Dewatering of the river has imperiled three native species
Colorado Parks and Wildlife aquatic biologist Jim White, left, wildlife officer Luke Clancy, right, and volunteer Pete Deren survey native and invasive fish on the lower Dolores River for the first time since 2019. (Courtesy of Colorado Parks and Wildlife)
As biologists with Colorado Parks and Wildlife watched the snow accumulate on the peaks above McPhee Reservoir this winter, their anticipation swelled.
“It’s very exciting (to) look at the snowpack and watch it grow,” said CPW Native Aquatic Species Biologist for the Southwest Region Dan Cammack. “There’s murmurings of ‘maybe we’ll be able to do Slick Rock Canyon this year.’”
Unlike the throngs of whitewater enthusiasts who flocked to the Dolores River this year, Cammack was not concerned with rafting for recreation. This year’s controlled releases from McPhee, the first since 2019, provide a critical tool to Cammack’s work.
The remote river – which is more of a trickling stream in low-water years – is host to three species of native fish that are imperiled by the low flows that percolate through the canyon most years.
“The Dolores River is a boom-and-bust drainage,” Cammack said.
Kristina Morben, Colorado Parks and Wildlife aquatic biologist, holds a sucker caught during the agency’s survey of native fish on the lower Dolores River. (Courtesy of Colorado Parks and Wildlife)
Bluehead suckers, flannelmouth suckers and roundtail chubs all thrive in cold water.
The slow, warm water that constitutes the Dolores most years is an idyllic habitat for the population of invasive and predatory smallmouth bass, which slipped out of McPhee Reservoir during an accidental spill in 1993.
The lack of water not only threatens the strength of the native fish population, but makes studying their status challenging.
“The Dolores is just a really notoriously hard place to sample,” Cammack said.
The Dolores River holds secrets and surprises for the biologists, who have not been able to access the canyon for work since the runoff four years ago. Willows and shrubs grow into the channel in dry years, only to be scoured by high water in wet years, flushing out sediment and debris; the topography of the channel can change drastically year to year.
And after four years away, scientists are anxious to know how the native fish have fared.
From hell to high water
An adult bluehead sucker caught in the lower Dolores River. The species has been at risk as a result of dewatering of the river and the threat of the invasive predatory smallmouth bass. (Courtesy of Colorado Parks and Wildlife)
Jim White, Colorado Parks and Wildlife aquatic biologist, and BLM employee Jake Blakely are on the row raft loaded with fishing equipment on the Dolores. Because of infrequent flows, a survey of the native and invasive fish population has not been possible since 2019. (Courtesy of Colorado Parks and Wildlife)
As is the case with development, wildfire management, agriculture and myriad other topics of concern in the West, water, and the lack thereof, is of ultimate concern in the Dolores River.
The Bureau of Reclamation has dedicated 31,795 acre feet of water, called the “fish pool,” to be released from McPhee to keep a trickle of water running through the Dolores riverbed. But in low-water years, when McPhee does not fill, the fish pool is proportionately impacted by the shortage like all the water rights holders. In dry years, the Dolores River can dwindle to flows as low as 20 cubic feet per second.
To study the fish populations, biologists float the river with electrofishing equipment to temporarily stun the fish. They collect the specimens, sometimes marking them, before tossing the native fish back and euthanizing the invasive bass.
The electrofishing equipment weighs down the rafts used to float the river, making studies impossible to conduct without ample flows.
“We’ve tried flows as low as 400 cfs, and I always jokingly say, I made my temporaries get out and walk through sections of the river because we just couldn’t get through with all the boulders,” said Jim White, CPW’s aquatic biologist in Durango.
With this year’s flows peaking at over 4,000 cfs, CPW biologists made two trips into the Dolores, one through Slick Rock Canyon and the other from Snaggle Tooth Rapid to the beginning of Slick Rock Canyon.
The population of flannelmouth suckers is doing relatively well, preliminary results showed. But the bluehead suckers and roundtail chubs are struggling with the river’s dewatering. In Slick Rock Canyon, the crew found just five or six blueheads.
