Deadly blue-green algae concerns close Colorado lake

Blue-green algae

Photo courtesy of Colorado Springs.

Another body of water in Colorado appears to have blooms of a deadly algae and is forced to close to visitors, according to Forest Service officials.

Willow Creek Reservoir in Granby is showing signs of the toxic blue-green algae. Officials closed the reservoir Saturday to any activities that require contact with the water.

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Algae are an important part of aquatic food webs, but some types of blue-green algae can produce toxins that may cause negative health impacts for humans and pets at elevated concentrations. Currently there is no method to remove toxins from lakes.

The Colorado Department of Public Health and Environment recommends keeping children and pets out of water where the algae is blooming, not drinking untreated water, and avoiding contact with the algae. Young children and dogs are at high risk because they are most likely to accidentally (or intentionally) drink the water, health officials said.

Blue-green algae is, also known as cyanobacteria, is described as a photosynthetic bacterium that is common and natural to waters found in all states, including Colorado. When water temperatures are warmer (commonly 68 to 77°F) and extra nutrients are present most commonly in sunny yet still waters, these algae can multiply to form algal blooms.

Some of the algae can start producing toxic compounds called cyanotoxins.

Recreational exposure can occur through incidental ingestion of water containing toxins during activities such as swimming, wading, surfing, jet skiing, and waterskiing. 

For more information, watch this video created by Colorado Parks and Wildlife explaining Blue-Green Algae or click here to visit the CDPHE webpage on Toxic Algae.

Willow Creek Reservoir is west of Highway 34 between Granby and Grand Lake and is normally open to non-motorized recreation only. 

Leslie James is all about Colorado when it comes to writing features, sharing adventures, and creating colorful galleries. She loves camping, hiking, mountain biking and snowboarding. Leslie joined OutThere Colorado in November 2020.


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(1) comment


The supposed wise species continues to poison the things indispensable for life.

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