Colorado Parks and Wildlife is warning residents and their pets to use caution and avoid contact with water at Stagecoach Reservoir after accumulations of blue-green algae were discovered last week.

The wildlife agency temporarily closed a section of the reservoir last week after samples collected Sept. 2, “indicated hazardous levels of toxins specifically in Morrison Cove with lower levels of toxins found at four other test locations within the reservoir,” according to a news release.

“Algae blooms are common at Stagecoach State Park,” said Craig Preston, Stagecoach State Park manager. “Because it is an annual event, we installed educational signs several years ago and we test the algae once a week all summer long. Additionally, we send samples to the (U.S. Environmental Protection Agency) once a month for further analysis.”

Preston added that “hazardous levels of toxins” are rare at the reservoir while “low levels are not uncommon late in the season. But we have not seen hazardous levels since 2014, the first year we started testing for toxins.”

As of Sunday, Stagecoach Reservoir is one of four water bodies in the state listed under a “warning” for toxins detected at high levels. Before the water body was built in 1989, planners warned that algae blooms fed by nutrients could be a long-term issue to contend with.

Toxic algae exist naturally in Colorado waters but become an issue when they multiply rapidly to create a “dense cyanobacteria concentration” or “bloom,” according to the state Department of Public Health and Environment.

Certain water bodies in the state are monitored and tested for toxic algae, which can make water look green, turquoise, gold or red.

The blooms can also appear “like thick pea soup or spilled paint on the water’s service,” according to the state’s public health agency, or can be made up of “small specks or blobs” floating at or just below the water’s surface.

In addition to weekly and monthly testing performed by CPW and the EPA, the Upper Yampa Water Conservancy District also contracts with a consulting firm to run monthly testing in five locations around Stagecoach Reservoir between May and November.

CPW said Friday that park staff are encouraging visitors “to still avoid any activities in the water where algae has accumulated and elsewhere around the reservoir.”

“Be especially mindful when walking pets near the reservoir, as blue-green algae can be toxic to pets. Keep your dog leashed at all times. Do not let them drink the water, or lick or ingest dried scum along the shoreline,” said CPW.

“If accidental exposure does occur, do not let your pet lick their fur or paws. Rinse them thoroughly with potable water and wear gloves for your own protection, if possible,” the agency added.

According to CPW, algae are an important part of the aquatic food system, but some types of blue-green algae are capable of producing toxins that may cause negative health impacts for humans and pets at elevated concentrations.

If individuals or their pets are exposed to an algae bloom, they should contact a medical care doctor or a veterinarian if they experience nausea, digestive stress, breathing problems or unexplained illness, the wildlife agency said.