For the first time in several decades, the water level behind the Coon Rapids Dam will not be drawn down this fall, which means big changes for the hundreds of homeowners along the 6-mile pool behind the dam.

They usually see the water drop about 6 feet in October and November, but this year the pool will be kept indefinitely at its higher summer level to prevent Asian carp from moving upriver past the dam.

The higher level affects the shorelines of 384 property owners above the dam, which straddles the Mississippi River between Brooklyn Park in Hennepin County and Coon Rapids in Anoka County.

Since the pool is normally drawn down each year, most have been able to leave their docks in place during winter, since the structures were high and dry and not threatened by river ice. That won't be true this year and in the future, said Dale Homuth, manager of conservation assistance and regulation for the Department of Natural Resources. He said some homeowners will need to remove docks and other structures to protect them.

State and county officials have taken steps to inform people, he said, but some may still be unaware.

Another potential problem, Homuth said, is that many have built illegal retaining walls at the edge of the water along the shoreline.

"They're going to get ripped out with ice pushing up against them," he said. "Some are pretty fancy and probably cost tens of thousands of dollars."

The plan is to keep the pool behind the dam at the higher level year-round to maximize the dam's effectiveness as a barrier against invasive species, especially Asian carp, he said.

Dam rehab set for 2013

Three Rivers Park District, which is based in Plymouth, owns and operates the dam but signed a joint agreement in January with the Minnesota Department of Natural Resources. It gives the DNR responsibility to decide the appropriate pool levels and manage a $16 million, two-year rehabilitation of the dam that will begin next year.

Northern States Power Co. built the dam 99 years ago for hydroelectric power and operated it until 1966. The dam and associated property on each side of the river were donated to the park district (then the Hennepin County Park Reserve District) in 1969. It became Coon Rapids Dam Regional Park.

Three Rivers sold 214 acres of parkland on the Anoka side to Anoka County last year and still owns 160 acres of parkland on the Hennepin County side.

The dam was modified in the mid-1990s with a new walkway and new gate system. But it developed large scour holes on the downstream side of the dam in 2005 and in 2009. A special dam commission in early 2011 recommended a major structural rehabilitation.

Homuth said that even without the concern about invasive species, the dam needs those structural improvements, including new steel gates and changes to its spillway. The Legislature appropriated $16 million in bonding in July 2011 to repair and renovate the dam.

Homuth said the project is on schedule, and is now in its design and engineering stage. The design engineer is getting bids for "something like $6 million or $7 million in steel gates," he said, that need to be ordered a year ahead of time.

The plan is to close about half the dam next spring by installing a temporary dam upstream to divert water, Homuth said, and then do the other half of the work in 2014.

While that work moves forward, a private firm is investigating whether a new hydropower plant might be feasible at the site.

Margie Dahlof, associate superintendent of strategic initiatives at Three Rivers, said district employees have had some preliminary discussions with the firm, Nelson Energy of Golden Valley, mainly to learn more about the potential project's scope, financing and other details.

Tom Meersman • 612-673-7388