It’s a familiar sight from the boardwalk of Washington Harbor – on a summer scorcher, a handful of friends diving into the Potomac River from boats nearby, water skiers wiping out near Memorial Bridge, or families tubing near Roosevelt Island, one or two brave swimmers briefly dipping into the murky depths of the river.

“What are they thinking?” whispers an onlooker from one of the outdoor bars. “That’s an hour-long shower right there.”

But when conditions are just right, one to three days after rain and runoff drains into the river, the Potomac is clean enough for swimmers to stay in the water as long as they please, a fact flying in the face of urban myths and longstanding beliefs held by countless denizens of the District.

“It's definitely safe to swim in the Potomac on a perfect summer day,” said Phillip Musegaas, vice-president of programs and litigation for the Potomac Riverkeeper Network. “If it hasn't rained very much in the last few days beforehand, absolutely.”

According to Musegaas, the District lacks a coordinated, efficient system to monitor river water quality, leaving residents without a clear picture of when it’s safe to swim.

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Cell phone video from this weekend’s rain storm appears to show a water quality alert beacon out of order. Two lights near the Watergate complex are supposed to flash when sewage flows into the river, events that occur when rain runoff mixes with untreated waste.

“Swimming right now, definitely a bad idea,” Musegaas said. “But up to three days after a rain event, you'll be fine if you're in the river for a few minutes, or for a couple of hours.”

Few have wanted to go anywhere near the river for decades – the Potomac was largely seen as an afterthought even in the city’s planning from the early days of the Republic.

But 40 years after the Clean Water Act and more attention from the Environmental Protection Agency, the river may not be pristine, but it is clean enough for swimmers who want to escape the region's steamy summer days.

“We encourage people to just know the last time it rained, and to go out and enjoy the river,” Musegaas said. “It’s one of our most precious resources, and I think more and more people are now just beginning to realize it.”