SLICKROCK, Colo. — A flash flood on the Lower Dolores last week has biologists with Colorado Parks and wildlife concerned because unusually high levels of silt in the water caused a die-off of native fish.
A team of CPW biologists conducted a native fish survey Aug. 18-21 as part of an ongoing effort to determine the health and habitat conditions of the bonytail chub, bluehead sucker and flannelmouth sucker.
On Aug. 20, heavy rains flooded Disappointment Creek, a Dolores tributary, bumping up the flow of the Dolores below Slickrock from a trickle of 11 cubic feet per second to a churning 400 cfs overnight.
“It’s in a flood stage right now, and that will make the survey more challenging,” said Harry Crockett, a CPW native fish species specialist, to a crew of 13 gathered on the river bank. “But we’re here so we might as well give it a go.”
Paul Jones, a CPW lead researcher, explained the previous days’ milder work conditions.
“The river was what we call a pearl-necklace, characterized by deep pools, connected by a sliver of water, and then a deep pool again,” he said. “Today should be interesting.”
On Wednesday morning, the Dolores was a muddy brown torrent, not unusual following a major rain event, but it’s what was in the water that the team later learned was problematic.
Dead catch
Canoes were loaded and gear organized, an efficient process carried out by an experienced group of career fish biologists and seasonal workers, some of the them environmental students from Western State College.
The crew was divided into two groups to capture, identify, measure and record fish in a three-mile section beginning at the Big Gypsum Valley boat launch. The teams leap-frogged each other downstream to cover more ground.
Jones monitored the survey from his one-man pontoon oar boat and assigned an identical rig to the Journal reporter so he could witness first hand how the work was done.
Wearing life-jackets, participants waded into waist-deep water, climbed onto boats and were quickly swept downstream to the first measuring point, a small island a couple of hundred yards away.
CPW fish biologists Jim White and Crockett tied off their canoe and pulled out a net, a scale, a fish-measuring device and buckets. The 15-foot by 5-foot net had flotation along the top and weights on the bottom. Two people deployed it from rods attached on either end, moving in tandem in an upstream sweeping motion along backwaters, eddies, and riffles.
White’s end of the net was in the current and he was suddenly swimming, but he had the strength to maneuver the net around towards shore.
“Yesterday, we were walking from pool to pool in an almost dry stream bed,” he said, completely covered in mud.
“Will we see any big fish?” the reporter asked.
“Not likely; this seine technique is more suited for capturing the juvenile fish,” White said. “We’re trying to learn under what conditions the native fish reproduce well.”
He added, “Low water here reduces their habitat and produces smaller adult fish. We’re seeing adult roundtails that are 6-7 inches when they should be 13-14 inches.”
The heavy net was hauled to shore, filled with flood debris: plant mulch, wood, deer droppings, sand and mud.
A dire problem began to become obvious.
Most of the inch-long bonytail chub were dead. So were the young suckers and non-native fish, including bullheads and smallmouth bass.
“They’ve been dead for a while,” Crockett observed. “There’s no color to them.”
But a few were found alive in the muck of the net, and the two biologist went to work measuring, identifying and writing the data on forms. A GPS reading was taken of the location. The small fish were returned to the water but would probably not survive.
Some were saved for later research, including genetic analysis.
“We want to know if the native fish are specific to certain drainages and tributaries or are more wide ranging,” Jones said. “Genetics can help us get to that answer.”
Their populations cannot be restored in hatcheries, he added. “They reproduce and spawn best in the wild.”
Non-native bullheads, with their needle sharp spines, were tossed on the bank. Smallmouth bass, an aggressive non-native that competes with natives, also are discarded when possible.
Suffocating silt
The group continued on, passing the other team on the shore as they were counting fish from their catch.
What the team witnessed next revealed what was causing the die-off. As the researchers floated on the rush of muddy water, all around were fish rising to the surface, gasping and writhing.
“Their mouths are agape,” White noticed. “They’re trying to get air because the can’t absorb oxygen into their gills from the water. The silt is so thick it’s suffocating them.”
Soon hundreds of dead fish were seen floating, and in between, others were in the thrashing throes of imminent death. Team members were covered in a crust of mud that did not wash off. A hand dipped in the water came out covered in a slimy, thick, brown slime.
Crocked examined a dead chub. Its gills, which should be pink, were brown.
“They’ve evolved to handle murky waters, but these conditions are overwhelming them,” he said. “You know it’s pretty bad if the bullheads are dying. I’ve never seen it this bad, and it is a serious concern. Now we have a better understanding of what they are going through during these heavy rain events.”
The biologists see the problem as two-fold: A lack of water limits critical dilution effects, and there is an unnatural build-up of silt due to infrequent flushing flows.
The lower Dolores is dam-controlled from McPhee reservoir, and only so much flow is allocated for the downstream fishery, known as the fish pool. Biologists release 31,900 acre-feet for the fish, stretched out over the entire year. During normal years, natural flows from tributaries keep the flow at a healthy level, but during a drought, shortages starve the river of water.
“It’s a consequence of not enough water,” White said. “During a flash flood event on Disappointment, the surge of debris-filled water flows into the Dolores River, but there is no water to help dilute the surge of silt-laden water.”
Compounding the problem, the buildup of sediment in the main channel of the Dolores is picked up and suspended in the current, creating an unusually thick flow of mud.
Rafting releases limited
In even an average winter, the lower Dolores gets a decent flush when rafting flows are released in spring and early summer.
But since 2001, there have been only six years when water was released for rafting, with the most recent in 2011. The lack of regular flushing flows is causing an unnatural build-up of sand and sediment and is a threat to native fish and their habitat, according to the CPW.
“The natural flush is important to regularly remove silt so when there is a flash flood there is not so much of it,” White said. “We did not fully appreciate the build up (of sediment.)”
More net sweeps proved fruitless, or turned up dead fish. The crew assembled and decided to cancel the rest of the day’s survey.
“We’re collecting more dead fish than alive. The nets are just trapping the mud in the water. It will be a challenge to analyze today’s data set, but we still learned something about the habitat conditions and what these fish go through when there is a flash flood event,” Crockett said. “Flow management from the dam is a big part of a healthy fishery here.”
Floating out, the reporter stalled out in an eddy and examine some of the dead fish bobbing in the water. Their mouths were frozen open, a sign of desperation, suffocation. Minnows, bullheads, suckers, chubs and bass of various sizes floated by the boat, belly-up, drowned in the chocolate-colored water.
Ahead the younger staffers swam and body-surfed the rest of the way out under sunny skies, laughing and rough-housing in the muddy current, a juxtaposition to the stark demise of a fishery struggling to survive all around.
“It is just another challenge for the native fish here,” Crockett said.
For boaters, the frequent rain deluges are a blessing. It is possible to run the 36 miles of canyons from the Gypsum Valley boat launch to Bedrock, but a straight-through run would be advisable.
“You could do it, but I wouldn’t stay overnight. If the level drops back to a trickle you would be a long way from nowhere,” said fish tech Nathan Jones.
