Boat waves are washing away the shoreline of Big Island, causing bluffs to collapse and trees to fall into Lake Minnetonka
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.
To solve the erosion, the project does two things: It installs large rocks, some of them boulders 2 feet wide, from just below the water’s edge to about 3 feet on shore. The rocks, known as riprap, form a sort of armor to reduce the impact of constant waves.
The second step, which began last week, involves planting layers of fast-growing vegetation behind the rocks.
Tory Christensen, owner of Wetland Habitat Restoration in Minneapolis, worked with a crew of four on what he called the “greening-up” part of the project. First the workers staked into the ground behind the rocks a series of long bio logs made of coconut fibers.
The biodegradable logs also help break the waves and can be planted with cuttings from willows, dogwood and other shrubs that tolerate wet soil and form dense roots to stabilize the soil.
“What we’re trying to do is get root mass and vegetation to hold everything in place and get it solidified,” Christensen said.
The dormant “live sticks” will be planted this week and will take root and grow quickly next spring, he said.
Behind the bio logs, compost and soil will be added by mid-month to form sheltered catchment areas. Bare root maple, cottonwood, highbush cranberries and other native species will be planted in those areas.
A similar project completed in 2007 on the nature park shoreline shows how the restoration should look in five to 10 years. There, clusters of willow and 15-foot maples and cottonwoods are growing in front of the bluffs, and soil from any rain or wind erosion has filled in behind the shoreline rocks, forming a more natural slope.
“It’s not actively eroding from the bottom anymore,” Clark said.