Cleanup of the Upper Blackfoot Mining Complex

Twilight of the Dam

LINCOLN - As the pool of water recedes behind the Mike Horse dam, so does the threat of another blowout.

At this time of year, the pool typically is at one of its lowest levels. And on a recent crisp fall day, amid howling winds and a hint of snow, a backhoe operator was carefully digging a V-shaped ditch 4 feet deep, just above the western boundary of the pool, which will completely drain it.

This is one of the first long-awaited steps toward restoring the headwaters of the famed Blackfoot River to its original state.

The work is oddly reminiscent of another man-made effort decades ago, when a crew carved a similar trench into the hillside to do what they're doing again today - rerouting Beartrap Creek around what most people call the Mike Horse dam.

Technically, the Mike Horse dam is an impoundment, not a bona fide dam constructed to hold back water. It originated in 1941, when ore was pulled from deep within the bowels of the nearby Mike Horse Mine. After the metal was extracted from the ore through a milling process, the wet tailings ran down a sluice next to Beartrap Creek, where they settled and excess water evaporated or seeped through the mound of tailings.

It's been decades since tailings were placed here, but the seeping is still taking place, sometimes at up to 500 gallons per minute. The giant slope that's the face of the impoundment is moist and spongy when trod upon; the metals that make their way through the ground with the water, now and in the past, have painted the soils bright orange, coal black and salty white.

At the base of the dam, where Mike Horse and Beartrap creeks meet, fluorescent green plants thrive in the metal-laden water.

"It only used to discharge during the springtime," notes Charlie McKenna, an engineer with the Helena National Forest. "But now, even when the water level is low behind the dam, water leaks through it.

"When the water level is high, it leaks like a sieve."

The Blowout

The impoundment and nearby mining sites are part of the Upper Blackfoot Mining Complex, 16 miles east of Lincoln and about 50 miles northwest of Helena. The land on which the Mike Horse dam sits is part of the Helena National Forest's Lincoln Ranger District.

Various entities have worked claims in the complex, but Asarco was the primary operator and created the ditch that funneled Beartrap Creek around the impoundment, according to a history of the site by David Stiller, who wrote a book about the Mike Horse and its impact upon Montana's environment.

On a cool, wet June day 32 years ago, the upper ditch gave way after days of powerful rainstorms cut loose landslides. The creek water mixed with trees, rocks, rain and melting snow, with the debris blocking the only spillway.

Cresting the 50-foot-tall impoundment, the water took bite after bite through a 100-foot-wide section on the east side, eventually carving a deep "V" through the dam and sending 100,000 tons of tailings into the headwaters of the famed Blackfoot River, immortalized in Norman Maclean's novel "A River Runs Through It."

The blowout caused a toxic stew of heavy metals to rush downstream, killing thousands of fish and all other aquatic life for 10 miles of the Blackfoot River. It became the poster child for environmental groups calling for the cleanup of the Upper Blackfoot Mining Complex, as well as other abandoned properties throughout Montana.

The Anaconda Co., which leased the mine and mill operation from Asarco, repaired the impoundment by November 1975. The new structure was 50 feet high, 500 feet long and 25 feet wide at the top. Upgrades made in 1980 raised the impoundment by two feet.

By this time, Beartrap Creek was flowing directly into the pool behind the dam. It's still considered a "dead" stream below the dam, devoid of fish.

Concerns were raised about the old impoundment in 1964, 1972 and 1974, before the blowout and dam repair.

Questions began circulating a few years ago about the new structure's stability.

A 2005 study by the U.S. Forest Service, which owns the land on which the dam sits, called the structure "compromised," and noted it had several large voids in it. Last spring, water was seeping through the dam at around 400 to 500 gallons per minute.

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