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2015 Temporally. the highest median activity of Cd+2 was predicted during pre-snowmelt and snowmelt runoff . (Figure 6). The lowest median activity of Cd+2 was predicted during the low-flow period. On average. about 65 percent of the dissolved Cd species existed as Cd+2 during the snowmelt-runoff flow regime. TItis relation can be attributed to the relatively low pH and alkalinity concentrations. and hence complexation potential. that were typical of snowmelt runoff. Conversely. about 58 percent of the dissolved cadmium was predicted to occur as com pic xed species during the post-snowmelt and low-flow periods, when alkalinity and pH generally were higher. Figure 6 near here. Unlike Cd. which exists primarily as the more toxic free cation, most dissolved Cu exists as metal complexes (Figure 7). The dominant Cu species. Cu(OH)2' accounted for over 90 percent of the dissolved Cu at all sites and all flow regimes. Activities of the Cu species, Cu(OHh and CuCO). generally increased between . Leadville and Buena Vista and then decreased downstream. The highest median concentration of Cu(OHh was predicted at the Granite site. The Cu+2 species accounted for less than two percent for samples collected from the Arkansas River at all sites. The lowest median activities of all species were predicted at the Portland site. The highest median activities of Cu(OH)2' CuCO). and Cu+2 were predicted during snowmelt runoff and the lowest median activities were predicted during the low-flow period (Figure 7). The Cu+2 species accounted for an average of 1 percent or less of the dissolved Cu species for all flow regimes. The largest percentage of the Cu+2 species predicted for one sample (15%) was for a sample collected from the Arkansas River at Buena Vista during snowmelt runoff. The inflow of low alkalinity water from Lake Creek did not significantly influence Cu speciation. . 14