The Idaho Department of Health and Welfare has issued a health advisory for harmful algal blooms at two southwest Idaho reservoirs, cautioning residents and visitors to avoid swimming, wading, or any water contact at the affected sites. Recent water samples revealed high levels of cyanobacteria producing cyanotoxins, according to a news release issued Friday.
The advisory covers Grasmere Reservoir, located approximately 35 miles south of Bruneau on State Highway 51, and the Bruneau Arm of CJ Strike Reservoir, situated northwest of the town of Bruneau. Both bodies of water are in Canyon County and the surrounding southwest Idaho region.
Health Risks and Visibility Challenges
Cyanotoxins pose risks to people, pets, and livestock. The toxins can cause illness ranging from skin irritation to gastrointestinal and respiratory problems with prolonged or repeated exposure. A particular concern is that harmful algal blooms may not always be visible to the naked eye—they can appear as surface mats, foam, discolored water resembling spilled paint, or foul-smelling scum, but blooms may also form on the bottom of reservoirs undetected.
Cyanobacteria naturally occur in Idaho’s recreational water bodies. Rising temperatures can trigger the growth and toxin-producing activity of these organisms, particularly during summer months when conditions favor their proliferation.
What Residents Should Know
The Health and Welfare Department recommends that residents and visitors stay out of the water at both Grasmere and CJ Strike’s Bruneau Arm until further notice. Those who have had contact with water at these reservoirs and experience symptoms such as skin rashes, stomach upset, or respiratory irritation should seek medical attention and inform healthcare providers of potential cyanotoxin exposure.
Pet owners should prevent dogs and livestock from drinking or wading in the affected waters. Additional updates on the advisory status will be provided as testing continues.