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As soon as the rain stops, work will resume on the last four steel gates needed to make the Coon Rapids Dam a better carp stopper.

Five gates were installed last year in the first phase of a two-year upgrade to keep Asian carp from getting past the dam and into the upper Mississippi River watershed and possibly the popular Lake Mille Lacs fishery. The invasive species has not been found this far up the river. However, a silver carp, known for its appetite and spectacular leaping ability, was found dead last August near Winona, Minn., the farthest north that species has been detected.

The nine steel gates, including six 97-footers, are replacing aging rubber tube gates, which are less effective at thwarting fish passage that is possible during serious flooding. To help ensure that any fish stay below the dam during and after the upgrade, the pool above the structure will be kept indefinitely at its high, summer level, said Jason Boyle, a dam safety engineer overseeing the project for the Department of Natural Resources.

The five spillway gates put in place last season are working fine, as is a 450-foot stilling basin installed in the apron below the gates, Boyle said. He said the project is staying close to its $16 million budget.

"Phase one is complete and we are starting earlier than we did last year. We didn't have the high [spring river] flows like last year," Boyle said. "We learned some things from last year that should make it go smoother."

The Mississippi's spring runoff has abated enough so that once the rain stops, workers will begin moving a barge and steel sheets to the Brooklyn Park side of the dam to prepare for installation of the last four gates, said Dennis Jonjak, project manager for Edward Kraemer & Sons, of Burnsville. The crew will also rehabilitate and paint a 103-foot-long control gate, he said.

A temporary coffer dam will be built above the dam to divert water around the work area. A tall crane, sitting on a barge, will lift 45-foot-tall metal sheets into place and then pound them about 20 feet into the river bed with a big vibrator hammer.

Once the coffer dam is secured to the riverbank and dam structure, water will be pumped out so workers and equipment can operate inside an island of relative dryness to remove the old rubber bladder gates and install steel gates.

The work is expected to be done by fall. The rebuilt dam is designed to last 50 years.

The dam is owned by Three Rivers Parks, Hennepin County's parks system, which agreed to let the DNR oversee the rehabilitation. The dam creates a popular upstream recreational pool that stretches 6 miles northwest to the Anoka-Champlin Bridge.

Built in 1913, the 1,000-foot-long dam was long used for hydroelectric power.

During the current construction, the public walkway across the dam will be closed, as will some trails on both sides of the dam and the boat launch near the Anoka County Parks Visitors Center on the east bank.

Jim Adams • 612-673-7658