Colorado health officials are tracking toxic algae blooms with warnings of health impacts 38%
By Shannon Mullane6%
7/18/2026, 9:42:00 AM
BS Summary: This article contains 19 faulty reasoning types, including Recency Bias, Appeal to Authority, and Availability Heuristic, with Post Hoc (False Cause) as the most egregious example at 16.7% saturation with 157 hits. Analysis detected 921 faulty-reasoning hits from 941 analyzed words, generating a BS Score of 44.1% and a BS Rank of 38% (10,980 of 17,596 articles). This article is better (less manipulative) than 62.40% of the article peer group.
Colorado public health officials are tracking nine algae blooms in reservoirs around the state, prompted in part by this summer’s hot temperatures and low water levels.
Colorado officials warned of a busy season for toxic algae earlier this summer, saying they might show up earlier than usual after a record-poor winter ended a month early.
Not all blooms produce the toxins that can harm people’s health, kill fish and shut down recreation at reservoirs.
But when in doubt, stay out, the Colorado Department of Public Health and Environment warns.
“Exposure typically happens through ingestion.
So dogs and young children are typically the most at risk because they just tend to swallow more water,” Sarah Erickson, CDPHE water quality specialist, said.
“There can also be skin exposures, so that’s where we see the rashes.”
Algae are natural parts of aquatic ecosystems, but when conditions are right, their growth can explode into what’s known as a bloom.
The algae blooms often form a thick, green layer on the surface near shorelines, like foamy pea soup floating on a lake.
These blooms can be unsightly or cause taste and odor issues that affect drinking water and recreation.
Several types of algae, like cyanobacteria, can produce harmful blooms that affect public health.
These blooms vary in color, often appearing blue-green, red, gold or bright turquoise.
(Toxic algae is typically not stringy or mustard yellow in color.
That’s likely pollen.)
Toxic algae can affect the liver, skin and neurological systems, according to CDPHE.
Health impacts vary by the type of toxin or exposure, but symptoms can include skin irritation, gastrointestinal upset, fever, headache, sore throat, muscle and joint pain, and liver damage.
Swallowing water and direct skin contact are the most common means of exposure.
Algae blooms, toxic or not, can also suck up dissolved oxygen under water and kill fish, Erickson said.
But, and this is key, people can’t tell by looking at the algae whether it is producing toxins, said Erickson, who has spent much of her summer sampling lake water and investigating reports of blooms.
“Algae is common and natural in Colorado, so we see algae everywhere,” she said.
“There are just certain types that can produce toxins.
They don’t produce the toxins all the time, so we have to use laboratory testing to determine if the actual algae is producing toxins.”
“A busy season so far”
This summer’s conditions have set the stage for rapid algae growth.
Algae love warm, stagnant, shallow water and higher concentrations of nutrients like nitrogen and phosphorus, which can be found on farms and in household products.
Temperatures have been unusually hot this summer, which heats up lakes, rivers and streams.
This spring started with a record-smashing heat wave in March that was 13 degrees Fahrenheit over the norm from 1901 to 2000, according to the Colorado Climate Center.
April was warmer than usual by 3 degrees, May was near normal and June was 4 degrees above normal.
After a record-low snowpack this winter, the spring snowmelt was earlier and less than usual.
That meant less water to refill reservoirs before the summer irrigation season, when many of Colorado’s lakes are drawn low for farms, ranches, lawns and communities.
Colorado’s statewide water storage is at about 65% of its norm for this point in the season.
Blue Mesa Reservoir, the largest in the state, is at historic lows.
Announcements about reservoir closures and limitations on fishing, boating and recreation have been coming out regularly.
“We’ve had a busy algae season so far for sure.
We’ve seen a lot of low water levels throughout this state,” Erickson said.
“Those low water levels can help to contribute to algae blooms.”
Algae blooms come and go, and conditions can change rapidly.
Boaters can often get out on the water and avoid boating through visible algae scum.
Fish can usually be eaten if cleaned properly with potable water.
Visitors should pay attention to any posted warning signs and follow local instructions, CDPHE said.
This summer’s algae blooms
The state doesn’t track all water bodies for signs of algae.
CDPHE and Colorado Parks and Wildlife monitor state parks and select wildlife areas during peak toxic algae season, May through September.
CDPHE also samples a selection of lakes monthly from July to September.
This season’s blooms of green slime started in May with a nontoxic bloom in Cherry Creek Reservoir that resolved within three weeks.
In June, several appeared, some of which are ongoing.
Out of 118 lakes and reservoirs, nine have either toxic or nontoxic blooms.
Here’s the list as of this week:
* **Deweese Reservoir**: The algae bloom at this reservoir, part of a state wildlife area in south-central Colorado, was first reported June 24 and was still producing toxins as of Wednesday.
Visitors should not enter the water and obey posted signs.
* **Jackson Reservoir**: This bloom was first reported June 8 and has produced toxins intermittently.
The reservoir, located in northern Colorado and popular for its swim beach, did not have toxins as of Wednesday, but CDPHE still advises people to obey posted signs and avoid entering the water.
The following lakes and reservoirs have had a potential bloom reported or may be waiting for test results about toxicity.
Visitors should proceed with caution and obey signs, CDPHE says.
* Barr Lake, northeast of Denver
* North Sterling Reservoir, northwest of Sterling
* Martin Lake, south-central Colorado near Walsenburg
* Cowdrey Reservoir, northern Colorado near Cowdrey
* Lake John, northern Colorado near Cowdrey
* Vega Reservoir, western Colorado near Palisade
* Windsor Lake, town of Windsor
Analysis
Hover over highlighted words in the article to view the associated bias or fallacy analysis.