Rocky Mountain National Park is shutting down a section of its west side this week as crews continue a multi-year effort to bring the greenback cutthroat trout back to its native waters.
From Monday, August 24, through Sunday, August 30, 2026, the park is closing all areas and trails within the Poudre Headwaters Project zone, including the Colorado River Trail and the Grand Ditch. The closures cover the area where biologists will apply rotenone, a piscicide widely used by state and federal land agencies, to roughly 14.3 miles of the Grand Ditch system and its connected tributaries. The chemical treatment targets non-native fish—chiefly brook trout, which compete with and prey on young greenbacks—clearing the way for the native species to eventually be reintroduced to the headwaters of the Cache la Poudre River.
This marks the second phase of the project inside the park, following an initial closure and treatment in August 2025. Park officials say this year’s work will closely mirror last year’s operation.
Despite the closures, much of the Kawuneeche Valley will stay open to visitors. The East Inlet, North Inlet/Tonohutu, Green Mountain, Coyote Valley, Onahu, and Timber Lake trails will remain accessible, as will the Holzwarth Historic Site and Harbison Meadows Picnic Area. Fishing will still be allowed along the Colorado River south of the Colorado River Trailhead, including near Holzwarth and Coyote Valley.
The Poudre Headwaters Project is a long-term, multi-agency undertaking led by the U.S. Forest Service, with Rocky Mountain National Park, Colorado Parks and Wildlife, and the U.S. Fish and Wildlife Service as key partners. Trout Unlimited and the Rocky Mountain Conservancy have also provided support. Once complete, organizers expect the effort to create the largest connected stretch of greenback cutthroat trout habitat in Colorado, a milestone for a fish that decades of stocking with non-native species had pushed toward extinction in its home waters.
