Sweet Briar Dam is experiencing a 60-gallon-per-minute seepage that is raising concerns and compelled the State Water Commission to lower the adjoining Sweet Briar Lake by five feet to reduce the risk of a possible dam failure.

Morton County Park Superintendent Vern Davis apologized to Morton County Commissioners for bringing them "bad news to start the new year" at a special meeting Thursday to wrap up year-end business. He said he has known about the situation for about two weeks and has met with the water commission, North Dakota Department of Transportation and North Dakota Game and Fish Department about possible consequences and how to go about fixing the problem.

A consulting firm out of Denver has been contracted by the water commission to prepare a field investigation report, according to Jason Boyle, who heads the construction section.

"We're not pushing the panic button on this," Boyle said. "The reason we're having the draw-down is because we don't know what's going on. There are a lot of unknowns. We saw with Mount Carmel, in northeastern North Dakota, what can happen if you don't correct internal seepage, and we don't want to have that."

A portion of the Mount Carmel dam gave way in 2003, and water washed out an area beneath the dam's spillway.

Sweet Briar Dam was built in 1960. In 1980, the state began inspecting it every six months after the Army Corps of Engineers started the National Dam Safety Program. There has always been some seepage, according to Boyle, and with that seepage, particles gather in piles on the downstream side of the dam. During the last inspection, bigger piles of particles than before were discovered, and in an area where they hadn't been noticed.

Acting on safety's side, it was decided to draw down the lake. The maximum draw-down is five feet from its normal level using a valve built into the system. The lake is already about a foot and a half below its normal level, so the actual draw-down will be about three and a half feet.

"To draw it down any further will take some work," Boyle said. "We would either have to demolish a wall or do some expensive pumping."

The plan is to start lowering the lake next week, at a rate of about a half an inch per day, Davis said.

"By lowering the lake, it's been determined to increase the safety factor about 14 percent," Davis said. "One possibility would be to do nothing. But what would happen if it were another Mount Carmel dam, the liability would be unbelievable. If the Interstate highway went down, consider all the rerouting of traffic that would have to take place."

Sweet Briar is located 20 miles west of Mandan on Interstate 94, and a section of the Interstate actually acts as the dam. Having to come up with alternate routes while the dam is being repaired would put a lot of stress on the county road system.

Commissioner Matt Erhardt said that Morton County put $15,000 into a Sweet Briar fix in 2003, which had a total cost of more than $100,000, and had been assured there wouldn't be other problems. But Davis said that the current problem isn't associated to the earlier situation and was unforeseen. The work, according to Boyle, was to fix a concrete inlet damaged by ice loading and was completely unrelated to the seepage.

While study of the problem could begin immediately, with the weather the area is experiencing, it would be difficult and expensive. Plans are to lower the lake now and begin an extensive study in the early spring. Depending on what is found, Davis is concerned that the lake might have to be lowered much more, which could have a dramatic effect on one of the state's most popular fishing locations.

The lake covers about 200 acres and is about 40 feet deep. It's open for shore fishing and boat fishing, with a variety of fish available, including bass, northern pike, muskies, walleye and crappie.

"It's got to be fixed," Davis said. "It's one of the most popular fishing spots in the state and is used 10 months out of the year. I'll bet you there are 20 ice-fishing houses out there now."

If the lake had to be lowered dramatically, there probably would be a major loss to the fish population and it would have to be restocked.

"It would likely improve the lake immeasurably for the next 20 to 30 years if all the fixes were made," Davis told the commissioners. "It's unlikely there would be any netting operations to save those fish now there, but they could be restocked."

It appears the clay core of the dam is giving away, Davis said, though core sampling and sounding will be done to determine the exact problem.

"Right now there really isn't a clue to what's causing the seepage," he said.

(Reach reporter Gordon Weixel at 250-8255 or gordon.weixel@bismarcktribune.com.)