Efforts to stymie the Asian carp will ramp up this spring with the start of a $16 million project to make the Coon Rapids dam a stronger barrier to the invasive species moving up the Mississippi River.

Five new steel gates — each 8 feet tall, up to 97 feet long and weighing up to 90,000 pounds — will be installed this summer on the Coon Rapids side of the dam, part of a two-year project. Another four gates will go in next year on the Brooklyn Park side, said Jason Boyle, a dam safety engineer for the state Department of Natural Resources (DNR), which is overseeing the state-funded project.

This month, the state awarded a $10.8 million contract to a Edward Kraemer & Sons of Burnsville to install the gates, made in Pennsylvania at a cost of about $3.5 million, Boyle said. The work is expected to begin by the end of May.

Kraemer & Sons also will replace the pocked 50-by-430-foot concrete apron below the century-old dam with a deep concrete stilling basin. The basin will dissipate erosive energy from the churning water pouring down before it hits the river bed.

Boyle said the closest to the dam that an Asian carp has been netted in the Mississippi in Minnesota was near Hastings in March 2012. A month later, a leaping silver carp, a variety of which can jump 10 feet, was caught near Winona.

However, silver carp DNA was found in three of 19 water samples taken from the Mississippi above the Coon Rapids dam in September 2011 and in 16 of 29 samples taken below it. The sensitive tests are designed to detect DNA from carp excrement or mucous in the river water.

The Mississippi River flows over the Coon Rapids Dam on Thursday.

Department of Natural Resources leaders have expressed alarm about the DNA findings, although none of the carp was found in tested areas. The agency has urged quicker action, such as the dam makeover, to block the Asian carp.

DNR spokeswoman Julie Forster said Thursday that another round of DNA river tests has been done but that results aren't yet available.

Massive gates

The new steel gates will replace rubber tube gates currently used to raise or lower the water level in the recreational pool above the dam.

The DNR's Boyle said the new gates will be more effective as a fish barrier because they are lowered gradually from the top, unlike the tube gates, one or more of which must be deflated entirely to reduce the water level.

Boyle noted that the 97-foot gates will arrive in two parts and must be bolted together on site.

During work on the dam, the walkway across it will be closed. So will some trails and the boat launch near the Anoka County Parks Visitors Center.

A temporary coffer dam will be built above the work area to divert water.

With normal maintenance, the rehabilitated dam is expected to last 50 years, Boyle said.

Pool will stay high

To help ensure that any large, leaping carp stay below the dam during and after the upgrade, the pool above it will be kept indefinitely at its high, summer level, Boyle said.

For about 40 years until this winter, the 6-mile-long pool had been lowered 5 or 6 feet during the winter and raised again after the spring runoff.

The dam is owned by Three Rivers Parks, Hennepin County's parks system, which has granted the DNR authority to set the water level above the dam.

The higher level affects the 380 or so residents with riverside homes above the dam in Brooklyn Park and Champlin in Hennepin County, and in Anoka and Coon Rapids in Anoka County.

Jerry Koch, a Coon Rapids City Council member, lives on the pool about 2 miles above the dam. He said river residents will have to reconfigure their docks to adjust them to the higher water.

At the same time, however, Rick Townsend-Anderson of Brooklyn Park said future ice damage may be reduced with a deeper pool, which makes it easier for jams to float downstream, he said.

Boyle said the pool has little water storage capacity and has filled quickly during past spring runoffs, even when the pool was low. He said the DNR will keep the level high except during very high flows, when the steel gates can be lowered as needed to avoid upstream flooding.

Anoka City Council Member Jeff Weaver is another riverside resident. He and Koch said their councils and many riverside residents are relieved that the dam, which authorities once had considered removing, is being rebuilt to preserve the pool for boating and fishing.

Jim Adams • 612-673-7658