Logan Canyon’s dams are prone to sludge buildup. An overdue disaster could make it worse.

The critical dams in Logan Canyon are choking on mud, and the threat of a long overdue disaster worsening the issue has experts on edge.

This summer, Logan used excavators to remove the accumulated sludge from one of the dams, with the goal of protecting its hydropower equipment from damage caused by sediment buildup. The only function of the dams within Logan Canyon — known simply as Second Dam and Third Dam — is to generate hydropower for the city.

“People call it Second Dam,” said Mark Montgomery, the city’s director of light and power. “We call it a diversion and a sediment trap.”

The sediment removal at Second Dam cost $1.2 million and took five months to complete. Montgomery said the dam, once deep enough for Utah State University students to jump in and earn the title of a “dam Aggie,” had become so shallow that people could wade into its center. In some areas, islands of sediment had formed.

Third Dam, located a few miles upstream on the Logan River, is in even worse shape, Montgomery said, with five to seven times the amount of muck in Second Dam. The city lacks the budget to remove sediment there.

“It’s like a river running through a field of cattails,” Montgomery said, “that should be an open reservoir.”

First Dam, which is located at the mouth of Logan Canyon and is owned by USU, faces the same issues as the other dams.

Although this doesn’t impact the city’s water supply, sediment buildup creates additional issues by disrupting the natural flow of the river and harming the ecosystem. Experts worry a blaze in the canyon could only intensify the threat.

Patrick Belmont, a hydrologist and geomorphologist at Utah State, said dams clogged with sediment can turn into “methane pools,” due to the high levels of carbon trapped inside. While this may not have immediate local consequences, it does contribute to global greenhouse gas emissions, Belmont said.

“When you see bubbles coming up from the bottom of the reservoir, often, that’s methane coming up,” Belmont said, “and that’s a really powerful greenhouse gas. It’s not a good thing to be releasing a lot of.”

The city transported the sediment from Second Dam to an unused gravel pit in Millville, just south of Logan.

An ongoing threat of wildfires

Sediment, which is brought downhill by the river and avalanches, has accumulated in the dams over time. However, Belmont said in the event of a major wildfire, a dam could be filled with sediment rapidly.

The Logan River Watershed, like many other areas in Utah’s national forests, hasn’t experienced a significant wildfire in a long time — a situation Belmont described as unnatural. He is concerned that the overgrown forest poses a severe risk, and under the right conditions, a fire could be “unstoppable.”

Unlike neighboring states, Utah hasn’t experienced a half-million-acre wildfire in decades, according to Larissa Yocom, a fire ecologist at Utah State. This has led to the accumulation of “fuel” in Logan Canyon, she said. While fires are necessary and beneficial for ecosystems, their impacts on humans can be severe, she said.

“I’m not necessarily worried for the animals and the plants themselves,” Yocom said. “What I would be worried for is the people and the effects on the downstream inhabitants, which are the people of Cache Valley.”

What does the canyon need?

If a fire does break out, Logan’s First Dam — a popular spot for fishing, paddleboarding and swimming — is the most at risk of filling with sediment. Due to its small size and location, it is the most vulnerable reservoir in the state, according to a study co-authored by Belmont that examined how wildfires could affect sedimentation at 137 large reservoirs across Utah.

In such an event, Logan Canyon’s other two dams would also likely fill with sediment.

Belmont said there is a need for preventative actions, such as controlled burns, thinning trees and beaver restoration projects, to prevent a catastrophic fire from taking over the entire watershed.

“We have so many environmental problems that we’re trying to solve now, and a lot of it’s because we’ve been working against nature in a lot of ways and doing things that are just very inconsistent with how nature would do things,” Belmont said. “We have a lot that we can learn from nature that’s going to make it a lot easier to maintain things moving forward.”