Boston's Charles River open for swimming for 1 day only

Why it matters: It's a state-sanctioned swim meant to demonstrate the progress made toward making the Charles consistently swimmable, a goal of the city, state and federal governments for decades.

• The river has improved from D-grade water quality in the '90s to B+ levels today.

Organizers see City Splash as proof of concept for year-round swimming access, just like Paris's recent Seine River swimming initiatives during the Olympics.

• The French capital reopened swimming in the once-polluted Seine last week.

What they're saying: "You get in the river, you're looking at your skyline from the water while you're bobbing up and down, and want to be able to do it more often, and you have a different relationship with the river," Laura Jasinski, executive director of the Charles River Conservancy, told Axios.

• Jasinski sees the annual event as proof that swimmers really do want to open up the Charles to more than boating.

The conservancy envisions permanent swimming infrastructure somewhere along the riverfront.

• That would require buy-in from the state to operate a swimming facility with lifeguards and bathrooms, as well as ongoing water quality testing and safety precautions.
• Just like other public beaches, swimming would be off limits on the Charles if bacteria levels were found to be too high.

For now, swimming remains prohibited outside permitted events like City Splash — not because of water quality, but because there's no permanent swimming infrastructure or lifeguards.

Dive in: Registration slots for the City Splash event sold out within hours of opening.

• Adults 18 and older registered for 25-minute swimming time slots in a designated deep-water area off Fiedler Dock on the Esplanade.
• The event will include food trucks, vendors and activities for swimmers and those not quite brave enough for the plunge.
• More than 1,300 swimmers have taken a dip in the Charles since the first event in 2013.