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1780 annu3J maximum during snowmelt runoff. Streamflow in the Arkansas River typically decreases during the reser- . voir-release period as runoff from snowmelt subsides and controlled releases of water from Pueblo Reservoir and John Martin Reservoir play significant roles in the streamflow characteristics of the river. Large amounts of water are also diverted for irrigation needs during this period. From July to September, streamflow can increase substan- tially for short periods of time following intense rainstorms and subsequent runoff from tributaries. DISCUSSION OF WATER.QUALlTY .~, >~~Ti~}~~if~~;~~SI:~?':~~. t. 0 ~1i~.~'r~.'L Dissolved Oxygen and pH ,. . . ...... ~:J::;:s:.=t ~'C' ~<~ViSlcn 0" ~ "OT QU"-:: '"'R "';::1 = " c.'=; ::",~/-:..., "\i' ',-" ~ 10. '...;/ I\.'-_~J'jl,~....,"- '" "A I!.. D'IPc ->"r ....':':T!...,.~o"'lor' ~~rovo ~y : '"':,.;'lo..f I ~ _. .". .Ii ';;:.:.1 .{""'~ l!l':: r..,~-.I","';""'I '::"r".-:.'j .'-'. """'_......~:::;,...-' 'IeIw .- Dissolved-oxygen concentration in water is inversely related to stream temperature and is affected by pho- tosynthesis, respiration, aeration, and waste loads. Overall, the waters of the Arkansas River Basin were well oxy- genated. Streamflow in high gradient streams in the upper basin supplied sufficient amounts of atmospheric oxygen to generally saturate the water column. In some cases, the water was supersaturated with respect to dissolved oxy- . gen because of increased photosynthetic activity. In the lower basin, the river continued to be well oxygenated even though stream velocities were slower and less aeration occurred (Dash and Ortiz, 1996). Aquatic-life standards set by the Colorado Department of Health (1994) for the Arkansas River stated that dissolved-oxygen concentrations should not drop below 5 to 6 mg/L; instantaneous dissolved-oxygen concentrations 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 8A (fig. 4). The pH of the Arkansas River in the Leadville mining district remained ne3J' neutral to slightly alkaline because mine-drainage water from the Leadville Mine Drainage Tunnel (LMDT) and the Yak Tunnel was neutralized before flowing into the river. The lowest median pH in the upper basin generally occurred in the vicinity of Buena Vista (fig. 4). Downstream from Buena Vista, the pH in the river increased because ofincieasing concentrations ofbicaroonate. In the lower basin, the median pH of the Arkansas River ranged from 8.2 to 8.5. The highest median pH occurred at Highway 227 (fig. 4) just downstream . from the confluence of the Arkansas River and Fountain Creek. The pH of the Arkansas River was typically lower 10