Long Lake in Lacey closed for swimming on Friday after health department officials detected a toxic algae bloom.
Algae toxin levels were measured at 21 micrograms per liter of water, which is more than three times the state standard for recreational use.
The lake will remain closed until algae levels measure at or below six micrograms per liter of water.
People and pets are asked to avoid contact with water that contains algae and avoid activities like swimming, jet skiing, kayaking, and paddle boarding.
Boating is still permitted, but boaters are asked to avoid areas with visible algae scum.
Long Lake Park will remain open.