SYDNEY (NYTimes) - The annual beach pilgrimage during the height of summer in Melbourne, Australia's second-largest city, is threatened by an unsettling phenomenon: shores where the tides are tainted with excrement.

The Environment Protection Authority in the state of Victoria said Monday that heavy rains had caused fecal pollution to wash into Port Phillip from rivers, creeks and drains. It advised against swimming at 21 beaches because of poor water quality.

"It's poo in all its luxurious forms that is causing the problem," said Anthony Boxshall, the agency's manager of applied sciences, noting that the waste was coming from people, dogs, horses, cows, birds and other animals.

Fecal pollution can cause serious health problems, including gastroenteritis.

Boxshall said much of the waste had been washed down the 150-mile Yarra River that runs through Melbourne into Port Phillip, affecting the city's bayside beaches the most.

The agency, which takes regular water samples, rates beaches. A "good" rating means that the water is suitable for swimming.

"Fair" means that rainfall has affected the water. "Poor" means people should avoid contact with it. Local councils in the city typically post signs warning beachgoers against swimming when the water quality is poor.

Residents said that the pollution had deterred them from indulging in a favourite summer ritual.

"When the temperature gets above 86 Fahrenheit, Melbournians typically pack the family in the car with food and drink and spend the day at the beach," said Sam Riley, who lives in the city. "I was going to take my two young boys to the beach myself over the summer, but now I'm concerned about whether the water is clean."

Boxshall said any improvement in the beaches' water quality was uncertain as long as the rain continued. The agency says it usually takes between 24 and 48 hours for the waters to clear after the rain stops.

"It's not going to be great tomorrow, so I would think about not going for a swim if you don't want to get sick," Boxshall said.