"There will be a first flush that will take all the contamination and debris from the sidewalks and streets and pick up all sorts of bacteria from there," Griesbach said. "That will go into the creek and come right out the outlet. It's pretty typical that those wet-weather flows do a lot poorer."
According to Heal the Bay, runoff can contain toxic heavy metals, pesticides, fertilizers, animal waste and human sewage. Potential health risks from swimming in contaminated ocean water include stomach flu, ear infection, upper respiratory infection and skin rashes.
The data in the annual Beach Report Card is collected from weekly water samples that measure the levels of three types of bacteria in ankle-deep water at the shores.
The beaches at the end of Bluebird Canyon Drive and Hotel Laguna also received among the lowest grades, with Cs during wet weather. Griesbach said people are advised not to swim in the ocean anywhere for 72 hours after rainfall.
"To have poor water quality during wet weather is pretty normal," she said.
Griesbach said some poor water quality issues are caused by things like sewage leaks, but others are just caused by normal runoff from the streets. She said people can help improve water quality by doing simple things like cleaning up after their pets.
Heal the Bay recommends people take eco-friendly steps like using a broom to sweep driveways instead of hosing them down with water, switching to reusable grocery bags and using compost instead of chemical fertilizers in yards and gardens.
Overall, Orange County beaches scored well, but even during dry weather, some did poorly. North Beach Doheny and Poche Beach, in San Clemente, scored Fs during dry weather, making the report card's "beach bummers" list of the top 10 worst-scoring beaches in the state.
Heal the Bay also posts weekly scores, so beachgoers can check up-to-date water quality info at beachreportcard.org.