City Cast Denver logo

What You Need to Know About Blue-Green Algae Blooms

Posted on August 9, 2023   |   Updated on September 30, 2025
Peyton Garcia

Peyton Garcia

Blue-green algae floats on the water surface of a lake.

Blue-green algae floats on the water surface of a lake. (Nicolas Armer / Picture Alliance / Getty Images)

Long streaks of intense summer heat often bring with it toxic blooms of blue-green algae to local ponds and lakes. Skin exposure to contaminated water is dangerous to humans and animals. Here’s what you need to know:

What is blue-green algae?

It’s a type of algae (or cyanobacteria) that grows when water temperatures warm up. Though it's a native, naturally occurring piece of the ecosystem, when it grows out of control (known as a “bloom”) it becomes dangerous to humans and animals.

How do you know if a body of water is contaminated?

Blue-green algae blooms look like thick pea soup or spilled paint, floating on the surface of water. It’s most often a blue-green color, but it can be gold, brown, or reddish, as well. The state health department tests different lakes and reservoirs every year, and many individual municipalities test their own waters, but the general rule of thumb is: “When in doubt, stay out.”

What happens if you’re exposed?

Skin-to-water contact for humans and animals is dangerous, leading to symptoms such as skin irritation, diarrhea, stomach pain, fever, headache, and a sore throat. For animals, like your pup, exposure can be fatal. If you think you were exposed to algae toxins, contact your healthcare provider or poison control.

Will it go away?

Blue-green algae blooms will recede as the weather cools. But as Colorado’s climate continues to heat up due to global warming we can expect more and more blooms in summer.

🔎 Learn More About Blue-Green Algae

Share article

Hey Denver

Stay connected to City Cast Denver and get ready to join the local conversation.

Can't subscribe? Turn off your ad blocker and try again.

The latest in Denver

Sponsored

Best Online Casinos for Real Money You Can Trust Today

Find the best online casinos for real money with fast payouts, big bonuses, and top games. Compare sites and start playing now.

UpVenture Logo
Keep It LocalSeptember 24

Where to Find All the Pumpkin Home Goods

It's pumpkin season! Here are the best places in Denver to grab your next pumpkin-flavored and pumpkin-designed fall goods.

White cake inside a box with orange writing that says “Hello Fall!” and red and yellow flowers with green leaves made from icing.
Keep It LocalSeptember 24

Where to Buy Actual Pumpkins

The only thing better than buying all the pumpkin-themed things? Buying actual pumpkins. Here's a quick and easy guide to shopping for yo...

A pile of pumpkins sits on hay bales in orange, beige, and green heirloom varieties with a blue sky in the background.
Keep It LocalSeptember 17

Where to Work Remotely in Denver

Grab a latte and a pastry at Little Owl Coffee in the hotel’s lobby, then head up the winding stairs to the cozy seating area with couche...

The airy upper-floor space at the Populus Hotel in Denver with a wooden floor, an exposed wood-beam ceiling, and ergonomically designed gray chairs and brown couches. A winding staircase is to the left. 🍃 Populu
Keep It LocalSeptember 17

Skip The Usual: Try These Local Remote Work Gems

The Bardo Coffee House has a location in Wheat Ridge with the same work-and-study energy, albeit with more seating options and a greater...

Brown leather couch with marble table in front of it at Bardo Coffee House in Denver. A bookshelf is to the left. The window facing the street is in the background.
Keep It LocalSeptember 10

The Best Denver Businesses for Music Lovers

Whether you want to learn an instrument, gift lessons to a loved one, or rent equipment for a gig, these Denver shops will nurture your m...

Inside a music shop with six guitars and memorabilia on the walls, red and black seating, with a coffee table at center on top of a rug. A window to the left filters light into the room.
Keep It LocalSeptember 10

Why Denverites Love Swallow Hill Music

Swallow Hill Music is one of the largest music schools for acoustic instruments in the U.S. — but it’s so much more than a place to take...

Outdoor performers under a white tent perform on stage while concertgoers sit on green grass listening. A brick building is behind the concertgoers to the left, and string lights run parallel to the crowd and stage.
Keep It LocalSeptember 3

Why Denverites Love Leevers Locavore

Buy a drink from Queen City Coffee at the café inside Leevers Locavore, plus a pastry to keep you alert on the drive up.

 Sign outside of grocery store with a funny saying that reads, “Beer may not solve your problems, but neither will milk or water. Open 7 AM - 9 PM,” and blue sky in the background.