An army of biologists and environmentalists is descending on the Lower Dolores River to study the ecological benefits of a rare high-volume release from McPhee Dam.

Biologists with Colorado Parks and Wildlife will do fish counts on native and non-native populations and conduct habitat improvement measures.

The Nature Conservancy, Fort Lewis College and American Whitewater will be studying geomorphology, benefits of flushing flows and recreational boating conditions.

“We have a lot of opportunity this year for fish sampling and monitoring,” said Jim White, a fish biologist with Parks and Wildlife, during a presentation last week at the Dolores Water Conservancy office.

His team will be studying population health of three native fish in the Lower Dolores: the roundtail chub, flannelmouth sucker and bluehead sucker.

One of their objectives is to measure the non-native small-mouth bass population, then work toward reducing them. Small-mouth bass are a threat to native fish, preying on their young and competing for food sources.

“We want to find out how widespread small-mouth bass are, especially if they are established in Slick Rock Canyon,” White said.

The bass have developed a stronghold upstream from Slick Rock Canyon to Snaggletooth Rapid. But the high runoff year has opened up an opportunity to try and take out small-mouth bass, White said. In mid-July, Parks and Wildlife plans a flush of 400 cubic feet per second for three to four days from its fish pool reserves within McPhee Reservoir to disrupt the small-mouth bass spawn.

“It would be delivered when male bass are guarding the fry on the nest and has the potential to hammer a major threat to native fish,” White said.

Parks and Wildlife manages a 32,000-acre-foot “fish pool” in McPhee Reservoir for minimum base flows below the dam.

During a whitewater release, the fish pool is not debited, White said, giving fish biologists more flexibility in how to use it. They will tap into 2,600 acre feet of the reserve for the bass-removal flush.

The Nature Conservancy is sending a team of researchers to the Lower Dolores for 10 to 15 days, said Colorado chapter representative Celine Hawkins.

Their work plan includes studying sediment transport and floodplain inundation, which is needed to widely distribute native seeds. They are especially interested in the impact 4,000 cfs peak flows will have on scouring the river channel.

“The information will be helpful to the Bureau of Reclamation planning” on future releases, Hawkins said.

The Nature Conservancy will be using drones to take aerial photos of the Lower Dolores before and after peak flows to track changes and compare them with past years.

Researchers are focusing monitoring efforts at Disappointment Creek, Dove Creek Pumps, Big Gypsum Valley and Bedrock.

Students at Fort Lewis College will be conducting ecological monitoring on the river as well, including on the alluvial groundwater aquifer.

Colorado Parks and Wildlife shared results of a 2016 fish study on the Dolores River.

A cold-water fishery sampling below the dam showed two-thirds brown trout and 16 percent rainbow trout.

Because of infrequent flushing flows, algae is abundant in the 12 miles of stream immediately below the dam. There is a concern it could have a negative impact on fish.

In June 2016, the 20-mile Ponderosa Gorge section (Bradfield Bridge to Dove Creek pump house) was surveyed. Of the 180 fish caught, 73 percent were brown trout, and roundtail chub was the second-most abundant. No small-mouth bass were found in the gorge.

Sampling at the Dove Creek pump station showed roundtail chub were holding steady, in part because they are an adapted pool species. The presence of bluehead and flannelmouth suckers was relatively low because they depend on a ripple environment. In 1992, fish sampling showed much higher numbers of native fish species, the study noted.

“The impact of flushing flows in (2016) was evident, and backwaters looked cleaner,” the study said.

For the past two years, Parks and Wildlife has been stocking bluehead suckers in the Lower Dolores. The fish historically relied on Plateau, Beaver and House creeks for spawning areas, but the dam and reservoir altered the river so suckers cannot reach those ephemeral streams. In 2016, 4,316 bluehead fingerlings were released downstream of the Dove Creek pump house. In 2013, a pit-tag array recorded one flannelmouth traveled 264 miles.

Farmers should be kept apprised of the native fish status on the Lower Dolores, said audience member Ed Millard.

“If they were ever listed as endangered, it could have an impact on the agricultural community,” he said.

jmimiaga@the-journal.com