In 2011, a razorback sucker, an olive-colored fish with a bony, razor-like keel behind its head that is found only in the Colorado River Basin, started its life in a hatchery. It was released into the wild as part of an effort to support the endangered species’ recovery with a small identification tag implanted under its skin. That tag allowed biologists to check in on this fish through its life, following its journey of nearly 1,000 miles.
Four years after its release near Green River, Utah, biologists caught it 291 river miles away on the Gunnison River in Colorado. The sucker’s identification tag was picked up again in 2017 by a biologist’s antenna in Lake Powell. Then, it swam upstream through Cataract Canyon, braving some of the biggest rapids on the Colorado River in peak runoff. A fish passage around the Tusher Diversion, on the Green River in Utah, allowed it to continue upstream to reach one of the most important spawning areas for razorback suckers, Razorback Bar, where strategically timed dam releases support spawning and newly hatched fish. In total, the Utah Division of Wildlife Resources estimates this razorback sucker traveled at least 942 miles over seven years, moving through several fish passages.
Razorback sucker
Courtesy of Mark Fuller, U.S. Fish and Wildlife Service (USFWS)
That single sucker’s journey illustrates both how widely fish species may migrate and how human management is enabling that movement. Other native fish in Colorado are also known to travel hundreds of miles, like the Colorado pikeminnow, but fish move on smaller scales, too, to access different habitats and food sources within a single river. Human and environmental stressors can limit that travel, with low streamflows, dams, irrigation diversions, and other infrastructure making it difficult or impossible for fish to move up and down rivers as they did historically. Human effort can also reconnect river miles, with success for fish, and for the ecosystems they rely on, often found through collaborative approaches.
Exploring Where, When and Why
There’s no asking a fish why it migrates, but biologists infer from when and where they’re found what they might have been seeking. Moving can bring access to more or different food sources. It can put a fish in habitat better suited to the season, like finding shady eddies with slightly cooler water in summer, or deep, slow-moving pools for over-wintering. They often move to spawn in specific locations. Slower water can give fish a chance to rest out of strong currents. Shallower pockets can offer small or young fish a place to avoid being eaten by larger species.
“The general population has a pretty good handle on the concept of habitat fragmentation and how detrimental it is for things like deer, elk and antelope. When you chop that [habitat] up into smaller and smaller pieces, you have fewer and fewer animals on the landscape,” says Jon Ewert, aquatic biologist with Colorado Parks and Wildlife (CPW). “Fish are exactly the same way … The more contiguous habitat you have that fish are able to move into and up and down and through, the better off the ecology of the whole system is going to be.”
Tracking fish can help researchers see which strategies to support fish migration are working, where problems are arising, and how populations are recovering.
An Epic Reconnection
Automated antenna systems in waterways detect passive-integrated-transponders, or PIT tags, small enough to surgically embed in fish and follow their movements around a landscape, as was the case with that roaming razorback sucker.
When the first fish found their way into the Colorado River Connectivity Channel in fall 2023, no equipment was required to see them. Within hours of flooding the re-created river channel that reconnected the Colorado River around Windy Gap Reservoir for the first time since 1985, people on the banks could see fish flickering through the water. In May, aquatic biologists estimated 848 brown trout and 221 rainbow trout were living in a one-mile reach of the new channel.
The Colorado River Connectivity Channel flows onthe right of the Windy Gap Reservoir. The re-created river channel reconnected the Colorado River in 2023 for the first time since 1985.
Courtesy of Northern Water
But biologists like Ewert are eying other signals for bigger changes in the river ecology. The connectivity channel reconnected fish habitat, but also restored natural river processes that direct how sediment, cobbles, boulders and logs move downriver. Windy Gap Reservoir’s wide, shallow pan trapped larger material, like cobbles and gravel that can rejuvenate riffles, but let fine sediment through. Downstream, that sediment filled in spaces between riverbed cobbles that were once habitat for macroinvertebrates and native sculpin, a small, stout fish that lives in cold, rock-bottomed tributaries.
“You’re just deleting that habitat from the system,” Ewert says. The species that depend on that habitat, like sculpin and aquatic insects, started to vanish, too. With them went the river’s food web. “What we were seeing over the course of decades is less invertebrate production, and therefore less trout production.”
