BOULDER, Colo. — Children and pets have been told to avoid multiple Colorado lakes due to potentially dangerous algae. 

Boulder residents are advised to keep their children and animals out of Wonderland Lake in north Boulder and a similar warning was issued for Quincy Reservoir in Aurora. 

Sloan's Lake is the only body of water in Denver that has tested positive for the algae. Signs are posted there warning people not to swim or wade in the lake, though health officials said they haven't heard of anyone getting sick from it. 

Thunderbird Lake in southeast Boulder also has reports of blue-green algae. Swimming and wading are not allowed there.

Blue-green algae is a bacteria found in non-flowing freshwater that can be fatal to animals if ingested. The algae naturally occurs in aquatic ecosystems and can appear rapidly – especially during the summer with hot weather and in slow-moving water bodies, such as lakes, according to the Colorado Department of Public Health and Environment. 

RELATED: Blue-green algae: What it is and how to spot it

Boulder said in a release Friday that it will install signs in the Wonderland Lake area that remind visitors to follow current regulations that prohibit swimming, wading and boating at the lake and to keep their dogs out of the water. 

RELATED: Woman's dog dies shortly after swimming; here's how water is tested

Fishing in the lake is allowed, but the city asks visitors to carefully follow state recommendations to clean any caught fish thoroughly and discard guts appropriately. 

RELATED: Algae blamed for contaminated drinking water in Greeley

According to the Colorado Department of Public Health and Environment, harmful algae blooms often have the following characteristics: 

  • May look like thick pea soup or spilled paint on the water's surface. 
  • Can create a thick mat of foam along the shoreline.
  • Usually are green or blue-green, although they can be brown, purple or white. 
  • Sometimes are made up of small specks or blobs floating just at or below the water's surface

Harmful algae blooms are not:

  • Long, stringy bright green grass strands that feel either slimy or cottony.
  • Mustard yellow (this probably is pollen).

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