Milltown Dam
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| Milltown Dam | |
| Impounds | Clark Fork |
|---|---|
| Locale | Montana |
| Opening date | 1908[1] |
Milltown Dam is an earth-fill gravity-type hydroelectric dam on the Clark Fork river, in the northwest part of the U.S. state of Montana.
The dam is located in the far northwest of Montana near the Idaho border. Downriver of Noxon Rapids Dam the Clark Fork is again impounded by the Cabinet Gorge Dam.
In March of 2008, the dam was carefully breached in the process of Superfund cleanup (see below); this breach is expected to lead to complete removal of the structure.
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[edit] History
Milltown dam was built in 1908 by copper mining tycoon William A. Clark, to supply hydroelectricity to his sawmills in nearby Bonner, Montana. Since the 1870s, the Anaconda and Butte areas had been mined as one of the richest deposits of copper sulfate ever found in North America.[1] Clark's sawmills supplied the giant timbers used to shore up the walls of the mine shafts.
However, in June of that year, a record flood on the Clark Fork washed tons of toxic mining sediment downstream, where it settled at the base of the dam (and where it remains to this day). Over the course of a century of mining, mining and smelting operations have created more such sediments. All told, the dam now retains over 6.6 million cubic yards of sediment contaminated with arsenic, lead, zinc, copper, and other metals.
[edit] Emergency draw-down
Officials of ARCO, who inherited the dam when they purchased the Anaconda Copper Mining Company in 1977, maintained for many years that the sediments were better left in place. This policy was changed after a February 1996 incident in which a 14-foot-thick chunk of ice broke loose and threatened to impact and possibly demolish the aging dam. Fearful of a major uncontrolled release of water (and sediment), officials quickly drew down the level of the reservoir, stranding the ice pack before it could cause any damage.[2]
Unfortunately, the rapid release of water had scoured a four-foot layer of sediment from the bottom, washing it downstream. The next spring, biologists reported that the number of catchable rainbow trout had declined "nearly two-thirds".[3]
[edit] Superfund cleanup
In 1981, it was found that the water behind the dam had forced arsenic into nearby groundwater, contaminating wells in the settlement of Milltown. In 1983, the area was listed as federal Superfund site.
In the Fall of 2007, work began on removing the dam. 700,000 cubic yards of sediment were removed, and a diversion channel was created, to allow for the slow, controlled breaching of the dam. In March 2008, that channel was opened, scouring some 300,000 more cubic yards of sediment downstream. (Contaminant levels are being monitored to prevent reaching levels toxic to the fish downstream.)
Over the next several years, it is planned that three million more cubic yards will be allowed to wash away, while an additional 1.5 million of the more toxic stuff will be removed by truck to landfills. The remaining sediment will be fixed in place once the dam has been removed. The dam's main structure is expected to be removed within two years, but the cleanup efforts will continue for several more. Overall, the cost of removing the dam is estimated at $120 million.[4]
