Milltown Dam could have just complicated Missoula’s current flooding

Posted at 12:12 PM, May 17, 2018

MILLTOWN – The highest water in 100 years is prompting some people to speculate that the current flooding situation on the Clark Fork River could have been much different if the old Milltown Dam was still in place.

But county officials say that opinion overlooks the fact that the dam was only designed to generate power, and never had any "measurable" flood control.

When the Milltown Dam was removed 10-years ago this spring, it marked an end to several years of debate about tearing out the dam and cleaning up the tons of toxic sediment that had settled in the river from more than a century of upstream mining operations.

One might think that the dam, and the acreage behind it, were useful in giving some flood control during high water like we’re seeing right now on the Clark Fork River. However, the Milltown Dam wasn’t what’s known as an "impoundment dam", primarily aimed at water storage. Instead, it was a "run of the river" dam, but to generate power.

"The amount of hydropower that it was capable of generating was solely based on the amount of flow that was coming through the river. And in fact these high flows were concerning for the structure of the dam itself," said Missoula County Environmental Health Specialist Travis Ross.

That was certainly the case during the only flood that’s seen higher waters than we saw just this last week. In the 17-foot flood of 1908, the dam was nearly destroyed.

"We know that in 1908 that significant damage was done during that flood to the Milltown Dam," Ross said. "I believe a third of the spillway was completely wiped out during that flood. So the dam was never designed to provide any measurable flood protection."

Old timers talk about there being "50-to-75 feet" of water depth behind the dam. But Ross says actually that capacity had dwindled to nearly nothing by 2008, as evidenced by the tons of contaminated soil that had to excavated and moved upstream.

Ross told MTN News that one of the reasons to remove the dam was the growing threat that the dam could "blow out" during a flood event like we’re seeing now, contaminating miles of river through Missoula and downstream.

"You know what capacity was in that reservoir was tremendously taken up by the contaminated sediments. And that was a continued threat to be released both in ice scour back then and high flow events," Ross said.

Ross also explained that the way the river has set up a "meander" with channel "braiding" through the former Milltown Pond site since the dam was removed is better for flood control than the old dam ever was.