The Idaho Department of Health and Welfare has issued a health advisory for Brownlee and Hells Canyon reservoirs following the detection of high levels of toxin-producing cyanobacteria in recent water samples. The advisory, issued Monday, warns residents and visitors to avoid contact with water in both reservoirs due to the presence of harmful algal blooms. This marks the third Idaho body of water to face such restrictions this month, following an earlier advisory for Crane Creek Reservoir in Washington County.
What the Advisory Means for Water Users
State health officials strongly recommend against swimming, wading, or allowing pets to enter the affected reservoirs. Boiling or filtering water from these bodies does not eliminate the toxins produced by cyanobacteria and may actually concentrate them, making consumption even more dangerous. Residents should avoid drinking or cooking with water drawn directly from Brownlee or Hells Canyon.
Fishing remains permitted, but the department urges anglers to wash their hands thoroughly after handling fish. Fish should be cleaned completely before preparation, as cyanotoxins can accumulate in fish tissue. People who consume fish from the reservoirs should rinse and cook them properly to minimize risk.
Health Risks and Symptoms
Contact with water containing cyanobacteria can trigger adverse health effects in both humans and animals. Common symptoms include skin rashes, hives, diarrhea, and headaches. Pet owners should be particularly vigilant, as dogs can become ill more quickly than humans following exposure. The department recommends washing pets immediately with clean water if they exit the affected reservoirs.
Because harmful algal blooms are not always visible from the water’s surface—sometimes appearing as mats, foam, or surface scum, and sometimes remaining submerged—residents cannot rely on appearance alone to determine whether it is safe to enter the water. The department urges all Canyon County and Treasure Valley residents to carefully evaluate current water conditions before any recreational activity at any lake or reservoir in the region.
Why This Happens
Cyanobacteria are naturally occurring microorganisms native to Idaho’s watersheds. During summer months, rising temperatures fuel increased cyanobacteria growth, creating blooms that can persist through the end of the season. The current advisories reflect seasonal conditions typical of this time of year, though the concentration of toxins in multiple reservoirs underscores the importance of water monitoring and public awareness.
Residents concerned about water quality at other regional bodies of water, including potential pollution threats to the Snake River, should stay informed through official state health department announcements and local water agencies.
What to Do
The Idaho Department of Health and Welfare recommends that all Idahoans check current advisories before visiting any lake or reservoir. Those who experience symptoms after water contact should seek medical attention and report the incident to local health authorities. The department will continue monitoring water conditions throughout the summer and issue updates as conditions change.