Sculpin were spotted last fall in the Upper Colorado River and the connectivity channel for the first time in several decades. When the connectivity channel reaches its full potential over the coming years, Ewert expects to see two keystone species, native sculpin and the giant stonefly or salmon fly, proliferating in this part of the Colorado River. Northern Water’s Municipal Subdistrict, Trout Unlimited, Natural Resources Conservation Service, Colorado Parks and Wildlife, Colorado Water Conservation Board, Grand County, the Colorado River District, and
the Upper Colorado River Alliance partnered
to create the project.
At the dedication ceremony in October, Bill Emslie, Northern Water Municipal Subdistrict president, explained that partnerships assembled the funding, design and action to make the channel possible. Local Grand County residents had conceived of the idea in the 1990s, not long after Windy Gap was built. The reservoir’s harm to the river quickly became visible, and locals persisted in gathering the allies and resources necessary for the work. Naturalist and fly fisherman Bud Isaacs is credited with raising the alarm with Trout Unlimited, starting the process of meetings and design work for a solution to the waning trout populations. The channel joined a $90 million package of projects that the municipal subdistrict has underway to both improve stream health, and secure Front Range water supplies, including construction of Chimney Hollow Reservoir near Loveland.
Windy Gap Reservoir also blocked trout from high-quality spawning habitat upstream, where fry hatch, get washed downstream with the first high flows, live out their lives, then work back upstream to spawn near where they hatched. But removing Windy Gap would only have given fish a few miles of river access before they were blocked by another diversion structure. In anticipation of the $33 million connectivity channel, Trout Unlimited, Colorado Parks and Wildlife, and other partners worked with the Town of Granby with funding from the U.S. Fish and Wildlife Service and Colorado Water Conservation Board to add fish and boat passage around a diversion structure for the town’s water supply, extending the river miles that trout and sculpin can access. Similar motives led to remodeling a concrete sill that served as a measurement weir on the Fraser River to a more fish-friendly design.
“Now, in theory, if you’re a rainbow trout in Dotsero, Colorado, which is many river miles downstream of here, you can start swimming upstream and not encounter any barriers to movement until you get to the Fraser River diversion at Winter Park, which, that’s a lot of river miles,” Ewert says. In total, by CPW calculations, the Colorado River Connectivity Channel opened up about 235 stream miles of contiguous habitat upstream of Windy Gap.
Similar projects are appearing around the state, with the success of one making the case for the next. A fish passage around the Redlands Diversion Dam on the Gunnison River has allowed more than 182,600 native fish access to more of their historic range. In a series of projects beginning in 2015, Trout Unlimited worked on Elk Creek and Canyon Creek, two Colorado River tributaries on the south side of the Flat Tops, to adjust irrigation diversion structures that had become barriers to spawning grounds. Subsequent efforts have moved upstream in collaboration with the Middle Colorado Watershed Council.
At Redlands Diversion Dam, a fish passage allows native fish to access more of their range.
Mike Gross/USFWS
Coordinating Releases
Other projects have focused on carving out some share of water for conservation efforts, including keeping water in rivers to support fish migrations large and small. Since the 1990s, the Upper Arkansas River Voluntary Flow Management Program has tried to release water into the Arkansas River at just the right pace to cover trout spawning beds while not flushing newly hatched fry downriver or diminishing the trout’s ability to search for food, which becomes harder in the deeper and faster water ideal for boaters. Brown trout were the focus, but rainbow trout are establishing a fishery there as well.
Five parties joined the effort — the Southeastern Colorado Water Conservancy District, Colorado Parks and Wildlife, Chaffee County, Arkansas River Outfitters Association, and Trout Unlimited — to support spawning trout in the Arkansas River headwaters while still meeting other water users’ needs, including those of boaters in the most commercially rafted section of river in the country.
“People long before my time said, ‘Hey, we should get all these groups talking and see if we could move this water in a way that’s beneficial for all interests,’” says Tom Waters, Arkansas Headwaters Recreation Area park manager. “What’s grown from it is what we have today.”
