982 lakes and rivers with a history of harmful algal blooms, coloured by the latest entry in each state's official data. Checked 24 October 2026. Open a state map for county lines and filters.
Highest level (Danger, high toxins or above EPA level)Warning, confirmed bloom or health advisoryCaution, precaution, suspicious bloom or bloom seenBloom reported or low toxin levelNo bloom in the latest entryBigger dots have more years with blooms.