The Colorado River and its tributaries were once a wild, muddy system of spring floods and warm summer shallows, and the fish that evolved in it were built for that. Four of them are found nowhere else on Earth: the Colorado pikeminnow, the humpback chub, the razorback sucker and the bonytail. The pikeminnow, the largest minnow in North America, is said to have once reached nearly six feet; it was a predator that migrated long distances to spawn. The humpback chub and razorback sucker carry odd shapes, a hump behind the head, a keel along the back, that probably help them hold position in fast water.
How they declined
The twentieth century rebuilt the river around them. Dams blocked their migrations and turned warm, flood-scoured channels into cold, clear water released from the bottoms of reservoirs. Diversions took water and fish into canals. Introduced species, among them smallmouth bass, northern pike and catfish, ate the young of the natives and competed with the adults. By the late twentieth century all four were listed as endangered. The pikeminnow was even renamed in the late 1990s, its old common name having carried an ethnic slur.
The recovery program
The endangered listings threatened every new water project in the basin, and in 1988 the governors of Colorado, Utah and Wyoming, the Secretary of the Interior and the Western Area Power Administration signed an agreement creating the Upper Colorado River Endangered Fish Recovery Program. Its premise was that the fish could be recovered without halting water development, if water users, states and federal agencies worked on it together.
Much of the work happens around Grand Junction. South of town, the Redlands Diversion Dam on the Gunnison River, built in the early 1900s, blocked fish from more than fifty miles of upstream habitat until the program built a fish ladder around it in the 1990s; fish that use it can be sorted so that natives pass and nonnatives do not. A national fish hatchery in Grand Junction raises razorback suckers and bonytail by the thousands in big indoor tanks for stocking. Reservoir operators time releases to give the rivers spring peaks again, irrigation companies have rebuilt their systems to leave more water in the river, and crews remove nonnative predators from key reaches.
Where they stand
The results are uneven. In 2021 the humpback chub was moved from endangered to threatened. The razorback sucker, whose numbers have grown with stocking, has been proposed for the same step. The bonytail survives largely through hatchery stocking, and the Colorado pikeminnow remains the hardest case of the four.
Standing on the riverbank at Grand Junction, below Colorado National Monument, you are unlikely to see any of them. That they are still in the river at all is the program's achievement.