An angler fishes on the Arkansas River, which boasts 102 miles of Gold Medal waters.
iStock
It’s a voluntary program, and subject to the whims of high run-off years, dry winters, and hot summers without monsoons. A testament to the program’s success comes in the overall increased biomass in the river — the quality and density of fish, says Paul Foutz, senior aquatic biologist with CPW. The Arkansas River headwaters also now counts 102 miles of Gold Medal water, a designation from Colorado Parks and Wildlife that recognizes the highest quality trout fishing streams.
“What was once not a Gold Medal river is now the longest stretch of Gold Medal water in the state,” Waters says, crediting both flow management and mine reclamation near Leadville.
Safer Passage
The Grand Valley Water Users Association’s Government Highline Canal
Greg Poschman
As they’re able to move up and down river channels, fish can run into human-created hazards, like canals flowing with water that, instead of reconnecting downriver, will see fish floundering in fields or trapped in vanishing pools as the canal is dewatered at the end of irrigation season. Some 272,021 native fish were rescued just from the Grand Valley Water Users’ canal in Grand Junction between 2004 and 2023, according to a 2024 report from the Upper Colorado River Endangered FIsh Recovery Program. A screen over the entrance tries to deter fish from entering the canal, but debris can clog that screen, leading the irrigation company to divert water around it or lift it away. Fish salvage operations rescue hundreds or thousands of stranded native fish each fall.
“It can be really difficult, particularly with really large rivers,” says Cat Adams, who studied fish screens as a research associate with the larval fish laboratory at Colorado State University. Her study looked at the Green River Canal in eastern Utah. The Upper Colorado River Recovery Program counted 700 endangered fish entering that canal on its way to water the town’s melon fields in 2012. In a single day, 301 razorback sucker became entrained. The recovery program installed a screen over the canal in 2019. Water flows over a fine mesh screen at an angle, redirecting fish and even larvae back into the river, Adams explained. A ditch rider clears the screen of algae, silt and vegetation. In the years since, no fish have been detected below the screen.
“In the areas where there’s four imperiled fishes, three of them currently endangered, in the river basin, stuff like this is particularly important as they face the other myriad issues, like non-native fishes, water modification, habitat loss, on top of being entrained in canals,” Adams says. “So, just another tool in the toolbox.”
Minimum Flows
When setting minimum instream flows, the Colorado Water Conservation Board often considers the depth of water a fish needs to swim from pool to pool or riffle to riffle in that waterway, as well as other factors.
“The idea behind every minimum flow is somewhat migrational — is fish passage — inherently,” says Tony LaGreca, stewardship manager with the Colorado Water Trust. The Colorado Water Trust works to secure water for environmental priorities, including fish and their migratory and habitat needs.
Since the 2010s, through the Yampa River Environmental Release Program, the trust has collaborated with the City of Steamboat Springs and the nonprofit Friends of the Yampa to see up to 7,100 acre-feet of water released into the Yampa River from upstream reservoirs to boost the water level during the hottest, driest summer months.
In shallower streams, water temperatures increase to levels that can stress and kill fish. To spare the fish further strain, the river closes for fishing and even wading, LaGreca says, a significant loss to the local outdoor recreation economy. Adding more water can keep river water temperatures at levels where the fishery remains open for anglers, and the river stays open for tubing, a popular pastime for Steamboat Springs visitors.
Sometimes, efforts to support fish habitat and movement take a creative turn, as with a project that benefits the 15-Mile Reach, a stretch of the Colorado River between Palisade and Grand Junction. This reach is recognized as critical for the recovery of endangered and threatened fish species.
There, the Colorado Water Trust’s leased water from the upstream Reudi Reservoir fuels hydropower generation at the Vinelands Power Plant, just above the town of Palisade. The power plant’s tailwater is released to critical habitat for the threatened and endangered fish that live within the 15-Mile Reach or downstream. These flows fill important habitat like backwaters, side channels, and eddies and pools at the bottom of riffles that might otherwise dry. At times, water from this project has accounted for half of the water in the reach. The excess power generated by the project is sold and accrues profits that will fund future infrastructure upgrades.
“It’s a really great project only because of the collaboration, and because we have so many partners who were willing to work together,” LaGreca says. “We see a lot
of folks coming and talking to us because they understand nothing gets done in water without everybody working together, and the environment is part of that.”
Independent journalist Elizabeth Miller writes about environmental issues in the American West for publications including The Washington Post, Scientific American, Outside, Backpacker, and The Drake.
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