Oregon health officials are warning residents to stay out of the Willamette River after they confirmed the presence of toxic algae.
The Oregon Health Authority said in an advisory that they recommend cordoning off a stretch of the river that flows to Sauvie Island, a popular beach spot.
"Drinking water directly from this stretch of the Willamette is especially dangerous at this time," the advisory said.
A preliminary test by the Oregon Department of Environmental Quality confirmed the presence of a toxic blue-green algae called Microcystis aeruginosa.
"The type of algae produces toxins that are harmful to humans and animals," the advisory explained.
When swallowed by people, the algae can cause numbness, tingling, dizziness, weakness and diarrhea.
Children and pets are especially at risk. Dogs could die within an hour of consuming the contaminated water, officials said.
Officials say public drinking water will not be affected.
Health officials do recommend, however, that people be careful when eating fish from affected waters. They also advise against eating freshwater clams.
Toxic algae blooms can occur when certain kinds of algae grow in response to changes in chemicals in the water. A similar algae bloom left 400,000 people in the city of Toledo, Ohio, without drinking water in August.