River Restoration Project Triggers Massive Fish Kill on the Rio Grande

Construction on a federally funded canal improvement near Del Norte, Colorado, dewatered 7.2 miles of the river, killing trout and native fish across all age classes in numbers too large for a nine-person crew to collect. Biologists say recovery will take three to five years. Images courtesy of CPW

Colorado Parks and Wildlife has confirmed a “large-scale fish kill” along the Rio Grande below Del Norte after a river restoration project dried up 7.2 miles of channel during a winter cold snap. The kill wiped out brown trout and rainbow trout from 2-inch fingerlings to 24-inch adults, along with native species that have inhabited the upper Rio Grande for millennia.

The disaster stems from the Farmers Union Canal Diversion and Headgate Improvement project, which received $1.27 million through a Bureau of Reclamation WaterSMART grant. The project—led by the Rio Grande Headwaters Restoration Project and the San Luis Valley Irrigation District—was designed to replace an aging diversion dam and headgates on the Farmers Union Canal, which bifurcates the Rio Grande into its north and south channels downstream of Del Norte. The new infrastructure was supposed to improve irrigation efficiency for 140 water users across roughly 43,000 acres while adding fish and boat passage.

Instead, a construction decision dewatered the river’s north branch.

What Happened

A landowner contacted CPW on February 3 to report that the north branch of the Rio Grande east of Del Norte was going dry and fish were dying. CPW investigators confirmed the dewatering was linked to the canal project. On February 16, a nine-person CPW crew walked the affected channel to collect dead fish. They ran out of capacity before they ran out of carcasses.

“There were too many dead fish for the team to collect them all,” said John Livingston, CPW’s southwest region public information officer.

Farmers familiar with the project told the Alamosa Citizen that a “hasty decision” was made to push construction forward during a cold spell this winter, drying the river through private corridors of the Rio Grande. Rapid dewatering in subfreezing conditions left fish stranded in shrinking pools, where they froze or suffocated. CPW said scavengers—birds, raccoons, skunks, foxes—moved in before biologists could tally the full toll.

Damage Beyond Trout

The kill extends well past the sportfish that draw anglers to this stretch. CPW’s investigation documented dead brook stickleback, longnose dace, fathead minnow, and white sucker. Northern leopard frogs and aquatic invertebrates—mayflies, caddisflies, stoneflies—also perished.

The reach harbors an aboriginal population of Rio Grande chub (Gila pandora), a Tier 1 Species of Greatest Conservation Need under Colorado’s State Wildlife Action Plan and a state Species of Special Concern. Historically the most common fish in the Rio Grande basin, the chub has declined dramatically due to habitat loss, water diversions, and predation by nonnative species. CPW and its partners have spent decades working to prevent federal listing of the chub and the related Rio Grande sucker — stocking nearly 1.2 million chub into 11 waters since 2002. Losing an aboriginal population in the main stem undercuts that effort.

The affected stretch also serves as nesting and rearing habitat for two federally protected bird species: the endangered southwestern willow flycatcher and the threatened western yellow-billed cuckoo. Neither overwinters in the reach, but both depend on its riparian corridor each spring and summer.

Recovery Timeline

CPW aquatic biologists estimate the fishery in this 7.2-mile section will need three to five years to recover. The kill eliminated multiple year-classes of trout, and brown trout eggs deposited in the riverbed gravel during last autumn’s spawn were most likely lost. That loss compounds the damage: the reach won’t begin producing its own wild brown trout recruits until surviving or recolonizing fish complete a full spawning cycle.

The affected reach lies just downstream of the Rio Grande’s Gold Medal section between South Fork and Del Norte—water CPW has designated for its trophy-caliber trout density and size. Fish move between the two stretches, and the loss of multiple age classes below Del Norte could slow recruitment into the Gold Medal corridor as well. Anglers planning trips to the San Luis Valley this spring or summer should expect diminished fishing in and around the affected reach and may want to focus on water well upstream of Del Norte until the population rebuilds.

A Restoration Project’s Unintended Consequence

The Rio Grande Headwaters Restoration Project has a strong track record. Since its founding in 2001, the nonprofit has replaced aging diversions, stabilized eroding streambanks, and restored fish passage along dozens of miles of the upper Rio Grande. Its Five Ditches project, completed in 2020, improved habitat and irrigation infrastructure for multiple ditch companies between Del Norte and Alamosa.

The Farmers Union Canal project was built on the same model: modernize infrastructure, benefit both irrigators and the river. The Bureau of Reclamation awarded the $1.27 million WaterSMART grant in late 2023, and construction was scheduled across the 2025–2026 window.

What went wrong was timing. Diverting flows during a winter freeze—when fish are physiologically stressed and have nowhere to go—turned a routine construction step into an ecological catastrophe. CPW has not yet publicly released a detailed finding on accountability or enforcement actions related to the incident.

The Alamosa Citizen reported that it was seeking comment from Daniel Boyes, executive director of the Rio Grande Headwaters Restoration Project. Neither the organization nor the San Luis Valley Irrigation District had issued a public statement addressing the fish kill as of this writing.

“CPW wants to thank the landowners along the river for their cooperation and for providing access to the river for this investigation, and CPW shares their concerns regarding this incident,” Livingston said.