Map of Mike Horse Dam

Mike Horse Dam Settlement

In April 2008, state and federal officials announced a $37 million settlement of litigation with Atlantic Richfield Co. and ASARCO LLC to remove the aging Mike Horse Dam and the contaminated tailings behind it, and to clean up and restore the Upper Blackfoot River and Mining Complex.

Under the terms of the settlement (PDF), ASARCO and Arco will each pay the state $8 million. The state will also receive a $19.77 million allowed claim in the ASARCO bankruptcy, and the U.S. Forest Service will receive $1 million to oversee the state's implementation of the project and a $230,000 allowed claim for past costs.

The dam sits in a floodplain at the headwaters of the Blackfoot River, and the tailings behind it will be moved to a repository on higher ground on ASARCO property. The project will also include cleanup of tailings along the Upper Blackfoot River, Beartrap Creek and Mike Horse Creek and the state will restore those streams to eventually bring back westslope cutthroat and bull trout.

Mine History

Mining of the Mike Horse Mine began in 1898 and was expanded into a larger operation in 1919.  In 1938, the Mike Horse Mining and Milling Company leased the mine and built a 150 tons-per-day flotation mill. In 1941, the Mike Horse Dam was built across Beartrap Creek to contain the tailings generated from the flotation mill.

In 1975, heavy rains caused a partial failure of the dam and high creek waters eroded contaminated tailings into Beartrap Creek and the Upper Blackfoot.

In 2005, a U.S. Forest Service Dam Safety Report determined that the dam was unsafe, and recommended that it be removed from service. In July 2007, USFS released an action memorandum calling for the removal and disposal of the dam, mine tailings and wastes.