Denver Parks and Recreation is saying a fish die-off that occurred last weekend and left at least 2,000 putrefying carp and crappies floating on the surface of Sloan’s Lake has subsided.
Toxic algae blooms caused by consecutive near 100-degree days in Denver spread over certain sections of the lake and “sucked oxygen” out of it that fish need to survive, Parks and Recreation spokesperson Stephanie Figeuroa said in an email Tuesday.
Water temperatures in the lake reached a high of at least 80 degrees over the weekend, Figueroa said. On Monday it registered 79.9 degrees, she said. Cold water from the Sloan’s Lake Park irrigation system was pumped into the lake in an attempt to cool the water, stop the algae bloom and save the fish. The die-off continued until Tuesday, when Figueroa said the department added “increased flows from the Rocky Mountain Ditch” that dropped the temp by another 0.2 degrees. By then, Figueroa said the dying stopped.
Sloan’s Lake Park sits in the Sloan’s Lake neighborhood in Northwest Denver. The lake is used for everything from water skiing to swimming to fishing and dog walking around its 2.6-mile perimeter. The popular Dragon Boat Festival has occurred there for 24 years, as it did last weekend. Figueroa said the festival “had no effect positive or negative on the fish kill. In fact the fish kill was isolated to the southeast end of the lake in Coopers Bay, away from the dragon boats, and no dead fish impacted their event.”
She added the sight of so many fish floating lifeless on the water “can be alarming,” but that last weekend’s die-off was “nothing new” in water bodies in urban settings.
Toxic blooms in recent history
A similar die-off occurred in 2020, when extremely low water levels coupled with hot weather caused thousands of fish to die in Sloan’s Lake, which averages 2.5- to 3-feet deep across and 5 feet deep in places, Figueroa said.
John Michael, a spokesperson for the Department of Public Health & Environment water quality division, added other blooms in 2019 (6), 2020 (12), 2021 (20), 2022 (11) and 2023 (20) to the list.
Michael said “it’s always difficult to predict what each toxic algae season will bring,” but that they usually run from July 4 to October.
“We did have some early blooms in June, so that may be an indication of a busy season ahead,” he added. And if temperatures like Colorado has seen over the past couple of weeks keep up, we can expect a bigger impact.
The Sloan’s Lake Park Foundation keeps close tabs on the lake, as a citizen-founded organization that wants to ensure “the environmental health, sustainability and beauty of one of Denver’s top urban natural assets.”
It says the lake is suffering “like many lakes in Colorado” from shallow, stagnant water “and a century of human development causing tons of runoff from parking lots, roads, etc. flowing into the lake in the form of sediment.”
Twenty-three storm drains and the Rocky Mountain Ditch funnel this polluted water into the lake. The foundation says the majority of existing sedimentation was caused by rapid urbanization within the South Platte Watershed prior to the Federal Clean Water Act of 1972, and that the impact has lessened with “much of the watershed fully developed and urbanized.”
But Kurt Weaver, the foundation’s spokesperson, said “if you drive around the neighborhoods in the greater Sloan’s Lake area, you still see lots of construction dirt that flows directly and unfiltered into Sloan’s Lake. That coupled with increasing temperatures has an inevitable outcome, and that is the deterioration of the lake.”
Weaver credits the parks and recreation department for doing all they can to keep Sloan’s Lake as healthy as possible, but says, “they are fighting a losing battle. The lake’s demise is imminent if we do not act to dredge it back to a healthy depth and improve the water flow,” evidence of which was on display last weekend.
Don’t drink oily, turquoise waters (duh?)
Toxic algae blooms are common in lakes and reservoirs across Colorado, and the physiological effects from exposure to them can range from mild to deadly. Shireen Banjeri, a clinical toxicologist and director of the Rocky Mountain Poison Center, said these vary based on how long someone submerges in bloom-infected water or how much of it they accidentally drink.
“If they just jumped in the lake and then they felt some itchiness on their skin and got out, we would consider that a minor exposure,” she said. “If they swam for a couple hours, they could have more prolonged effects, but I would still expect that to go away within 24 hours.”
Certain strains of the toxic algae tend to look like oily paint poured on the surface of a lake or pea soup on the water, and are red, orange, turquoise or other green-adjacent colors. And they can cause gastrointestinal symptoms like vomiting, diarrhea or abdominal pain if ingested.
“Or you could have itching, sore throat, maybe watery eyes or burning eyes,” Banjeri said. Accidentally drink enough, and you could have liver or neurological failure. And because dogs ingest a lot more water than humans when they’re paddling around in lakes, Banjeri says the effects can be exacerbated.
But there’s only so much any agency can do to keep people and dogs out of the algae.
Avoiding blooms is on you
Figueroa said her department didn’t prohibit access to the park or the lake, and likely wouldn’t, “just because some folks can’t follow certain guidelines or suggestions.”
The city of Denver employs rangers who patrol daily from 3 a.m. until 10:30 p.m., by foot, bicycle and motorized vehicles, the 20,000 acres of city and mountain parkland, around 250 individual urban parks and about 850 miles of paved and off-street bike trails.
“So if a ranger is [at Sloan’s Lake] and they see someone swimming, they’ll tell them you should not swim in the water and, probably, get out,” Figueroa added. “Then they’ll point them to the signs about the health effects that could happen.” And between messaging from the Colorado Department of Public Health and Environment and parks and rec to let people know when blooms are around, “we can get it pretty much under control, and get the right signage out and and keep people away,” she said.
Colorado Parks and Wildlife deals with toxic algae blooms in waters across the state, and they use their own signage to try and get their message across. On Tuesday, they reported a bloom in DeWeese Reservoir in Custer County. They said they’ve put up signs warning visitors to keep kids and pets out of the water, refrain from swimming, skiing, paddle-boarding and wading and that drinking the water could result in death.
Oddly enough, “You can still fish during an algae bloom,” wrote Justin Krall, CPW’s wildlife manager for the district DeWeese is in. But “it’s important to take care when handling and cleaning any fish caught in DeWeese,” because toxins accumulate in the liver and guts of fish, they said.
On Wednesday, Figueroa said 99.9% of the dead fish had been removed from the lake and disposed of. But Weaver countered her, saying, “if you walk the lake, there are still 400 just floating there, and that’s after they took out bags and bags of them.”
Figueroa said she hadn’t visited the lake personally but that her “team is certain there are no more dead fish.”