Walk the dirt path behind the old tennis courts at CU Boulder South this week and the place still looks like it always has: gravel underfoot, a dog nosing through the grass along the fence line, cottonwoods leaning over the creek. By early October, according to the university's own project page, all of it disappears behind a closed gate. Hiking trails, the twelve tennis courts, a warehouse building, all 308 acres.
Boulder has closed trails before. What makes this one different is the clock it runs on.
A Closure Measured in Years, Not Weeks
Compare it to what just happened forty minutes up the canyon. In March 2026, a hole opened up under a bridge abutment on the Boulder Canyon Trail, and the county closed the span with no detour available. Cyclists and commuters were left riding a shoulderless stretch of Highway 119, the same gap where a rider was killed in 1991, and cycling groups pushed hard for a fix. Boulder County Parks and Open Space had it patched and reopened twenty days later.
Or look south to Marshall Mesa. That trailhead shut down in late October 2024 for state coal-seam mitigation work and a full rebuild, and it stayed closed until it reopened on March 27, 2026, a stretch of about seventeen months.
CU South doesn't have a comparable number to count down. The City of Boulder's own announcement describes construction beginning in early October 2026 and projected to take between three and five years to complete. There's no bridge patch coming. There's no rebuilt trailhead reopening on a specific spring morning. The site closes and stays closed for the length of a construction project measured in years.
What's Actually Getting Built Under the Closure
The trails aren't closing because they're damaged. They're closing because the ground underneath them is about to become a flood detention system.
The City of Boulder calls it the South Boulder Creek Flood Mitigation project, and city officials have described it as the city's top priority flood mitigation project. Construction is led by Concrete Express, Incorporated, a contractor selected in fall 2025, and the total cost is estimated at $76.5 million, funded through City Council appropriated stormwater revenues in partnership with the Mile High Flood District.
The work includes an earthen flood detention dam along the north and west edges of the CU South campus built to store floodwater, along with floodplain restoration, all designed to preserve emergency access along Highway 36 if a major flood hits. Once finished, the city says the project will protect roughly 2,300 residents and 260 structures, along with transportation corridors that include Highway 36 and Foothills Parkway.
Part of the construction footprint requires removing CU's twelve tennis courts and a warehouse currently sitting on the site. The university has committed to covering the cost of replacing both.
The Trails Are the First Payment on a 2021 Deal
None of this is a surprise decision made this year. It's the execution of an agreement that's been sitting on paper since 2021.
That's when the CU Boulder South annexation agreement brought the 308-acre site into city limits. Under the 2015 Boulder Valley Comprehensive Plan update, only 129 of those acres are slated for development, plus an additional 30 acres set aside for recreational fields. The agreement caps what can go there too. No football stadium, no high-rises, no roadway bypass connecting Highway 93 to Highway 36. What CU has described wanting to build is mostly housing for faculty, staff, and non-freshman students, aimed at easing Boulder's tight housing market and cutting down on commuting.
But under the terms of the deal, flood protection has to go in before any of that housing can. CU's own FAQ page states it plainly: practically speaking, development at CU South won't begin until at least 2030. The trail closure isn't a detour from that plan. It's the first visible step of it, five years after the ink dried.
Where the Regulars Are Headed Instead
The city's guidance for the closure is straightforward: find alternative recreation locations through city parks and other open space trails. A few specific options are close enough to actually become the new routine.
| Trailhead | Where it sits | What you'll find | Dogs |
|---|---|---|---|
| Bobolink | Baseline Road at Cherryvale Road | Flat paved and dirt paths tracing the northern stretch of South Boulder Creek, wide enough for a jogger and a dog walker to pass comfortably | Leashed, allowed between Baseline Road and South Boulder Road |
| South Mesa | Eldorado Springs Drive, also known as Highway 170 | A larger trail network reaching South Boulder Creek, popular enough to fill its lot on weekends, no bikes on-site | Leashed, allowed on most trails |
| Marshall Mesa | Off Highway 93 near Marshall Road | Gentle mesa loops that reopened in March 2026 after seventeen months closed for coal-seam mitigation, open ground that dries out faster than shaded foothill trails | Leashed, allowed |
Bobolink is the one most likely to absorb the daily dog-walking traffic that used to head toward CU South. The trail follows South Boulder Creek through cottonwood woods, and the flat, wide surface is why it's long been popular with dog walkers and joggers looking for an easy out-and-back. It's also stroller-friendly, and a dog station at the trailhead makes cleanup simple. The one wrinkle is a seasonal one: fields along the trail serve as nesting ground for the bobolink, the migratory songbird the trail is named for, from mid-May through summer, though that doesn't affect access this time of year.
South Mesa trades flat and easy for bigger. It connects into a wider network of trails with varying difficulty and its own access to South Boulder Creek, which is part of why its 48 standard parking spaces fill up fast on weekends. Bikes aren't allowed at South Mesa itself, though the Doudy Draw area directly across Highway 170 picks up the mountain biking crowd instead.
Marshall Mesa is the furthest afield but arguably the most versatile pick heading into fall and winter. The signature loop stitches together the Marshall Valley, Marshall Mesa, and Coal Seam trails for about 2.7 miles across open grassland, with wide views back toward the Flatirons. Because the mesa is open and sunny rather than shaded, it dries out faster after rain or snow than the foothill trails do, making it one of the more reliable shoulder-season options once the higher trails turn to mud. It's also working ranch land, so cattle sharing the trail isn't unusual. Riders looking for more distance can link into the Community Ditch, Cowdrey Draw, and Greenbelt Plateau trails, including the Dirty Bismark loop that's become a regular pick for mountain bikers in the area.
None of these fully replicate what CU South offered, but between them they cover the three things people used the closed site for: an easy flat walk, a bigger network for a longer outing, and a reliable option when everything else is too muddy to use.
The city hasn't published an exact reopening date beyond the three-to-five-year construction window, so there's no single day to circle on a calendar the way there was with Marshall Mesa. The most reliable way to track it is the city's OSMP closures page, which gets updated as work milestones are hit rather than on any fixed schedule.
CU South's closure is the kind of change that reshapes a daily walk long before it shows up in anyone's headlines. Laura Melquist keeps track of shifts like this across Boulder because they shape the ground her clients are buying and selling around. If you want to talk through what a multi-year project like this means for the area near it, book a free consultation.