The survey also showed that the population of roundtail chubs has declined “significantly” since the 2019 survey.
A juvenile flannelmouth sucker caught in the Dolores River. The species has been fairing relatively well despite low water conditions, an initial summary of this year’s fish survey indicates. (Courtesy of Colorado Parks and Wildlife)
Cammack attributes this to heavy monsoon rains that flush out Disappointment Creek, a sediment-laden waterway that converges with the Dolores just above Slick Rock Canyon.
It is “basically just puking sediment into the river,” he said.
Although the sediment creates inhospitable conditions for the resident roundtail chubs, the impact is even greater on the smallmouth bass.
“We think that Disappointment Creek is sort of keeping them (the bass) in check downstream,” he said.
Still, Cammack says, the high, cold flows are “the best tool we have for managing the smallmouth bass invasion.”
White explains that the fish are poorly adapted to cold water – meaning they merely try to survive rather than reproduce. When temperatures ultimately do warm later in the summer, the spawn will not have enough time to gather nutrients before the cold winter.
Colorado Parks and Wildlife Native Aquatic Species Coordinator Jenn Logan tags a fish on the agency’s survey of several native species on the lower Dolores River in early June. (Courtesy of Colorado Parks and Wildlife)
In those conditions, White says the winter mortality of those young bass is “very high.”
Just as this year’s ample snowpack cannot reverse the impact of a 20-year drought, one spring of heavy runoff is unlikely to shore up the future of the three imperiled native fish species.
Sustainable native fishery a ‘pipe dream’
The Dolores has long been considered for preservation given the river’s immense beauty and biological and historical significance. But with respect to the native fishery, Cammack says its future is in jeopardy.
“A natural flow regime with consistent high flows, the spring pulse most years – something that’s pretty inhospitable for smallmouth bass – and then something that reinstates some natural geomorphology in the channel to improve fish habitat, that’s the only way, I think, to ever get back to a really sustainable native fishery,” he said. “And that’s a pipe dream. That’s not going to happen.”
The fish pool, which sustains just a trickle of water throughout the summer, is not sufficient. The warm, isolated pools it creates in the river bed, as opposed to a flowing stream, sustain the invasive bass and allow plants and sediment to build up in the riverbed.
White has said in the past, and continues to stand by, an estimate established in the 1996 environmental assessment of the fish pool, which says the minimum pool size should be 36,500 acre feet, allowing for minimum flows of 78 cfs year-round.
Even a yearly flush of cold water could be enough to keep the bass population at bay by disadvantaging their spawn. White says the tactic has been somewhat successful in other similar scenarios, but the situation in the Dolores is “thorny.”
“Some of these other projects can do it because they have senior water rights holders downstream of the dam, and then they can release water to supply those senior rights,” he said. “That doesn’t really exist on the Dolores because most of the water rights go over to the San Juan River Basin side, so we don’t really have that opportunity like other river basins.”
Still, White and Cammack say they are considering alternative ways to enhance the native fishery with the resources available. In the most recent survey, crews made two passes over the same reach of river in order to tag and recapture fish – something Cammack said should help build understanding of the accuracy of their surveys.
With respect to conservation, Colorado’s elected officials are still scheming ways to protect the Dolores River. A Wild and Scenic River designation could have endangered upstream water rights holders.
But Colorado Rep. Lauren Boebert and Sens. Michael Bennet and John Hickenlooper have jointly introduced legislation to Congress that would designate the lower Dolores River as a national conservation area. The proposal would protect the river from future dams and natural resource extraction, but would not jeopardize existing mineral or water rights.
Cammack seemed insistent that something must be done.
“If they give them (the fish) half a chance, they’ll make it work,” he said. “But I think where we’re at is, are we going to give them half a chance?”
rschafir@durangoherald.com
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Runoff means first survey of Dolores fish since 2019 - The Durango Herald
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