Abandoned mine drainages in Colorado
According to a survey completed by the Colorado Geological Survey between 1991 and 1999, the number of abandoned mines in Colorado is 18,382. The Arkansas Headwaters, Las Animas River, Rio Grande Headwaters, Alamosa, and Uncompahgre were the priority watersheds studied in this survey.
Source: Wikipedia — Abandoned mine drainages in Colorado (CC BY-SA 4.0)