CS30B - Concurrent Session 30B: A River Runs Through It - The 1975 Failure of Mike Horse Tailings Dam

Abstract Description: The Blackfoot River in western Montana was made famous by the 1976 novel and 1992 movie “A River Runs Through It.” However, the once thriving fishing river, as affectionately storied in the tale, has been impacted by decades of upland mining and an environmentally catastrophic tailings dam failure in 1975. The Mike Horse Mine in Lincoln, Montana was established in 1898 to primarily mine zinc, lead, and silver. The Mike Horse Dam was constructed around 1940 to contain waste from mining operations. It was constructed using upstream construction methods and metals-laced tailings to a height of above 60 feet. Heavy rain and flooding in 1975 resulted in a slope failure that blocked a diversion intended to prevent headwaters from inundating the impounded tailings area. As the headwaters flowed into the impoundment, the metals-laced tailings were discharged downstream and eventually into the Blackfoot River. An estimated 200,000 cubic yards of harmful tailings were released and mobilized 10 miles downstream. The mine’s corporate owner, American Smelting and Refining Company (ASARCO), rebuilt the dam shortly thereafter to a height of approximately 65 feet. The stability of the reconstructed dam was observed to deteriorate in the following years, and it lacked proper hydraulic capacity. Around 2007, the United States Fish and Wildlife Service issued an Action Memorandum calling for removal of the dam and impounded mine waste. A settlement with ASARCO prompted a $40 Million initiative to remove the dam and tailings, which was performed from 2010 to 2021. Restoration efforts continue today, and the objective is to return the Blackfoot River to the storied fishing river it once was. In this paper, the authors will seek information on the Mike Horse Dam failure from sources such as the Mine Safety and Health Administration, the Montana Department of Environmental Quality, and other affected stakeholders. Based on available information, the root cause of the failure will be discussed, and the impacts, which continue to be addressed today, will be explored.