By Tom Henderson, Andrew Ray, Pete Penoyer, Ann Rodman, Mary Levandowski, Alysa Yoder, Shane Matolyak, Mary Beth Marks, and Autumn Coleman Mary Beth Marks, and Autumn Coleman
During operation of the McLaren Mill, tailings disposal was problematic as overflow from the tailings impoundment flowed downstream into Yellowstone National Park. Inspections by park rangers documented a regular pattern of leaks and breaks in the earthen dike surrounding the tailings impoundment (Glidden 2001). As the daily operation of the mill tended to give a milky appearance to Soda Butte Creek, the frequent breaks and washouts of the impoundment had more serious consequences (Johnson 1949). A dam break occurred during summer 1950 that was caused by a series of heavy rainstorms and flash floods in the upper Soda Butte Creek basin. A ranger inspecting the area on 28 June of that year documented repairs made to the impoundment but noted that similar breaks in the dam occurred each spring and more breaks could be expected with continued operation of the mill (Johnson 1950). Years later, Meyer (1993) mapped bright orange-red sediments containing elevated levels of iron, copper, and lead from Cooke City more than 15 miles (24 km) downstream and concluded that the likely source was the 1950 release.
The Yellowstone fires of 1988 and concerns with failure of the tailings dam resulted in a heightened awareness of the potential threat of McLaren site to the park (Kauf and Williams 2004). Given the history of dam failure and added threats associated with altered runoff patterns following the 1988 fires, McLaren site was designated an Emergency Response Action Site by the US Environmental Protection Agency. Response measures included work to armor the margins of the impoundment, improve the stability of the dam, and reduce the amount of water flowing onto the impoundment (MTDEQ 2002a). At that same time, the Montana DEQ was completing the Water Quality Restoration Plan for the Cooke City TMDL Planning Area to improve water quality to a level that would restore beneficial uses (MTDEQ 2002b).
During this period, the US Forest Service established 13 long-term surface water monitoring stations as a component of the New World Response and Restoration Project (USFS 1999a; USFS 1999b). Monitoring generally occurred at or near winter base flow conditions (April), during high flow conditions (June), and during fall low flow conditions (September and October). The monitoring network included multiple sites on Soda Butte Creek and sites in a major tributary (Miller Creek). Importantly, these monitoring data can now be used to characterize variations in water quality over the decade preceding the reclamation of McLaren site (see fig. 2).