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Sunday, March 1, 2015 |  Madison, WI: 15.0° F  Light Snow
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Saving Cherokee Marsh
Officials hope a new technique will restore the shoreline and improve water quality
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Credit:Mario Quintana

For more than a century, the shores of Cherokee Marsh have slowly been washing away, making the marsh - north of Lake Mendota - more and more like "open muddy water with no plants."

Several factors - including the damming of the Yahara River and the invasion of carp - have combined to eat away at the shoreline. About one square mile of wetlands has since been lost.

For many years, environmentalists and local government officials have tried to protect and rebuild the shoreline, with limited success. A wooden boardwalk that crossed the marsh was removed in 2005, for instance, because of concerns that it was damaging the wetland.

But now, a new collaborative approach by UW-Madison, Dane County and Madison has sparked hope that the shoreline and natural ecosystem can be restored at Cherokee Marsh.

The hope comes in the form of structures made of wood and coconut mat called "floating bog interceptors," or "FBIs." They were designed by Chin Wu, a UW-Madison professor of civil and environmental engineering. In May, Dane County paid for 20 of the FBIs to be installed along the shores of Cherokee Marsh. Funding is being provided for more.

Scott McDonell, chair of the Dane County Board, says that building up the shoreline will help reduce sediment and pollution not just in Cherokee Marsh, but in all the lakes that it feeds into.

"It's so inexpensive on the scale of other things we're doing," McDonell says. "There's a lot more sediment moving around in Cherokee than they originally thought. That sediment is carried on through the whole water chain. We have to stop the phosphorus from coming into the lakes."

Russ Hefty, conservation resource supervisor with Madison's Parks Division, says the problem is a "situation of unintended consequences" that was created when the first dam was built on the Yahara River some 150 years ago.

That raised the water level in Lake Mendota and Cherokee Marsh by about four or five feet. "It backed water up into the Cherokee Marsh, into these peat wetlands," Hefty says. This in turn caused the peat bog lining the shores of the marsh to float on top of the water.

With the peat afloat, pieces large and small routinely break off into the water.

"It's not natural to have peat-floating wetlands. They've been calving off and degrading and eroding away," Hefty says.

If unchecked, the wetland would continue to erode until it disappears completely, Hefty says. "You'd be replacing it with open muddy water with no plants. It's a very poor ecosystem with very little habitat. You would have three or four feet of muddy water."

The wetland bog plays an important role in trapping nutrient-rich runoff from the surrounding farmlands.

"That sediment, which is usually quite fertile, is not going to make its way downstream to feed algae growth," says Hefty.

Hefty has spearheaded the city's efforts to reinforce the marsh over the past decade by planting water vegetation just off the shore. He's had the most success with American lotus water lily - a hearty native plant - in narrow parts of the marsh. In the past 10 years, the city has been able to grow about 80 acres of water lilies on the shoreline.

However, in wider areas, where the waves are stronger, he's had less success. "If you have a windy day it plucks plants out of that soft bottom, and they're dead."

In other parts of the country, the Army Corps of Engineers has built small cement walls to break waves and reduce shoreline erosion.

But professor Chin Wu wanted to try a different approach in Cherokee Marsh. Concrete "looks terrible," Wu says. "I was thinking, how can we do this in a way that's more environmentally friendly?"

Wu and his students designed the floating bogs to mimic the natural ecosystem. Dane County provided about $80,000 for the effort. The bogs are wood squares, lined with coconut mat, that float on top of the water. They're anchored to the bottom with concrete, but they allow water to flow underneath.

Native vegetation is planted inside the bog. As the plants' root systems grow, they will help trap silt along the shoreline. The floating bog gives the plants the support needed to withstand waves.

Wu says he was surprised by how fast the bogs started to make a difference. "I always thought it would take a while, but after 21 days there was almost a foot of sediment [accumulated on the bottom]," he says.

The city is going to add to Wu's efforts by planting water lilies in between the shore and the floating bogs, to trap more silt and build up the shoreline.

"We're trying to catch sediment and put it to use to eventually establish a natural transition," Hefty says. "Eventually we'll have a gradual transition from the wetland into the water body."

Another detriment to both the shoreline and the water quality of the lakes has been the invasive carp that live there. The carp muddy the water by stirring up the soil when they eat vegetation. "Carp get hungry and eat everything, like the rabbits in my backyard," says McDonell.

McDonell says this winter the county will hire commercial fishermen to remove the carp. This is easily done, because the carp don't like the cold and congregate in warm areas of the marsh, he says. The county paid to have carp removed from Lake Wingra in the winters of 2008 and 2009, and the water quality improved immediately and dramatically.

The fishermen got only about 50% to 60% of the lake's carp - about 7,000 fish. Says McDonell: "You can never get them all, so it's something you have to keep doing."

Until winter, Wu is studying Cherokee Marsh to see which degrades the shoreline more - carp or waves. That verdict is not yet in, but Wu hopes to use the information to help lakes around the region deal with similar water-quality issues.

"They want to learn how it happened, not just make it better," McDonell says of the UW's efforts. "But in 10 years, Cherokee Marsh should be noticeably better."

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