in concentrations that occurred among sites were significantly different. Dissolved Oxygen and pH Dissolved-oxygen concentration in water is a function of temperature and pressure, and to a lesser degree, the concentration of other solutes (Hem, 1985). Overall, the waters of the Arkansas River Basin were well oxygenated. Streamflow in high-gradient streams in the upper basin supplied sufficient amounts of atmospheric oxygen to saturate the water column. In some cases, the water was supersaturated with dissolved oxygen because of photosynthetic activity. In the lower basin, the river continued to be well oxygenated even though stream velocities and aeration decreased (Dash and Ortiz, 1996). Aquatic-life standards set by the Colorado Department of Health (1994) for the Arkansas River state that dissolved- oxygen concentrations should not decrease below 5 to 6 mg/L; instantaneous dissolved-oxygen concentra- tions for all main-stem sites in the basin ranged from 6.1 to 14.7 mg/L. The pH of the Arkansas River was near neutral to slightly alkaline; median pH values at main-stem sites in the upper basin typically ranged from 7.9 to 8.5 (fig. 3). In the Leadville area, pH of the Arkansas River was slightly alkaline because mine-drainage water from the LMDT is neutral, and the Yak Tunnel drainage, which is acidic, was neutralized before reaching the river. The lowest median pH in the upper 8 Waler-Quallty Assessment of the Arkansas River Basin, Southeastern Colorado, 199~3