The Crested Butte News Serving the Gunnison Valley since 1999
Water quality in Gunnison River and Tomichi Creek decline
After a yearlong study of the insect life in the Gunnison River and Tomichi Creek, Western State Colorado University professor Kevin Alexander showed the Gunnison Board of County Commissioners data that suggested the aerial application of the pesticide permethrin in 2012 and the aquatic insect mortality that followed resulted in some significant changes to the river system.
At a meeting Tuesday, August 26, Commissioner Phil Chamberland recalled playing golf at Dos Rios on the day the pesticides were being sprayed from an airplane passing back and forth over the city and the river corridor. “I remember looking up and thinking that wasn’t a very good idea and that maybe I should go inside,” he said.
Shortly after the pesticides were applied ahead of the Fourth of July holiday in 2012, Alexander started getting reports of large numbers of dead stoneflies collecting in eddies and appearing in the drift.
He said it would be hard to definitively link the spraying that was conducted as part of the county’s mosquito control plan to the changes in insect life and water quality that were observed on both rivers with such a short-term study. But the early indications were pretty clear.
Alexander and Corbin Bennetts, a Western State student who led the research effort for nearly a year, showed the commissioners their analysis suggested a link between the aerial spraying of pesticides and a rise in the number “less desirable” midges floating in the current, or drift, and fewer of the mayflies, caddis flies and stoneflies that are further up the food chain and more beneficial to fish, birds and the riparian ecosystem.
As part of their research, Bennetts and other students maintained nets that collected insects drifting in the current in areas upstream and downstream of the places where pesticides had been applied. They then counted the number of insects collected.
The researchers even coordinated with the company contracted to apply pesticides on the ground with truck- and backpack-mounted spraying equipment to run tests while spraying was taking place.
Bennetts said permethrin has a relatively short half-life and binds easily with soil particles in a way that prevents most of the pesticides sprayed on ground level from ever reaching the river system. As a result, the ground-based application of pesticides, which has been used exclusively every year since 2012, didn’t seem to have an adverse impact on aquatic insects.
“When we did the truck spraying, we saw no difference. We couldn’t detect any change. So we didn’t see anything there. But that was based on a one-time no increase in drift,” Alexander said. “But we did see an overall decline in the baseline of macro invertebrate-based water quality indicators in both Gunnison River and Tomichi Creek the year after the aerial spraying was conducted.”
Alexander explained that there have been a number of studies over the years that show the first aquatic insects to be impacted by poor water quality “are those big, long-lived insects and those indicators are based on the number of mayflies, stoneflies and caddis flies. If your midges go up and those go down that’s a bad sign.”
In the case of midges, a rapid life cycle means they can rebuild their ranks quickly after a short-term change in water quality causes widespread mortality. Insects that take longer to grow will take longer to repopulate.
“What’s really going on there is the midges will come in really quick when there’s been a disturbance in the past … now it’s dominated by a slightly less desirable group of organisms,” Alexander said, adding that high numbers of midges also indicate there are fewer of the larger, carnivorous aquatic insects to eat them.
