NETT LAKE- An aging, ailing concrete dam controlling the waters of Nett Lake is about to be demolished and replaced with a new design that will have ecological benefit for the lake and the Nett River.
The need to replace the dam, which was built in 1987 about three miles downstream from the lake, has been apparent for a number of years, Bois Forte Natural Resources Programs Director Chris Holm said.
“That old concrete and hardware isn’t as waterproof as it used to be, and you get a tremendous force from water and from ice buildup,” Holm said. “You have galvanized hardware that was holding on dam gates just breaking away and we’d find 5,000-pound gates just lying in the river. So then we have to call in construction crews and put it back together. After so many years of doing that it’s really become just cosmetic, putting lipstick on a pig to try to fix it.”
Holm was surprised that the dam withstood this past spring’s historic flooding.
“We really thought the whole thing was going to go,” Holm said. “It was completely inundated, the headwalls that anchor it to the shoreline were completely inundated, the water was up to the walkways, concrete was developing big cracks – I said it’s going to go. And it didn’t.”
The possibility of the dam collapsing is of great concern when considering the importance of Nett Lake’s annual wild rice crop.
“Nett Lake has amazing seed beds,” Holm said. “If the concrete dam were to let go and we had a major flush of water out of the basin, we were worried about losing all the water very quickly, not being able to recover it, and either freezing out or desiccating the seed beds. If that happened and we lost the rice forever, that would be unthinkable.”
Holm said the Bois Forte Tribal Council has been taking a hard look at replacing the dam for the past two years.
“We’ve had engineering companies in here doing full feasibility studies and getting cost estimates on whether to replace or repair, and it just makes sense that it’s time for that to go,” Holm said.
The council also allocated close to $1 million for the project, including federal American Rescue Plan funds, but with rising construction costs, that wasn’t enough.
“A year ago, the estimated construction cost was $780,000. Now it’s $1.2 million,” Holm said.
But last week the Iron Range Resources and Rehabilitation Board kicked in an additional $350,000 for the project, giving the Band what it needs to cover the $1.2 million dam and about a $100,000 for contingencies and engineering fees.
Rock arch dam
When the decision was made to replace the dam, that opened up an opportunity to consider alternatives.
“The issue became are we going to build another one of these old-school monstrosities that create their own set of ecological problems or are we going to get a little progressive and do what other areas of the state and nation are doing,” Holm said.
The concrete dam was designed to block flow, not facilitate it, and that created issues for the Nett Lake basin, Holm said.
“You can never draw down water in the basin far enough to get a good cleaning of the basin, removing the rice straw and sediment that builds up from year to year,” he said. “You create a dead water space, a stagnant space that encourages filling. Nett Lake on a good year will produce thousands of acres of rice which turns into rice straw. You’ve got to be able to get rid of some of that stuff and we haven’t been able to do it for 35 years.”
The concrete dam also prevented fish from moving in and out of the shallow basin.
“There’s been a fishery crash out there,” Holm said. “Nett Lake is really shallow, like three-feet average depth, and come wintertime there’s no refuge for fish, so they freeze out.”
To solve those issues, Bois Forte chose to go with something that doesn’t even closely resemble traditional concrete dams.
They’re installing a rock arch dam, also known as a rock riffle. It’s a series of boulders that stretch across the water in descending rows, with pools of water in between.
“It’s engineered pilings of rocks on a three-to-five-percent gradient, and the elevation is designed to maintain a middle-of-the-road optimal elevation for rice,” Holm said. “It creates pools and riffles that fish and animals can use to get in and out of the basin. You can actually have water level control and encourage ecological connectivity without a big old concrete wall blocking the flow. It’s a very low maintenance way of reconnecting fragmented aquatic systems.”
Holm, the tribal council, and others visited such a structure in the Park Rapids area, and Holm said that helped the council to decide to go with the rock arch construction.
An additional benefit of the new dam will be reduced maintenance, Holm said.
“I’m going to say that dam is something like 20 miles away by car,” he said. “It’s out in the sticks and there’s no electricity to it. There’s just a secondary access road to get in. To do water levels for rice, somebody has to go out there and manually crank mechanical gates up and down to adjust water levels. This thing gets rid of all the mechanical functions and all the potential damages and repairs.”
Demolition of the concrete dam is scheduled to begin next week, Holm said, and the target date for completion of the rock arch replacement is Dec. 31. Holm said that getting the work done now should help next year’s wild rice development.
“We’ll be back in the business of being able to move water out of the basin and regulate it for ricing again,” Holm said.