For most of the Charles River’s civic history, bathhouses and beaches crowded its shores. Places like Magazine Beach in Cambridge, Mass., and the Esplanade in Boston offered recreation and relief for swimmers on the hottest days of the year. But in the mid-1950s all of that changed. Due in part to an antiquated runoff system that integrated both stormwater and raw sanitary sewage, the river had become too polluted for swimming.

Efforts to clean up the river commenced in the 1960s, but only recently have those efforts begun to pay off. According to research by civil and environmental engineering associate professor Ferdi Hellweger, the river is now swimmable 70 percent of the time. The challenge is knowing which 70 percent, said Hellweger, whose team is developing a computational model to forecast river water quality.

Such a system would be indispensable if the Charles River Conservancy gets its way: The environmental advocacy group hopes to soon restore regular swimming to the river. On Saturday, Hellweger participated in a historic step toward realizing that goal when he joined nearly 150 local residents, young and old, to plunge in the river’s waters for the first community swim in more than half a century.

Like many of the participants, Hellweger was pleasantly surprised by the experience: “It was wonderfully refreshing, surprisingly so. When you are on the Esplanade in a heat wave, you don’t realize relief is just a few feet away,” he said.

The event marked the first in a multi-year roll-out plan that will eventually provide regular access at multiple sites throughout the summer, said Hellweger, who has partnered with the CRC on a number of water-quality research investigations.

Perhaps the biggest challenge to making the Charles a recreational hotspot is residents’ perceptions. Boston’s unofficial theme song—“Dirty Water” by the Standells—sums it up. “It doesn’t even occur to people to swim in it,” said Hellweger. “It’s been more than 50 years.”

But with public awareness events like this weekend’s, Hellweger and the CRC hope that will eventually shift. In 2006 the Stony Brook Sewer Separation project modernized a vast portion of the water run-off system.

Since then the surrounding communities have persistently worked to identify illegal sanitary discharges into the storm sewer system, including one that Hellweger’s team helped discover. A whole apartment building was discharging its sanitary sewage into the storm sewer, from where it entered the Muddy River, and then onto the Charles. Together, these efforts have cut pollution by 99 percent, said Hellweger.

Despite these improvements, public perception remains dour, unsurprisingly given the river’s cultural position in the city.  “But once you realize it’s amazingly pleasant, people value it as a resource,” said Hellweger. He plans to swim again next year and every year after that.