Scooteney Reservoir between Connell and Othello has been closed to swimming because of toxic levels of Microcystis aeruginosa, which is considered especially dangerous for small children and animals.
The Bureau of Reclamation called the Benton-Franklin Health District to the site June 8 because of an algae bloom, according to a health district news release.
Staff collected samples for testing and ordered the reservoir closed Friday.
People and animals are exposed to the bacteria by ingesting water, the health district said. Symptoms can take 30 minutes to a day to appear. They include jaundice, shock, abdominal pain, weakness, nausea and vomiting, severe thirst, rapid or weak pulse and death.
People are asked to keep children and pets out of the water until testing shows the toxins are reduced to safe levels.