By Forrest Adams

Carp and clean water dominated discussion at the annual Riley-Purgatory-Bluff Creek (RPBC) Watershed District meeting Dec. 2 in Chanhassen.

The meeting at the Chanhassen American Legion detailed watershed district efforts to improve area lake water quality.
The annual Riley-Purgatory-Bluff Creek (RPBC) Watershed District meeting featured speakers from CH2M Hill, the engineering firm hired one year ago to study and suggest ways to improve lake water quality, along with University of Minnesota professors Peter Sorensen and Prezemek Bajer, who are studying carp in Lake Susan, Lake Riley and Rice Marsh Lake to create an invasive species (carp) management program.

Said Mark Enochs, vice president of CH2M Hill, “We’ve been focusing on making sure we have good data and managing that data well and improving lakes, and we’ve seen some improvements so far.”
CH2M Hill is on a cycle monitoring Lake Ann, Lake Susan and Lotus Lake in Chanhassen, Lake Riley in Chanhassen and Eden Prairie, and Round Lake and Mitchell Lake in Eden Prairie. The firm is in its second year of reviewing these lakes. The firm is also monitoring the streams and creeks that run through the district. Six lakes are part of the three-year study.

The projects include assisting with the invasive species control project, analysis of Lotus Lake bounce after rain events, Lotus Lake and Mitchell Lake water quality projects and Round Lake improvements. This fall Lake Riley outlet improvements were completed.

Other major activities in 2008 included a Solar Bee mixing study on Mitchell Lake, sediment oxygen demand analysis in three lakes and plant harvesting that involved removing about 200 tons of Eurasian Milfoil from Mitchell and Round Lake. No milfoil was found on Lotus Lake.

Solar Bees

The Solar Bee units did not improve lake water quality, the district found, because “the mixing velocity was not high enough,” said David Austin from CH2M Hill.

The rental units were installed in the spring. They were meant to mix lake water from the top and bottom of the lake and in so doing suppress cynobacteria, green-blue algae, in the water.
The district found the units to not be effective because they were sheltered from the wind and therefore did not mix the water enough.
“The Solar Bees are a more subtle technique. They need to work in concert with the wind,” said engineer David Austin. “We do not think they are a good fit to the situations that we’re looking at right now.”

Carp project

The carp study that is being conducted in Lake Susan and Rice Marsh Lake has found that carp travel between the lakes, so now CH2M Hill and University of Minnesota researchers have decided they need to install a fish barrier between the lakes to prevent the fish from leaving Lake Susan. The barrier could be electric or air bubbles.

Rice Marsh Lake is east of Lake Susan, and the two are connected by a stream that flows under Highway 101 in Chanhassen. The flow goes from Susan to Rice Marsh, down to Lake Riley.

2009 projected projects

In 2009, look for the watershed district to continue monitoring the lakes. Also in the plans are harvesting curly leaf pondweed in Mitchell and Lotus, a lake restoration pilot study, sediment studies and investigation in biomanipulation, which means stocking a lake with northern pike fingerlings that would eat carp minnows.

Attendance

More than 90 people signed in for the annual meeting.

Readers can contact Forrest Adams at fadams@swpub.com.