The next two weeks will be a hectic period during which engineers, construction workers and scientists will undertake the complicated task of unleashing millions of gallons of water while trying to minimize environmental damage and avoid flooding.
While the $100 million project has a long way to go, the rivers' release will mark perhaps the biggest milestone in the effort to clean up the toxic mine and smelter wastes that made the upper Clark Fork River basin the nation's largest Superfund site.
The powerhouse and abutment wall, which make up half of the dam, have been demolished and replaced by a temporary earthen coffer dam supported by sheet metal. The coffer dam allowed workers to stay dry during demolition.
The rivers are currently flowing through the dam's radial gate, which along with the spillway and divider block make up the other half of the dam, where demolition is slated to start this fall.
The spillway area will be the final river channel, but for now the rivers will be routed through a temporary channel where the hydroplant's powerhouse once stood.
Starting March 24, workers will remove the coffer dam's sheet metal, dig a notch in the earthen dam, raise the reservoir level and then allow the rivers to wash away the remnants of the dam.
The breaching is expected to draw a large crowd on the bluff overlooking Milltown Reservoir and dignitaries near the dam site below.
Rather than a spectacular wave of water, there likely will be an incremental flow over several hours that restores the Clark Fork, Montana's largest river, and the Blackfoot, made famous in Norman Maclean's book, “A River Runs Through It.”
“It won't be a huge wave, but it's a designed dam failure, so it's always possible” it will break through quickly, said Russ Forba, the U.S. Environmental Protection Agency's Milltown Superfund manager.
After the breaching, the rivers will release a surge of water that scours an estimated 300,000 tons of sediment from the mouth of the Blackfoot River.
The sediment will contain little to none of the mining contaminants that have been deposited behind the dam by the Clark Fork River, project officials said. Workers are continuing to remove those metal-laden sediments.
No flooding is expected when the rivers are released, but officials cautioned boaters, anglers and others to stay away from the water downstream because the muddy surge will carry logs and other debris.
Warning signs are to be posted and police have been notified to keep people away from the water.
The water level is expected to rise about 4 feet just downstream from the dam and gradually diminish as the river settles to its natural level.
The water level is projected to rise about 3 feet at Pine Grove an hour after release, 2 feet at Deer Creek an hour and a half after release, 1 1/2 feet in downtown Missoula two hours after release and 6 inches at the Bitterroot River 5 hours after release.
The reservoir water level already has been lowered 12 feet and dam breaching will lower it another 12 to 14 feet, which will lower the level of the groundwater and drinking water wells in the surrounding area. The EPA is replacing wells, well pumps and filtration systems of residents who are affected.
Meanwhile, workers are preparing for Tuesday morning's opening of the bypass channel.
The channel, which was constructed to dry out the contaminated sediment area behind the dam, is blocked by earthen dikes and sheet metal walls.
The channel has two openings - the inlet along the Clark Fork River about half a mile upstream from the dam and the outlet along the Blackfoot River where it joins the Clark Fork just above the dam.
The channel's metal walls have been removed and workers will notch the outlet dike Tuesday and notch the inlet dike Thursday or Friday.
Most of the rivers' flow will continue to go downstream, but they also will gradually fill the bypass channel - the Blackfoot flowing upstream and the Clark Flow going downstream.
The bypass channel has been lined with rocks and turf mats to reduce erosion and boulder clusters to give fish resting places.
The old powerhouse site where the rivers will flow temporarily also has been lined with rocks and bolstered to reduce erosion.
The dam breaching will allow fish to swim upstream, restoring the natural ecosystem for the first time since Milltown Dam was completed in 1908.
A large number of fish and other aquatic organisms are expected to die in the river in the short term because of the sediment load, accumulated stress from heavy metals and not being able to migrate, and bacterial and fungal infections brought about by high water temperatures.
Officials are monitoring water quality, sediment loads, copper, arsenic, fish and insects upstream and downstream from the dam.
Reporter John Cramer can be reached at 523-5259 or at johncramer@missoulian.com.
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