Thunder Mountain Bike Park Was Built on Dirt That Can Handle Juneau Rain

How does a brand-new Southeast trail network get through its opening day in the rain? Thunder Mountain Bike Park, the Mendenhall Valley's new set of trails at the end of Jennifer Drive, held its official opening on Sunday, Aug. 30, 2026, and riders of all ages splashed through the mud to try the new features.

The rain didn't ruin the day because of the ground the park sits on. Southeast Alaska is a hard place to build trails, and the Under Thunder area has soil that works with the rain. Six weeks after opening, that's still the main thing to know about the park.

The shale under Thunder Mountain

Reid Harris, president of the Juneau Mountain Bike Alliance, explained to the Juneau Independent why riders kept coming back to this hillside. The draw is the crushed shale coming off Thunder Mountain.

"Anywhere in Southeast it's a jungle — you got three feet of spruce roots and vegetation and stuff — so you actually get down to any kind of dirt that's actually good for building and holding a form. But right here it's just right on the surface and that pile replenishes itself every year."

In the same interview, Harris said "you have to engineer the trails to handle a lot of water," and in most of the region that means wooden features to bridge streams and gaps. The difference at Under Thunder is that the building material is already at the surface, and the mountain tops it up every year. You don't have to dig through feet of root mat to find dirt that will hold a berm or a lip.

That's also why the park exists at all. The trails weren't planned on a blank map and then cut into forest. The Juneau Independent reports that local mountain bikers found the area on their own and experimented there because the shale was so good to ride.

Five years from riders' lines to official park

The good dirt explains why the trails began here. Making them official took a lot longer. Harris told the Independent that riders first built without official approval, until the city asked them to pause and organize formally. The group set up a 501(c)(3) nonprofit and signed a memorandum of understanding with the city, and building picked back up about five years after it started.

The City and Borough of Juneau's project page for the park gives the official timeline:

1. 2022. A trail plan was drafted with input from JMBA members and the Parks and Recreation Advisory Committee at a public meeting. The plan was built around trails that had already been "created and used for many years."
2. July and August 2023. JMBA and Trail Mix ran community volunteer days.
3. June 2024. More volunteer days.
4. May 2026. A final round of volunteer work. Across these phases, the partners also hired an expert trail builder to build three new trails and rehabilitate two existing advanced trails.
5. Aug. 30, 2026. Official opening. Permanent signs now mark each trail's difficulty.

The Juneau Independent identifies the professional builder as contractor Gary Paasch. Harris put the total cost at about $320,000, mostly covered by grants from the Juneau Community Foundation and the Rasmuson Foundation. He also told the paper the project took "hundreds of volunteer laborers." At the ribbon cutting, Mayor Beth Weldon handed ceremonial keys to JMBA board member and volunteer Craig Good and to Harris.

Harris also told the Independent why the Valley was the right place: "75% of our population is right here." Before this, the Valley had no dedicated place for kids to ride. Now there's one on ground that can take regular use in the rain.

A trail system that teaches you to ride it

Easy-to-shape dirt let the builders lay out a real progression, not just a handful of expert lines. According to the Independent, the eight trails split three green, two blue and three black. Harris was candid that builders left to themselves "might just do black trails," and said the goal was for everyone else to learn too.

No public source we reviewed lists all eight trails with their ratings. Here's what local reporting and Trailforks confirm:

Harris told the Independent that once riders are comfortable on the green trails, "you can start to step up to the blue trails, just two of those." Longtime Juneau rider Phil Moser described the layout to the paper as a progression that goes "about as high as you want to go," with "always an alternative" for getting down.

How you get in, and which way you ride

All eight trails come off one spine. The Independent reports that every new trail is fed by the existing Under Thunder trail. On Trailforks, Under Thunder is a 1.9-mile green doubletrack, open both directions to hikers and bikers, with CBJ Parks and Recreation as land manager.

The jump trails run one way. CBJ's project page sets the rules:

• Helmets are required on every trail.
• No uphill traffic, on foot or on a bike, except on the "Uptrack."
• No motorized vehicles in the area or on the trails, electric motorcycles included.
• Parking is at the trailhead at the end of Jennifer Drive, with overflow at the Miller Ballfields next to Glacier Valley Elementary School, also on Jennifer Drive.

Trailforks says the Thunder Mt Uptrack connects to a CBJ access road and is the usual climb from Jennifer Drive or for lapping Dream Zone. CBJ's page also says JMBA and Trail Mix can help with a free helmet for anyone who needs one.

What tears up a trail made of shale

The loose shale that makes building easy also makes the trails easy to damage. Harris told the Juneau Independent that motorcycles on the trails have caused damage and put other riders at risk. "The surfaces are not hardened for motor use," he said. "They get torn up, and it's actual structural problems." He added that the jumps and features are designed for speeds of 15 to 25 miles per hour.

Harris also said that because use has been so heavy, volunteers "can't be out here working right now." The ground refills, but the shaped berms, lips and wooden features still depend on volunteer maintenance. The downhill-only rule and the motor ban are what keep a soft-surface park in usable shape between those work sessions.

Six weeks in, riders are still out

As of Oct. 9, 2026, people were still logging rides. Trailforks activity logs show recent rides on both Under Thunder and Dream Zone. We haven't found published hours or official snow-season guidance for the park. For plans after the first freeze, JMBA and Trail Mix are the contacts the city lists.

The Valley also got a second trail back this month. CBJ announced that Kaxdigoowu Heen Dei, also known as the Brotherhood Bridge Trail, reopened along with the Dimond Park Footbridge on Oct. 8, 2026. Most of the trail had been closed in September so construction crews and trucks could reach the bridge. The footbridge passed an Alaska Department of Transportation & Public Facilities inspection on Friday, Oct. 2. Parks & Recreation then smoothed the walking surface and put up temporary fencing around the approach before reopening. Valley residents now have a jump park on Jennifer Drive and the Brotherhood Bridge Trail and Dimond Park Footbridge, both open as of this week.

In Southeast Alaska, what's under the surface decides what a piece of land can be used for. That holds for a bike park, and it holds for a cabin lot or a waterfront parcel. At Beloved Alaska Realty, we pay attention to drainage, access and terrain when we market property across the region, and we film it with drone video and Matterport 3D tours so distant buyers can see it before they visit. If you want a close look at a Southeast parcel, request a virtual tour or a local consultation, and we'll walk through what the ground there can support.