LOS ANGELES -- Less winter runoff has allowed visitors to enjoy mostly clean water at California beaches, according to a report released Thursday.

An estimated 93 percent of the 445 beaches along the state's coast that were tested for bacterial pollution from Memorial Day through Labor Day had good to excellent water quality.

It was the fifth consecutive year that beachgoers have seen incremental improvement in beach water quality during summer months.

The annual beach report card, issued by the Santa Monica-based environmental group Heal the Bay, studied bacteria levels found in water samples taken by local agencies.

Heal the Bay attributed the improved water quality to extremely low rainfall in Southern California and infrastructure upgrades. Lack of rainfall means less urban runoff into the ocean.

Most bacterial contamination occurs during winter, when heavy rains overload storm drains and sewage systems, washing pollutants into the sea. Swimming in polluted waters can cause ear infections and gastrointestinal problems.

The results bode well for the upcoming summer beach season after a drier-than-normal winter in Southern California.

Among the state's 10 beaches with the worst water quality, four were in Los Angeles County, the report found.

Testing showed the dirtiest beach was at the city of Avalon on Santa Catalina Island off Southern California. The environmental group said it expects water quality to improve once Avalon completes an upgrade of an aging sewer system that has been blamed for persistent high pollution levels.

The second-most polluted beach in the state was Cowell Beach in Santa Cruz County followed by Poche Beach in Orange County.

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Report: Most California beaches had clean water in 2012