Flash flood alters river flow through Gunnison Gorge
The quick-hitting storm that spawned what some Hotchkiss residents are calling a “mini-tornado” in that town on Aug. 19 also sent a wall of water flooding the Gunnison Gorge.
The flash flood, estimated at reaching more than 2,000 cubic feet per second, washed away campsites, changed several major rapids and possibly killed thousands of tiny rainbow trout.
No one can accurately say how much water sped through the Gunnison Gorge from the fast-moving storm, but the signs of severe flooding were obvious.
“I was down there last Wednesday, less than a week after the event, and there was a lot of impact from the storm,” said Edd Franz, outdoor recreation planner for the Gunnison Gorge National Conservation Area. “A couple of campsites in Ute Park were really affected by debris and Boulder Garden Rapid was significantly changed.”
The Gunnison River was flowing at 600 cfs prior to the flood.
A tributary that enters the Gunnison just below Boulder Garden Rapid pushed a wall of debris into the river, backing up water and temporarily making the rapid a lake.
“It slows the water (through the rapid) quite a bit,” Franz said. “Instead of being pretty challenging, for now it’s pretty easy. At least until the next storm.”
The sudden high flows likely impacted the fishery but how much won’t be known for some time, said Dan Kowalski, Colorado Division of Wildlife fisheries biologist.
“We’ve seen no indication of a massive fish kill,” Kowalski said, referring to stories circulating through the angling community. “There was certainly some dead fish immediately after this flood, but I have no idea the extent of the impacts this storm had on the fish population in the gorge.”
Kowalski said he will have a better idea of the population impacts after he finishes his annual fish survey in October.
It’s doubtful there was a complete kill, Kowalski said. However, the smallest rainbow trout likely were the most affected.
“The big impact probably was to this year’s rainbow trout fry,” he said. “It’s very, very bad timing for rainbow trout fry.”
Rainbow trout fry (about 100 millimeters, less than 1 inch long) are hit hard twice by the high flows.
One, the small fish, which hang out near the bank to avoid predation and find slow-moving water, can be killed by the high water velocities associated with the fast-rising waters and then stranded on the flood plain when the water recedes.
The small trout fry simply are too small and weak that soon after emergence to survive higher water velocities.
All signs indicate the flood came and went quickly.
“We figured it went from 600 (cfs) to 2,000 in about 15–20 minutes,” said Ben Olson, manager for Black Canyon Anglers near Austin. “It was pretty serious.”
There are no stream gauges in the gorge, and the main gauge near Delta registered a 1,000 cfs jump, but that gauge records only every 15 minutes.
It’s possible, said several sources, the height of the flood passed the gauge between readings.
Kowalski, extrapolating from the data available, said it’s entirely possible the river quickly jumped 1,000 to 2,000 cfs.
A story in Monday’s Montrose Daily Press, quoting a river runner who was on the river during the flood, said the river rose 4 vertical feet in less than an hour.
Mike Haas of Snowmass Village told the Daily Press that the river sounded like “a freight train on top of an earthquake.”
Haas could not be reached Monday.
“There’s a lot of uncertainty about what happened,” Kowalski said, referring to the lack of accurate river readings. “But it could have been 1,000 to 2,000 cfs and maybe even more.
“It was a massive flash flood.”
The timing of the flood was reminiscent of a similar flood that swept down the gorge in August 1989, Kowalski said.
That flood resulted in a 40-percent decline in the river’s fish biomass population, he said.
“We hardly ever see fish biomass decline so abruptly in one year,” Kowalski said. “By far, this was the worst possible time (the flood) could have come for the young-of-the-year trout fry.”
A couple of weeks earlier, Kowalski had stocked 20,000 whirling disease-resistant Hofer rainbow trout fry at Ute Park and another 97,000 fry in the river below Chukar Trail.
“This is very, bad luck for the newly stocked rainbow fry, but we can’t speculate on the full extent of the impacts until we collect more data,” he said.