The unstable dam has been sitting there on the headwaters of the Blackfoot River for decades, a toxic time bomb of sorts, just waiting for the inevitable mountain downpour or sudden thaw to wash it away -- dumping tons of mining waste into one of Montana's most revered trout streams.

The Mike Horse Dam about 16 miles east of Lincoln, built in 1941, already has failed once. The 1975 washout killed fish and other aquatic and riparian life for 10 miles downstream.

A repeat of that disaster was bound to happen sooner or later, but for years now the Forest Service has hemmed and hawed, leaving the dam, and the river, in a dangerous sort of limbo.

That limbo came to end Tuesday when Regional Forester Tom Tidwell signed a decision requiring that the dam and the poisonous waste behind it be carefully hauled away to an Asarco-owned site a mile and a half away.

And, the Forest Service said, Asarco is responsible for paying for the job, which is estimated to cost $27 million. That's because the company bought the Mike Horse Mine and Milling Co., which had produced lead and zinc along the Blackfoot until the early 1950s. Preliminary preparations for the removal could begin as early as this fall, the agency said.

The decision is a victory for those who argued against alternative measures that fell short of total removal. But, most of all, it is a victory for the Blackfoot, not to mention the valley and the lifestyle it runs through.