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Potentially toxic algal mats have been identified in multiple Sonoma County waterways, and officials with the North Coast Regional Water Quality Control Board are warning those who boat, fish, or swim in the affected areas to exercise caution.

The discoveries were made this week in both the Russian River and Santa Rosa’s Spring Lake.

The harmful algae blooms are caused by algal or cyanobacteria that grow floating in water, though some grow under the water at the bottom of waterways and can form colorful algal mats that range from bright to dull green, orange, brown and maroon, the water board said. These mats can become detached and end up stranded on banks.

Water quality officials said people should avoid touching or inadvertently ingesting the material, especially children and dogs, who are particularly susceptible to serious health impacts from the contact. Dogs who have come into contact with the blooms should be washed immediately. People who have ingested the material should see a medical professional and make sure to tell them about being exposed to a cyanobacteria.

Sonoma County will be posting “Toxic Algae Alert” signs at affected recreation areas. Swimming and wading at Spring Lake are prohibited. However, the water at the swimming lagoon in Spring Lake Regional Park is treated and therefore safe, county parks officials said.

The county and water board will be posting updates on the situation online. To report a bloom, people can call 844-729-6466.