The Santa Monica Pier area and other California beaches made the list of dirtiest beaches because they have poor water quality due to pollution.
Heavy rain causes sewage and other pollution to flow into the ocean through storm drains, making the water unsafe for swimming.
Some beaches, like those near urban areas or harbors, are more susceptible to pollution because the water doesn't flow away as easily.
Heal the Bay, a non-profit organization, monitors water quality and recommends avoiding swimming near storm drains and staying out of the ocean for a few days after heavy rain.
