Hundreds of boats jam the waters of Cruiser's Cove next to Big Island on Lake Minnetonka on summer evenings and weekends, but by late last week there was nothing to be seen but a barge moving through fog to remove some of the lake's last docks before winter.
On the island, however, crews are hard at work on an off-season project. They stand along the shoreline before bare, vertical bluffs that are collapsing and pulling trees into the water. The problem is erosion caused by too many waves, especially those produced by ever-larger boats that pound the shoreline relentlessly, said John Barten, director of natural resources management for the Three Rivers Park District.
"The banks in many cases are 8 to 10 feet high, and the soil underneath those gets eroded away by wave action until the trees and the upper bank collapses," Barten said. "So tons and tons of sediment get carried out into the lake."
Three Rivers and the Minnehaha Creek Watershed District have joined forces and split costs on a $267,000 project to restore 1,200 feet of the shoreline and protect it from additional damage. The eroding bluffs are on the north side of the District's Arthur Allen Wildlife Sanctuary, a 62-acre preserve that is not open to the public.
It sits next to Big Island Nature Park, owned by the city of Orono, where crews did a similar face-lift along 3,000 feet of shoreline in 2007.
"This is the middle of the metro area, close to 120 acres in two parks that we're preserving for generations," said Gabriel Jabbour, the Nature Park's volunteer custodian. "Doing this project is a great, great asset to the community and to the state."
The problem with the erosion, said Renae Clark, project leader for the Minnehaha Creek Watershed District, is that all the soil that flushes into the lake clouds the water and covers the bottom with silt. That area closest to shore, up to 15 feet deep, is where fish lay eggs, aquatic plants flourish and insects proliferate. "It's the most sensitive and most productive area of the lake," she said. The eroding soil also carries nutrients, she said, including phosphorus that contributes to excessive algae growth.
In addition, the park district is losing vegetation along the upper banks, Clark said, sometimes including huge trees that fall into the water as the bluffs slump, or blow down more easily in the unstable soil. On a visit to the island last week, park signs at the top of one bluff were only inches from the edge, showing that the bank had receded about 4 feet in recent years.