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Rehab or decommission? City of Missoula mulls future of 10 Rattlesnake wilderness dams

Little Lake Rattlesnake
McKinley Outflow Rattlesnake
Posted at 12:15 PM, Apr 09, 2021
and last updated 2021-04-09 14:15:37-04

MISSOULA — Now that the city of Missoula has removed the lower dam on Rattlesnake Creek, it’s starting to toy with what to do with the 10 dams higher up.

On Wednesday evening, the city of Missoula hosted the first information meeting on the future of the 10 city-owned dams on lakes in the Rattlesnake Wilderness and National Recreation Area.

It’s been less than a year since the lower Rattlesnake Dam was removed, but the city, Montana Fish, Wildlife & Parks, the Lolo National Forest and Montana Trout Unlimited want to get the ball rolling on possible projects in the upper reaches of the creek.

“There was quite a collaboration that came together with (the lower Rattlesnake dam) project. The one thing that was really essential for that project was really good collaboration on a holistic vision for that site. That is something that we really hope to carry with this project up into the wilderness,” said Morgan Valliant, Missoula city conservation lands manager.

Rattlesnake Creek winds south to Missoula from 10 impounded lakes in the Wilderness that were once Missoula’s water source. But after a Giardia outbreak in 1983, the Missoula Water Company transitioned to well water, leaving the lakes high although not dry.

The impoundments on eight lakes – two lakes have two dams each – are ancient constructions of dirt over wooden cribs filled with rocks. They’ve been maintained somewhat 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.

But working on dams in a wilderness adds complications, such as the need to use manual tools and the difficulties in accessing the high lakes. It all takes a lot of time, effort and money for infrastructure that isn’t being used for its original purpose.

So the city is taking a hard look at whether the dams should be rehabilitated or decommissioned, and it wants public input. The partners have already gathered a lot of data to evaluate possible alternatives.

Altogether, the 10 dams store about 2,600 acre-feet of water, which is a drop in the bucket compared to the 30,000 acre-feet of water that Missoulians use every year. They don’t do much for people, but they could help maintain healthy flows in Rattlesnake Creek in low-water years.

That could be critical for the native trout, including bull trout, that spawn in the upper reaches.

There’s also a question of what to do with all the storage water rights. Since the water rights have filing dates from the 1920s, they’re senior to many other rights, which means they have priority. That makes them valuable so the city doesn’t want to lose them just because they aren’t using them.

But should they be changed to something like instream flow or mitigation rights if the dams are decommissioned?

“WGM is helping with the water rights,” Valliant said. “We’re still evaluating options of what to do with those. We’re not going to remove any lakes until we have a plan in mind for how we’re going to process those water right changes.”

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 a dam and maintain it over time cost 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.

McKinley Outflow Rattlesnake
The impoundments on eight lakes – two lakes have two dams each – are ancient constructions of dirt over wooden cribs filled with rocks.

But it doesn’t have to come down to all or nothing, said Trout Unlimited project manager Rob Roberts.

“The point is the consultants came to the conclusion that most of the dams in the wilderness should probably be decommissioned because it’s really not worthwhile in the long term to maintain those dams, maintain those water rights given the cost,” Roberts said. “The other thing they concluded was that there may be an opportunity to rehabilitate one, two or three dams that have the largest water storage volume.”

The two largest lakes – Sanders and Big – hold more water than the other six combined, said Ladd Knotek, FWP fisheries biologist. They would be the best candidates for rehabilitation.

But could the stored water help keep the creek flowing as hoped?

In August 2019, the partners released water from Big Lake to measure what happened to flows and water temperature downstream. The good news is Rattlesnake Creek appeared to retain the additional 5 cubic feet per second of flow all the way down to the stream gage in Greenough Park for about four weeks.

However, the stream temperature didn’t decrease with the additional water. Roberts said the outflow valve appeared to pull from the warmer water at the top of the lake, instead of the cold water at the bottom.

“Is there some fix that would allow us to release colder water?” Roberts said.

For now, the city is considering a pilot project that would decommission one of the smaller dams so the partners can learn all the aspects they’d need to consider if they did more. It would be a slow process including public participation, Forest Service consultation and fundraising. So any actual work wouldn’t occur until probably the summer of 2023.

For now, public comment is needed. Those interested in shaping any plan to work on the dams should go online and take the survey.

“The more input we have, the better,” Valliant said.