Laura Lundquist

(Missoula Current) The city of Missoula is one step closer to starting a pilot project that would decommission one of the ancient dams in the Rattlesnake Wilderness.

The Public Works Committee of the Missoula City Council unanimously approved an agreement this week to allow Montana Trout Unlimited to begin using grant money on a project to decommission the earthen dam on McKinley Lake. The council as a whole will consider the agreement on Monday.

Montana Trout Unlimited project manager Rob Roberts said the city already signed an agreement in February with Montana Trout Unlimited and Montana Fish, Wildlife & Parks to allow for the removal or rehabilitation of 10 dams in Rattlesnake Wilderness and National Recreation Area. Then the city applied for and received a $125,000 Renewable Resource Project Grant from the Department of Natural Resources and Conservation to decommission the first dam.

“We needed an additional agreement to authorize TU to manage the grant. Anytime we have an expense, we’ll work with the city to seek cost recovery from the grant,” Roberts said. “This is kind of like the seed money, the start of the foundation for fundraising for this particular project. We can go to other funding sources and say, ‘Hey, we already have $125 grand, we need X amount more, we’re going to match it with this money.’ We’ve already shown that people are interested. It just snowballs from there.”

McKinley Lake is one of eight lakes in the Rattlesnake Wilderness that feed Rattlesnake Creek and once served as Missoula’s municipal water source. However, they store less than 10% of the water that Missoula demands now. A Giardia outbreak in 1983 prompted the Missoula Water Company to transition to well water, making the dams on the eight lakes mostly unnecessary.

The impoundments - two lakes have two dams each - are ancient constructions of dirt packed over wooden cribs filled with rocks. They’ve been minimally maintained as required by the Lolo National Forest but most haven’t aged well and are in need of repair. The city is also required to maintain the roads and trails that access the dams.

In April 2021, the city started considering its options as to what to do with the dams.
A 2018 feasibility study compared the cost of rehabilitating each dam as opposed to decommissioning them. Depending on the dam, rehabilitation could include replacing outflow pipes and walkways, installing liners to prevent dam seepage and regarding the slope of the dam to improve stability.

In every case, the cost to rehabilitate and maintain a dam over time was at least three times as much as decommissioning. It would cost about $7 million to rehabilitate all 10 dams or about $1.2 million to decommission them.

It costs less to decommission all 10 dams, however, two largest lakes – Sanders and Big – hold more water than the other six combined. They would be the best candidates for dam rehabilitation if the city wanted to retain water to augment the flow in Rattlesnake Creek during drought years.

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McKinley Lake was chosen as the pilot project because the 2018 feasibility study found its dam has deteriorated the most. The emergency spillway was significantly eroded, threatening the dam’s stability. So several years ago, the outlet was left open permanently to prevent more erosion.

The plan is to breach the dam, creating a 7-foot-wide hole, and construct a new stream channel that could handle a 100-year flood. But the lake will essentially become a glacial cirque lake that spills only sporadically, Roberts said.

“In the future, it will rise and fall only about a foot. All the shallow lake edge and flat areas (that used to get covered by water) should have a chance to colonize with native vegetation,” Roberts said. “We should see some wetlands develop and some forested riparian edge zones. That will be part of the restoration."

Work on the dam is slated to take place next summer. Roberts said the permitting and construction details are coming through slowly because working in a wilderness has several limitations. Roberts has to get waivers if he wants to use mechanized equipment.

It’s also going to cost more than $125,000, Roberts said, so completion of the project will depend on how quickly the money can be raised. But Roberts has been through it before with the removal of the Rattlesnake Dam, which came through on time and within budget.

“We’re not exactly sure when we’re going to be able to tee this up and get it on the ground. If this was a front-country project, we’d do it in two weeks and be done. But there’s just so many logistics to take care of,” Roberts said.

Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.

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