The Lake Creek watershed in Colorado is an excellent, unique, natural outdoor laboratory for studying naturally occurring acid rock drainage (ARD) and other hydrogeochemical processes involving metal transport and deposition. ARD originates in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream, headwater tributaries of Lake Creek that drain low-grade Cu-Mo hydrothermal systems associated with the Grizzly Peak Caldera, host of an Oligocene, calcalkaline porphyry. The watershed receives no visible contribution of ARD from mining or other anthropogenic disturbances. The pH in Peekaboo Gulch, Sayres Gulch, and Sayres Bowl Stream starts in the 2.5-3.0 range, but is naturally neutralized by freshwater tributaries to Lake Creek and ultimately attains a pH of 7.7 at the confluence with the Arkansas River, 37 kilometers downstream.

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