Rocky Mountain National Park is taking an aggressive step to remove non-native fish from the 14.3 miles of the Grand Ditch system (century-old water diversion project in the northwestern corner of the park) and will utilize a poison that has been in use for thousands of years.
In an effort to remove invasive fish to make room for the reintroduction of the native Greenback cutthroat trout, aquatic biologists will apply a piscicide called rotenone to the waterway and its upstream and downstream tributaries. Rotenone is a naturally occurring chemical compound found in the roots and stems of various tropical and subtropical plants and has been used by indigenous peoples in Central and South America to catch fish for time immemorial. Rotenone will essentially function as a reset button for the Rocky Mountain National Park aquatic ecosystem and hopefully provide a clean slate for the native Greenback cutthroat trout to comeback and thrive.
During the implementation areas of RMNP will be closed to the public but fishing access will remain open along the Colorado River located south of the Colorado River Trailhead. These areas will be clearly marked and easy to avoid. Waterways treated with rotenone may appear red in color and should be avoided. RMNP officials advise guests should do not pick up dead fish, consume fish, or drink water that has been treated with rotenone. Learn more below.
Rocky Mountain National Park Press Release:
Rocky Mountain National Park will be implementing the Poudre Headwaters Project during the week of August 25-31, 2025. This project involves the removal of all non-native fish from the Grand Ditch and upstream and downstream tributaries inside RMNP. Once removed, native Greenback cutthroat trout will be reintroduced into its native range in the headwaters of the Cache la Poudre River.Â
To implement this project, some areas on the west side of RMNP will be closed to all uses, including hiking and fishing access. To remove non-native fish, aquatic biologists will apply a piscicide called rotenone to the 14.3-mile Grand Ditch system and its upstream and downstream tributaries. Rotenone is commonly used by state and federal land management agencies. This project will be led by biologists with the U.S. Forest Service. Other partners include Colorado Parks and Wildlife and U.S. Fish and Wildlife Service. Additional funding support is provided by Trout Unlimited and the Rocky Mountain Conservancy. Â
Trout that are not native to RMNP, including brook trout, currently occupy these waters. They pose threats to native greenback cutthroat trout if not removed as they could move into the greenback recovery area. Because of the level of activity associated with the implementation of this project, all areas and trails within the project zone will be closed to all uses beginning on Monday, August 25 through Sunday, August 31. The Colorado River Trail and the Grand Ditch will be closed.
While this project is underway, the following trails will remain open in the Kawuneeche Valley: East Inlet Trail, North Inlet/Tonohutu Trail, Green Mountain Trail, Coyote Valley Trail, Onahu Trail, and Timber Lake Trail. The Holzwarth Historic Site and Harbison Meadows Picnic Area will also remain open.
Fishing access will remain open in all areas along the Colorado River located south of the Colorado River Trailhead, including near the Holzwarth Historic Site and Coyote Valley. Water in the Colorado River may appear brown in color due to the treatments upstream. This water is safe for people and fish.
Waterways that have been treated with rotenone may appear red in color. For your safety, do not pick up dead fish, consume fish, or drink water that has been treated with rotenone. These areas will be clearly signed.
