Two Denver area lakes are closed indefinitely after health officials found toxic levels of blue-green algae in the water.

Rocky Mountain Lake — located at 3301 West 46th Avenue in Denver — closed Thursday after recent testing found toxic levels of algae around the shoreline, the Denver Department of Public Health and Environment said in a statement on social media.

“This algae can make people and pets sick, so stay safe and stay out of the water,” Denver health officials said, adding that warning signs had been posted around the lake.

Recent routine testing at Lake Arbor in Arvada also revealed blue-green algae was approaching toxic levels, forcing the city to close the lake indefinitely Thursday, Arvada officials said in a news release.

Arvada officials will continue to test the water for levels of blue-green algae and — once it’s safe — said they will reopen the lake for fishing and non-motorized boats such as kayaks and small paddleboards.

Body contact with the water at Lake Arbor is not permitted, Arvada officials said.

The number of algae blooms will increase as Colorado’s climate becomes warmer, according to previous reporting. The blue-green algae found in the lakes are naturally occurring and an important part of the ecosystem, but the blooms can produce toxins if they grow big enough.

Harmful algae looks like thick pea soup or spilled paint with a green, red, gold or turquoise color. They also often have foam or scum.

Symptoms of exposure to toxic levels of algae include skin irritation, diarrhea, stomach pain, fever, headache or a sore throat. The toxins can be fatal to pets, like dogs, that swim in the waters.

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Originally Published: July 12, 2024 at 9:16 a.m.