On Friday, March 28th, hundreds of people gathered to watch the breaching of Milltown Dam just upstream of Missoula, MT.  The dam had impounded the confluence of the Blackfoot and Clark Fork rivers for a century.   During that time the dam trapped mining waste from upstream that contained toxic chemicals like arsenic.  The pressure of the reservoir drove arsenic into nearby wells, and to top it all off the dam was deemed a high risk for failure.   Based on nearly three decades of work by local citizens and organzations, as well as county, state, and federal agencies, the dam finally passed its last water on March, 28th. 

The removal has been a complex challenge.  The reservoir was lowered and sediment removal began two years ago.  Last year a temporary earthen coffer dam was built just upstream of the powerhouse and part of the dam on river right.  Those structures were then removed, leaving the coffer dam and the river-left half of Milltown Dam blocking the river.  A bypass channel was then constructed out of rock to allow the Clark Fork to flow through the reservoir area without cutting into toxic sediments.  Water was released into the bypass channel earlier this year, partially dewatering the old Clark Fork channel.  On Friday, March 28, 2008, crews dug a small channel through the coffer dam.  At 11:45 am, with a woman at the helm, an excavator removed the final scoops of soil blocking the river and the Clark Fork began to flow over the coffer dam through the small channel.

The water quickly began eroding the coffer dam (which was built out of clean sediments).  A waterfall formed at the downstream end of the channel and began working its way upstream.  The erosion carved off huge crowd-pleasing chunks of sediment.  Eventually, the waterfall (known as a head cut) worked its way through the entire coffer dam and found a new equilibrium with the Clark Fork both above and below the dam site.  Once this happened, the reservoir and remnants of Milltown Dam were dewatered. 

The next phase of this huge project will be the removal of the rest of Milltown Dam, and the removal of vast amounts of toxic sediments.  These sediments are being taken by train back upstream to an existing closed-basin repository. Once the reservoir site is restored to the old floodplain levels, native plants will be planted and a new channel constructed.  Then, the bypass channel will be removed and the river will once again flow in a naturally sinuous and dynamic channel. 

The dam removal went off without a hitch.  Congratulations to all who have worked on this great project!

You can learn more from the Clark Fork Coalition or Missoula County.

AW staff and friends were present for the removal and filmed it in time lapse from a bluff above the river.  Check out the video: