Eighteen metal-mine drainage sources have been located in that part of Gilpin, Clear Creek, and Park Counties, Colo., lying within the Missouri River basin. At least 13 of these sources are known to contain high acidity and (or) trace-element concentrations or to contribute water to adversely affected streams. From January 1976 to March 1977, drainage from the Argo Tunnel in Idaho Springs--one of the major metal-mine drainage sources in the study area--exhibited variations in discharge from 0.35 to 0.55 cubic feet per second (0.010 to 0.016 cubic meters per second), a relatively constant temperature of 16 degrees Celsius, and variations in specific conductance from 2,680 to 3,410 micromhos per centimeter at 25 degrees Celsius (though a value of about 3,100 micromhos persisted throughout most of the period of record). High, but relatively constant, total concentrations (in micrograms per liter) of arsenic (100 to 180), cadmium (140 to 170), copper (5,000 to 6,000), iron (160,000 to 200,000), lead (less than 100 to 200), manganese (80,000 to 110,000), and zinc (40,000 to 49,000) were measured in the Argo Tunnel drainage from March 1976 to March 1977. Except for lead, the trace elements were mostly dissolved (82 percent or greater) and appear to represent baseline concentrations. Long-term degradation of water flowing from the Argo Tunnel is shown by increases of at least 2.5 to 8.0 times for dissolved solids, dissolved iron, calcium, magnesium, and sulfate since 1906. The acidity has changed from neutral in 1906 to a median pH value of 2.9 in 1976-77. Comparison of current Argo Tunnel data with those collected previously by other investigators indicates that spring chemical flushes containing higher than baseline trace-element concentrations occurred in 1973 and 1974, but not in 1975 or 1976, and probably not in 1972. The spring chemical flushes appear to be associated with increased infiltration from snowmelt in the catchment of the Argo Tunnel. Because of the wide ranges in mine-drainage quality and quantity expected for discharges from abandoned mines in the study area, each situation must be examined individually, and the management alternative chosen for mine-drainage abatement must be tailored to solve the particular mining and hydrologic problems at a given site.