RAPIDAN — A Blue Earth County campground was evacuated, a reservoir lowered and an observation deck closed after federal inspectors found what they suspect is a hole in the base of the Rapidan Dam Thursday night.

Employees from the Federal Energy Regulatory Commission were doing a routine inspection of the county-owned dam, which North American Hydro uses to create electricity, when the problem was found in the middle portion of the dam, said Al Forsberg, Blue Earth County engineer.

That’s when the precautionary decision was made to evacuate about 10 campers from the campgrounds in the county park on the Blue Earth River below the dam. The dam has been labeled a “low hazard” dam, meaning there’s little risk to human life if it gives way, Forsberg said. So the campground was the only area that needed to be cleared out.

The next step is to take a closer look at the base of the dam, both from the inside and the outside, and determine how serious the hole is, he said. The base of the dam was probed below the water line by inspectors Friday and a diver has been called to check the dam’s base today. The water behind the dam has been lowered between 10 feet and 12 feet.

It could be several days before inspectors determine if the dam is at risk of crumbling, Forsberg said. Until then, the campground will remain closed, the water behind the dam will be kept low and park visitors will be kept off the observation deck on top of the dam.

“We need to find out where the void is, how big it is and whether it’s a risk to the dam or not,” Forsberg said.

The dam’s safety has been a focus of federal, state and county officials for years.

Mel Sinn, head of technical resources for the Minnesota Department of Natural Resources Waters Division, has recommended removing the dam, a job that would cost an estimated $20 million to complete.

Jim Hruska, co-owner of the nearby Dam Store restaurant, and others have said removing the dam would send tons of silt down what is now a popular stretch of rapids.

Those against removing the dam, built in 1910, also point out its historic significance and ability to create what they describe as “green” electricity. The estimated cost of making the long-term repairs the dam needs, however, is $8 million.

That doesn’t include the thousands of dollars in repairs that have already been poured into making short-term fixes to the dam’s base when problems have been found in the past. Either option would cost more than could be raised through county and state funds alone, Forsberg has said in the past.

It’s too soon to tell now how this week’s concerns about the dam’s base will influence future Blue Earth County Board decisions about the dam, said County Commissioner Will Purvis.

“Until we find out exactly what the problem is, it’s hard to comment on how we’re going to proceed,” Purvis said.

Tony Cornish, state representative for the district around the dam, said it’s time for county officials to move forward with the study. He helped get $60,000 in state money to fund the $239,000 U.S. Army Corps of Engineers project. Another $60,000 has been pitched in by the county and the Corps is paying $119,000. Cornish is among the group of people who want the dam to stay.

The problem, he said, is the study is expected to take several years, which means there’s more potential for “patchwork” type repairs such as the county is facing again now that a possible hole has been found.

“They need to make a decision now before they have to do more patchwork,” Cornish said. “My advice to the county is to engage in the study immediately and get it done.”

Not dealing with the dam also is creating harm to the environment, Hruska said. When the Blue Earth River’s water behind the dam was drained quickly Thursday night, fish were left lying on the silty ground. The river is known to area anglers for its wide variety of fish, which also has made the campground behind Hruska’s restaurant a popular fishing spot.

“If they kill a bunch of fish, it will hurt us for years to come,” he said.