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,. 2008 General water-quality characteristics . The general water quality of the upper Arkansas River is influenced by many factors including ground water, direct runoff from snowmelt or rainfall, mine drainage, Iransmountain diversions. and water use within the basin. These factors result in distinct spatial and temP<Jra! patterns of surface-water quality in the area. The pH uf the upper Arkansas River typically falls in the range 7.5-8.5 and tends to increase downstream. Due to the low buffering capacity of snowmelt runoff, pH values tend to be smallest during this flow regime. Dissolved-oxygen concentrations generally are near saturation throughout the basin. Alkalinity ranges fmm as low as 20-30 mg/L as CaCO) in the upper reaches of the basin to about 170 mgJL near Portland. The smallest alkalinity occurs at Granite and is a result of low-alkalinity inflow from Twin Lakes Reservoir. Alkalinity tends to be smallest during snowmelt runoff and increases during other flow regimes. Throughout the upper basin, water in the Arkansas River is a calcium bicarbonate water type, although the concentrations of the major solutes tend to vary spatially (Figure 3). Dissolved-solids concentrations are smallest in the upper reaches of the basin because of the predominance of . chemically-resistant igneous and metamorphic rocks. Calcium and bicarbonate concentrations decrease about 40% between Empire Gulch and Granite due to dilution by inflows from Lake Creek. Dissolved-solids concentrations tend to increase downstream from Granite due to the increasing proportion of less chemicall y-resistant sedimentary rocks in the tributary drainages. Figure 3 near here. During the study period. concenlrations of total-recoverable and dissolved metals exhibited substantial spatial and temporal variability (Figure 4). Total-recoverable metals increased between Leadville and Empire Gulch during all seasons presumably due to mine drainage. Between Empire Gulch and Granite. total-recoverable metals decreased due to dilution by Lake Creek. Downstream from Granite, total-recoverable metal concentrations generally decrea.~ed during all flow regimes except snowmelt runoff. Total-recoverable metal concentrations . typically were largest during snowmelt runoff and tended to increase downslrcam from Granite. The increase in 9