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1788 Trace-element concentrations in the upper Arkansas River exhibit marked temporal variability due to the . influence of streamiiow regime. The downstream, seasonal variability of trace-element concentrations in the upper basin is described in terms of four distinct seasonal streamiiow regimes. The periods depicted include the follow- ing: pre-snowmelt runoff (April), snowmelt runoff (May-June), post-snowmelt runoff (July-September), and low flow (October-March). During the low-flow period, streamflow is relatively small and is maintained by natural baseflow and reservoir releases. During pre-snowmelt runoff, streamflow begins to increase, as lower elevation snowmelt commences and begins to flush the abandoned mines, mine dumps, and tailing piles of trace-element enriched water. The volume of water that actually flows into the river from these sources during the pre-snowmelr period is relatively small, but because flow in the river is also small, the impact on trace-element concentrations can be very substantial. During snowmelt runoff, streamflow is at its annual maximum and increases substantially in the downstream direction because of reservoir releases and tributary inflow. The post-snowmelt runoff period includes the receding limb of the snowmelt runoffhydrograph in July and a period dominated by releases of stored water from Twin Lakes Reservoir (plate 1) in August and September. Additionally, during the post-snowmelt run- . off period, streamflow can increase substantia\Iy over short periods of time following intense rain storms. This def- inition of seasonal streamflow regimes serves as a general guideline for the interpretation of seasonal trace-element concentrations and is subject to some degree of variability due to year-to-year variability in weather conditions and reservoir operations. The primary cause of elevated trace-element concentrations in the upper Arkansas River is acidic, metal- laden drainage from abandoned mines and mine tailings. The most substantial sources of mine drainage are located in the Leadville area and include the Leadville Mine Drainage Tunnel (Li\<IDT) and the Yak Tunnel (Moran and Wentz, 1974; Wentz, 1974; Bunting, 1989; Clements, 1991). Nonpoint sources ofrrace-element enriched drainage in the upper basin are also substantial because of the wide distribution of mine tailings and mine waste piles throughout the upper basin. The LMDT discharges to the East Fork of the Arkansas River . upstream from Leadville (plate I) and the Yak Tunnel discharges to California Gulch near Malta (plate I). In the . spring of 1992, water-treatment plants were installed at both sites. This action coincided with-the last year 'Of data collection for this study. Both plants utilize a Chemical-preciPitation'PfUr~~rrlRMl Subject to Revision pO ~OT QUOTE OR RElEAS;:: ;- ,;;rldmg Approval by Director, . .' U.S. Geological Survey 18