Fern Ridge Reservoir is under a recreational use advisory as of Wednesday.
The Oregon Health Authority said a partner agency reported a harmful algae bloom there.
Cyanotoxins are not absorbed through the skin, but OHA said people shouldn’t swim or power boat in areas with algae blooms, because water or spray could potentially be inhaled.
Drinking the water in affected areas is especially dangerous, and children and pets are at high risk for illness.
OHA also recommends not eating fish caught in areas with cyanotoxic blooms.
The agency is waiting on water testing results, and will either confirm the advisory or lift it in the coming days.