At its peak production, Iron Mountain ranked as the tenth largest copper production site in the world, sixth in the U.S. and first in California. During its operation, from 1879 - 1963, ten different mines throughout the site's 4,400 acres were the source of not just copper, but also silver, iron, gold, zinc and pyrite (iron sulfide).
A century of active mining at Iron Mountain took an environmental toll. The first documented consequence of mining was fish kills in the Sacramento River in 1899, followed by severe air pollution from the open-air heap roasting and smelters that stripped the land of vegetation for 14.4 km south, 5.6 km north, 3.6 km west, and at least 8 km east of the smelters near the town of Keswick along lower Spring Creek. As mining operations increased, so did pollution. Acid mine water seeping into the Sacramento River, sizable fish kills, and sediment deposits in the Spring Creek Arm of Keswick Reservoir have all plagued the area. To compound the issue, the city of Redding receives its drinking water from the Sacramento River, downstream from the Iron Mountain site. An uncontrolled release of Iron Mountain acid mine drainage could potentially threaten the quality of the drinking-water supply.