A sunbather relaxes on a dock along the Charles River, where the first public swim in over 50 years was held. (Elise Amendola/AP)

When you think of the Charles River, a lot of things come to mind. But probably not, say, the backstroke.

However, that changed Saturday morning. It has been unthinkable for decades but members of the general public actually swam in the Charles near the Hatch Shell in Boston. Recreational swimming in the river has not been legal since the 1950s, when it was banned because the Charles was so polluted.

WBUR's Weekend Edition host Sharon Brody spoke to the president of the Charles River Conservancy, Renata von Tscharner, about the big swim.

Sharon Brody: First of all, today's madcap, zany high jinks — your idea?

Renata von Tscharner: Well, it has been a dream for 13 years to be able to swim in the Charles, yes.

We remember that the Charles has been a very polluted waterway. Now it's clean enough for at least today's swimming. How did that happen? What were the big factors in the transformation?

A lot of work and about $500 million have been spent on cleaning up that water. Much of it was to separate sewage from storm water and to separate those pipes so that the sewage goes into the island and we don't have raw sewage going into the Charles River. That probably is the biggest effort.

Yeah, I would say getting rid of the raw sewage into the river, that probably helped.

Yes it does.

The water still appears to have that yellow-brown color. Is that an indication of less than perfect cleanliness?

I call it tea-colored. So, I think we should think of the Charles River as swimming in tea. Because it traverses areas with a lot of leaves it has tannin in the water, but the color in the water does not give an indication of the bacteria. So that is not an indication that it's not safe to swim. But it is not safe to swim unless it is a sanctioned event like we have today, so it's not alright to just go and walk into the river, it's not alright just to jump in because there are many shallow spots. So, it is a celebration swim today but it's not time for anybody to jump into the river unless it's a special event.

What are among your biggest challenges in changing the public perception of the Charles River?

Well, there is the song about that. "Love That Dirty Water." So, whenever people hear that song, and there are many reasons in Boston, we love to hear that song often, whenever we hear that song we need to think, "Oh, this is a different era. Times have changed, the river is cleaner, we can swim in the water."

So, yes, perception is a big thing and the sediments are still a big obstacle to overcome. The sediments are very toxic. While the water is clean enough to swim, you do not want to touch the sediments. So we need to find technological solutions of how to handle that. You know, there's dredging which is expensive and messy, there's covering it up. Right now, for this particular swim, we're just avoiding it because the swim is in deep water and nobody will be touching the sediments.

Today's swim is for registered swimmers only. Do you anticipate doing more swims for the public in the future?

Yes. Today we have 144 swimmers registered. We are full. That's all we could take. But we have a wait list and we encourage people to sign onto this wait list. They can sign up on our website, thecharles.org, and then that means we have a reason to have other swims and they will be first when we have future community swims in the Charles.

Right. Well, Renata, are you swimming today?

Absolutely, yes. I grew up in Switzerland swimming in rivers and when I first immigrated in 1979 that was my first wish. I then became a volunteer water tester and then when I started the conservancy in 2000 that was the top thing on my agenda, to bring swimming back to the river.

Well, thank you and your Swiss roots.

Well, it's wonderful to bring this, to make that happen here on the Charles River.