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HUDSON, Wis. – It took nine years, thousands of volunteer hours and a great deal on wood, but now the first few miles of what's billed to become one of the largest mountain bike trail networks in the region will have its grand opening Saturday at Willow River State Park.

It took so long to get going, in fact, that between trail proposal and approval, the sport of mountain biking grew from a mostly fringe activity to one of the fastest-growing high school sports in the country.

"It's one of those things that grinds on forever and then all of a sudden it goes 'Zoop!' " said Rita Thofern, president of the Friends of Willow River and Kinnickinnic State Parks, one of the groups supporting the trail's construction.

The new trail system will run inside two Wisconsin state parks separated by 15 miles — Kinnickinnic and Willow River — with plans calling for about 15 miles at Kinnickinnic and some 25 miles at Willow to be built in the next three years or so.

Tom Gujer, a mountain biker, coach and volunteer trail builder, said plans first came together years ago, when he and other volunteers from the nonprofit Friends of the Willow River and Kinnickinnic State Parks helped build a 3-mile demonstration trail at Kinnickinnic, a 1,280-acre state park that includes steep ravines and St. Croix River waterfront across from Afton, Minn.

They hoped the trail would persuade the Wisconsin Department of Natural Resources to support development of a larger system.

It worked, sort of. The staff loved the trail, but the DNR couldn't just sign off on a more extensive network. "The unfortunate part was the parks ran into some formalities that wouldn't allow them to put mountain bike trails in the park until their master plan was renewed," said Gujer.

Years passed, and mountain biking saw the advent of high school racing, with a Minnesota league forming in 2012. As demand grew, Gujer and others found more support for trails at Willow River, a 3,000-acre park just north of Hudson.

The nonprofit Friends group formed the Willow Kinni Mountain Bike Club, or WIKI, so that when the trails got approval they would have the volunteers needed to build them.

Last year, the DNR finished updating the park's master plan and greenlighted the trails. The DNR kicked in $30,000 in Knowles-Nelson Stewardship grants, and the Friends matched it with dollars and volunteer hours.

The money covered professional design from Trail Source, a Twin Cities company, and rental of mini-excavators.

It also helped when trail builders ran into a surprise: extra wetlands. The trail's route, chosen in consultation with the DNR staff, seemed to indicate that about 100 feet of boardwalk would be required to cross wetlands. It turned out they needed to build about 600 feet.

"We just didn't anticipate the magnitude," said Ben Swenka, president of the Willow Kinni bike club.

With wood prices reaching historic highs this past summer, the project looked like it had hit an expensive hiccup.

The plan was saved by two fortunate turns: first, Willow River park staff knew of a cache of green-treated pine timbers owned by a local resident who was willing to sell it cheap. Once used as posts to secure cables for a highway guardrail, the 6-inch-by-8-inch posts offered plenty of potential boardwalk material, but it needed to be cut to size.

A mill owner who lives about 30 minutes from Willow River agreed to mill the timbers to the needed 2-inch-by-6-inch dimensions, and for a few thousand dollars the trail builders had their boardwalk.

The first 3.5 miles of new trail at Willow River State Park will open Saturday at 2 p.m., with another 1.5 miles expected by year's end. The grand opening runs from 2 to 6 p.m. at the Willow River Mountain Biking Trailhead. A new 2-mile trail opened at Kinnikinnic State Park earlier this summer, with another 1.5 miles expected by the end of the month.

Thofern said the Friends group will continue to seek grants and donations, and volunteers from the WIKI club will keep building, eventually constructing close to 40 miles of trail between the two parks.

For Willow River Park Superintendent Aaron Mason, it's all a welcome development. The DNR, with three full-time staff at Willow River, simply doesn't have the bodies to build an extensive trail network. Coordinating with volunteers made it possible to add one more attraction at the popular park, balancing the needs of park visitors, he said.

On a recent afternoon, riders with the Hudson High School mountain bike team tested out the new trails, their bikes clicking and whirring through the trees.

"It's kind of been a long time in the works," Mason said.

Matt McKinney • 612-673-7329

Correction: A previous version misidentified the mountain biker in the caption of the photo.