A stretch of the Animas River south of Durango has turned fluorescent green from an algae bloom.
The growth is most pronounced downstream of the South Durango Sanitation District treatment plant, 434 Trestle Lane, about a mile south of Home Depot, and stretches several miles downstream, past the Weaselskin Bridge on La Posta Road (County Road 213).
Shirley Isgar, who lives about a mile south of the wastewater treatment plant, said she and a fisherman noticed the algae growth early last week. A dead fish, about 16 inches long, was lying belly up Tuesday on the riverbed behind her house. Isgar has lived there for several years, and her mother has lived there for decades, and they have never seen the river taken over by algae quite like this.
“The river is always low this time of year, every year – always low, never green, never algae growth like this,” Isgar said.
San Juan Basin Public Health was unaware of the algae growth when reached Tuesday afternoon by The Durango Herald. Brian Devine, water program manager for the health department, visited the site that afternoon and confirmed the algae bloom.
The Southern Ute Indian Tribe, which monitors that section of the river, said algae grows naturally in rivers and streams across the Four Corners, including the Animas River during sustained low-flow and clear-water conditions.
It didn’t respond to specific questions about the most recent growth and whether it presents a health hazard for humans or wildlife.
Algae blooms can occur naturally or as a result of human activity. Some algae blooms can deprive fish of oxygen or be a sign of degraded water quality.
Several factors can contribute to algae growth in rivers, including high temperatures, low river flow, plenty of sunlight and an influx in nutrients contained in fertilizers and wastewater discharge, Devine said. Often, it is a combination of all four factors that play a role, he said.
The health department was unaware of any discharge problems or anything else being done differently at the South Durango Sanitation District treatment plant that may be affecting water quality, he said. The plant is permitted to release effluent, and it’s possible nutrients contained in that discharge are contributing to the growth, he said.
“With the water level as low as it is, even if they’re doing everything right, the nutrients that they’re adding to the system may be contributing to algae growth,” Devine said.
The river was flowing below 250 cfs on Tuesday. By comparison, the median flow is 375 cfs and the average flow is 500 cfs this time of year, Devine said.
“It’s not necessarily that anybody is doing anything wrong or dumping nutrients into the river inappropriately,” he said. “It does happen this time of year. The water is below-average flows right now.”
The health department hasn’t notified New Mexico or downstream water users of the algae growth, saying the bloom appears to be localized, and the only major water user in the area – the East Mesa Ditch – pulls water from above the highly concentrated area of algae.
“I don’t have any health concerns at this point,” Devine said Wednesday.
Any reports of dead fish are “more concerning,” he said.
Operators for the Durango South Sanitation District said they were unaware of the algae bloom below the treatment plant. They also were unaware of any mishaps or issues at the plant that may be causing algae.