|Remediation Review: How to Remove a Dam|
After 20 years of study, debate and planning, on-the-ground work began at the Milltown Reservoir Superfund site in the summer of 2006. In just three years, the dam and most of the sediments proposed for removal are now gone. Here's how that happened:
The first step for Milltown cleanup work was lowering the water level by roughly 12 feet in the reservoir so that construction crews could begin work on the site. Remediation work began with the construction of haul roads, flood berms, a rail spur for shipping and a bypass channel. This rip-rapped temporary passage -- built to withstand a 100-year flood -- diverted the Clark Fork around the contaminated sediments, isolating and preventing them from being washed downstream during the cleanup.
Demolition of the powerhouse began in mid-January of 2008. Envirocon first pulled the generators from the powerhouse and Northwest Energy hauled them away for re-use, donating one for future display in Milltown.
In order to keep the work area dry, a temporary earthen cofferdam was built just upstream of the powerhouse and a work pad was constructed downstream. The cofferdam was reinforced with long sheets of steel (sheet piling) driven down into the bed of the river. Once powerhouse demolition was completed, the footprint was lined with rock and re-graded to create a temporary channel for the river.
Then excavators started chipping away on the north abutment and the powerhouse itself. The building's roof and outer walls came tumbling down in a little over a week. Concrete rubble, bricks, steel and timbers from the dam and abutment were salvaged to the extent possible and the remainder hauled to an on-site landfill located in the park area to the north.
In March 2008, the Clark Fork River upstream of the dam was redirected into the newly completed bypass channel next to I-90, and a pilot channel was excavated just upstream of the powerhouse area, leading from the Blackfoot River toward the temporary channel in the powerhouse footprint. The cofferdam was removed to the extent possible, and the pilot channel breached in late March by raising the level of the reservoir.
That momentous event came on March 28, 2008. After speeches by assembled dignitaries to mark the occasion, and with several hundred spectators on both sides of the river, Governor Schweitzer yelled "Let ‘er run!" and an excavator scooped a couple buckets of dirt to open a small ditch on the upstream side of the earthen cofferdam.
As water began to course down the channel, it cut deeper into the sediments, picking up more and more water until, after about nine hours, it widened and deepened enough to capture the entire flow of the Blackfoot and Clark Fork Rivers. For the first time in 100 years, the rivers flowed free at the confluence.
Once the north abutment and powerhouse were removed and the rivers are temporarily routed through their old footprint, Envirocon prepared to demolish the other half of the dam: the spillway, radial gate and divider block.
Envirocon created a dry working area and work pads for excavators and trucks, and built a cofferdam upstream from the spillway radial gate and divider block to isolate them from the river. Envirocon workers also excavated a 20-foot-deep pile of contaminated sediment just upstream of the spillway and built a new, permanent channel for the river in the footprint of the spillway. Demolition began in the late summer of 2008 and continued through autumn.
In late March of 2009, just shy of a year after the breach that set the Clark Fork and Blackfoot Rivers free, the last visible vestige of the Milltown Dam was removed and the earthen cofferdam built to isolate the concrete spillway was breached. The combined flows of the Clark Fork and Blackfoot found a new, and quite possibly the original, channel up against the Milltown Bluff.