Colorado's Cinnamon Gulch releases acid rock drainage (ARD) from anthropogenic and natural sources. In 2001, the total discharge from Cinnamon Gulch was measured at 1.02 cfs (29 L/s) at base flow and 4.3 cfs (122 L/s) at high flow (spring runoff). At base flow, natural sources account for 98% of the discharge from the watershed, and about 96% of the chemical loading. At high flow, natural sources contribute 96% of discharge and 92 to 95% of chemical loading. The pH is acidic throughout the Cinnamon Gulch watershed, ranging from 2.9 to 5.4. At baseflow, nearly all of the trace metals analyzed in the 18 samples exceeded state hardness-dependent water quality standards for aquatic life. Maximum dissolved concentrations of selected constituents included 16 mg/ L aluminum, 15 mg/L manganese, 40 mg/L iron, 2 mg/L copper, 560 ??g/L lead, 8.4 mg/L zinc, and 300 mg/L sulfate. Average dissolved concentrations of selected metals at baseflow were 5.5 mg/L aluminum, 5.5 mg/L manganese, 14 ??g/L cadmium, 260 ??g/L copper, 82 ??g/L lead, and 2.8 mg/L zinc.