Last year’s rainy summer brought a slight drop in water quality to Greater Boston’s beaches, but they remain among the cleanest urban strands in the nation, according to a new water quality report by Save the Harbor/Save the Bay.

“We’re lucky to live in the Bay State,’’ said Bruce Berman, director of strategy and communications for the group. “Clean water is a core family value here. And we should be pretty proud of ourselves this Memorial Day weekend.’’

In 2017, Boston-area metropolitan beaches were safe for swimming 94 percent of the time on average, down from an average of 96 percent in 2016, according to the report, which tested water at 15 beaches throughout last summer. The cleanest beaches were in South Boston; the dirtiest were on the North Shore and in Dorchester.

There are a million things that affect water quality, Berman said, but the primary driver is the amount, intensity, and frequency of rain. Stormwater carries high bacteria levels, and it can flush whatever is in the streets and sewage systems into the water. The summer of 2016 was unusually dry, and the summer of 2017 saw more than twice the rainfall during the sampling season.

Because the beaches around the city are generally pretty clean, Berman said, the rain did not have as serious an impact as it could have if it were coming down on dirty areas. And in fact, he said, the storm patterns from 2017 — which included three major storms dumping a full quarter of the season’s rain all at once — may have washed out the sewer systems.

The cleanest beaches were M Street and Carson in South Boston, which were found to be safe 100 percent of the time, continuing a years-long trend. Pleasure Bay in Castle Island and Nahant Beach were also found to be safe 100 percent of the time, up from slightly lower findings in years past.

Two of the most popular beaches south of Boston — Nantasket and Wollaston — showed slight drops in quality, but overall were rated very clean. Nantasket was swimmable 98 percent of the time in 2017, compared with 100 percent the year before. And Wollaston was swimmable 92 percent of the time in 2017, versus 93 percent in 2016.

Tenean Beach in Dorchester came in last, with a safe-to-swim percentage of just 81 percent.

That beach is in a tough spot, said Berman — it’s near both the Neponset River and the dog-friendly Victory Road Park. Save the Harbor/Save the Bay partnered with volunteers from Boston Properties and officials from the Department of Conservation and Recreation to remove more than a ton of dog waste from the park just a few weeks ago, Berman said.

The biggest surprise in a report that otherwise revealed year-to-year consistency in beach cleanliness was the spike in water quality at King’s Beach in Lynn and Swampscott, which has consistently lagged behind other area beaches since 2012. In 2017, it was swimmable 92 percent of the time, compared with 83 percent in 2016.

“It’s too early to say whether it’s an anomaly or it means something,’’ said Berman. But both communities have been doing some work to improve the beach, he said, and it’s possible they were able to fix whatever was causing the low water quality.

The test, said Berman, is not what the year-to-year numbers say, but what the average cleanliness of the beaches is over a period of years. And most of Boston’s beaches pass that test.

Evan Allen can be reached at evan.allen@globe.com. Follow her on Twitter @evanmallen.