Colorado Tracks Two Toxic Algae Blooms Amid Record Heat

The Colorado Department of Public Health and Environment is tracking two toxic algae blooms and seven other non-toxic blooms across state reservoirs as of July 15, 2026. This monitoring effort comes in response to a busy summer season driven by record-high temperatures and historically low water levels.

Public health officials issued warnings earlier this summer, noting that these blooms may appear sooner than usual due to a record-poor winter that ended a month early. While not all algae blooms produce the toxins capable of harming human health, killing fish, or shutting down recreational activities at reservoirs, the department’s guidance remains consistent: when in doubt, stay out.

Sarah Erickson, a water quality specialist with the Colorado Department of Public Health and Environment (CDPHE), explained that exposure typically occurs through ingestion. Consequently, dogs and young children are identified as the most at-risk groups because they tend to swallow more water during play. Skin exposure is another common vector, leading to visible rashes on those who come into contact with affected waters.

Algae are natural components of aquatic ecosystems, but under the right conditions, their growth can explode into what experts describe as a bloom. These often form a thick, green layer on the surface near shorelines, resembling foamy pea soup floating on a lake. While some blooms are merely unsightly or cause taste and odor issues for drinking water, several types of cyanobacteria can produce harmful blooms that directly affect public health.

The visual appearance of these blooms varies, often appearing in shades of blue-green, red, gold, or bright turquoise. Toxic algae is typically not stringy or mustard yellow in color, a distinction Erickson notes is likely pollen. However, visual identification alone is insufficient for determining toxicity.

“Algae is common and natural in Colorado, so we see algae everywhere,” Erickson said. “There are just certain types that can produce toxins. They don’t produce the toxins all of the time, so you have to look at it.”

The health impacts of toxic algae can affect the liver, skin, and neurological systems. Symptoms vary depending on the type of toxin and the method of exposure, but they can include skin irritation, gastrointestinal upset, fever, headache, sore throat, muscle and joint pain, and liver damage. Swallowing contaminated water and direct skin contact remain the most common means of exposure.

Beyond human health, algae blooms — whether toxic or not — can deplete dissolved oxygen underwater, leading to fish kills. Erickson has spent much of the summer sampling lake water and investigating reports of blooms to determine which reservoirs pose the greatest risk.

The current tracking effort highlights a complex picture for Colorado residents. With nine blooms currently under observation, the state is balancing the natural presence of algae with the specific risks posed by toxic varieties. The variation in color and texture does not always correlate with toxicity, requiring ongoing sampling to ensure accurate warnings for the public.

As summer temperatures persist and water levels remain low, the likelihood of continued blooms remains high. The CDPHE’s active monitoring provides a layer of security for locals, even as the visual landscape of the state’s reservoirs shifts with the changing seasons.

The water looks deceptively calm, but beneath that green surface, the chemistry is shifting, waiting for the next swimmer or dog to take a chance.