Water quality problem? Remove the water

David Brewster, Star Tribune

Mike Bendsten is one of the homeowners on Lake Anderson and is leading the neighborhood response to the curly leaf pond weed problem.

Water will start draining from Anderson Lakes in August in an experiment to kill off invasive pondweed.

By LAURIE BLAKE, Star Tribune

Last update: July 15, 2008 - 9:09 PM

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A widely watched experiment in natural lake-weed control will begin in early August with the draining of two of the three Anderson Lakes near Hwy. 169 in Eden Prairie.

The goal is to rid the lakes of curly-leaf pondweed and reduce the phosphorous that feeds the algae, which turns the lake water pea green.

The success of the chemical-free weed treatment will depend in part on the coming winter. With the water gone, officials are counting on a dry, cold winter to freeze and kill the weeds in the lakebed -- delivering cheaper, longer-lasting results than dousing the lakes with herbicides.

The Nine Mile Creek Watershed District will remove the stops from a lake-level-control structure and allow the lakes to drain by gravity into Nine Mile Creek, said watershed district administrator Kevin Bigalke.

The lake level will probably drop by about 18 inches after the plug is pulled. In late August or early September the district will begin using electric pumps to remove as much of the remaining water as possible.

In the northwest lake, 18 to 19 acres of water will be left in the eastern bay as a concession to nine Bloomington homeowners with lakeshore property. Chemicals will be used to kill the weeds in that part of the lake, Bigalke said. The residents have given their consent.

Curly-leaf pondweed -- a non-native plant with jointed stems, floating leaves and greenish flowers -- covers 45 to 50 percent of the two lakes. The plant has been growing in Minnesota for 100 years, according to the Department of Natural Resources.

In a growing cycle out of sync with native plants, it's the first thing up in the spring and first to die in July -- leaving vegetative tangles on the water surface that block light to native plants and then decompose quickly in the heat, adding phosphorous to the lake. Algae thrive on the phosphorous, turning the lake pea green.

Emptying lakes to control pondweed is new to the metro area and can be considered only when lakes are shallow and easily drained. Bloomington's Mount Normandale Lake west of Normandale Boulevard is the next candidate in the Nine Mile Creek Watershed District. "We want to get the Anderson Lakes project done before we do Normandale because all the water flows into Normandale," Bigalke said.

Using the drawdown and chemical herbicides side by side will provide a good comparison of the effectiveness of the methods, said John Barten, director of natural resources for the Three Rivers Park District.

The district has monitored the water quality of the lakes last summer and this summer and will take similar water-quality readings next summer to determine the effectiveness of the drawdown, Barten said. "Presumably and hopefully, there will be a significant improvement in water clarity."

The park district also will measure the change in the curly-leaf pondweed and document any changes in native plant life in the lake after the drawdown, Barten said.

Early results may be available next June or July, he said.

The electric pumping will be done from a metal raft that will float around the lake. Pipes will take the water to a storm sewer running under Interstate 494 and into the creek, Bigalke said.

The slow draining of the lakes should have little impact on the creek, he said.

Laurie Blake • 612-673-1711