Swimming safety is usually determined by the concentration of bacteria in the water, and sometimes by the amount of certain algae that may produce compounds that can make humans sick. Bacteria is primarily tested by county health departments at public swimming beaches, as well as by select Riverkeepers, and displayed on the Waterkeeper Swim Guide Map/App.
As a general rule of thumb, don’t go swimming if the water looks unusual (overly green, red or brown). Avoid water contact for at least 48 hours after heavy rains. If you have open wounds or a compromised immune system, it is best to stay out altogether.