Saturday is Earth Day, a day when we celebrate and reflect on the health of the planet we all call home. And that got us wondering about a famous piece of the earth in the city that we call home: The Charles River, the inspiration for the song Dirty Water. It got us wondering just how dirty the water of the Charles really is. Edgar B. Herwick III, the guy behind GBH's Curiosity Desk, joined GBH’s Morning Edition hosts Paris Alston and Jeremy Siegel to discuss. This transcript has been lightly edited.

Jeremy Siegel: So, Edgar, the Charles River — much maligned. But as we come upon Earth Day 2023, I mean, how dirty is the Charles?

Edgar B. Herwick III: It's less dirty than it was in the '50s, which is when swimming was first restricted, and that was due to pollution. It's less dirty than it was in 1966 was when The Standells released that song.

Paris Alston: Jeremy did a deep dive, speaking of water, into this last year, by the way.

Siegel: They had never been to Boston when they wrote the song.

Herwick: And their producer Ed Cobb, had been mugged in Boston, which was part of the inspiration for doing this. He wrote the song. He also wrote "Tainted Love," by the way. So it's less polluted than it used to be. If we look at it now, the EPA has been giving a report card to the Charles River for decades. So this is sort of a good measuring stick in essentially assessing its cleanliness. There's lots of ways to look at how clean is a river. Their measure is like basically, how many days of the year is it theoretically swimmable and boatable, safely? So if you look back to the mid-to-late '80s, the start of this report card, the Charles River was a C and D student. It was not a great student. Its report card did not look very good. But in more recent years, they've now sort of split this report card out. They measure five different segments of the river, give each a grade. And in the last few years it's mainly now an A and B student for most of those segments. The one segment of the river that continues to lag a little bit is the Muddy River part. And even in 2022, that was up, from a D to a C, So that's improving as well. So generally not as dirty as it used to be.

Alston: Okay. Well, what did it do? I mean, well, not the river itself, but what happened to improve its cleanliness?

Herwick: Well, I mean, it's a number of things: Awareness, effort and money. I mean, that's kind of essentially it. So you can't understate the importance — we're talking about Earth Day, but these things that happened on the heels of that very first Earth Day in 1970: The creation of the EPA, the passage of the Clean Water Act, and the creation of an entire framework, legal and regulatory, for environmental protection issues. None of that existed when the first Earth Day happened. So that has been huge. I also spoke with Laura Jasinski, who's the head of the Charles River Conservancy. They're a nonprofit focused mainly on the lower Charles and its health, and the health of the environment around the river. And she talked about how infrastructure improvement, which has cost millions of dollars from lots of places. And they have done a better job of separating sewer drains from storm drains. So that was a huge issue for a long time in that when you'd get a lot of rain, the storm runoff would mix with sewer water and that would all end up in the river. And so you need an infrastructure where that doesn't happen. They've done a better job of that. And then there's been cleanup of the bank's areas. There's been efforts to manage the land near the river, and newer efforts, like a pilot program that they launched a few years ago where they've created a floating wetland in the lower basin.

[Previously recorded]

Laura Jasinski: By using basically an artificial island planted with native wetland species, we can add back some biodiversity to the sea wall edges that you see along Memorial Drive by MIT or down East Cambridge. And by adding back some of that biodiversity you can help absorb some of that pollution, and also add a lot of habitat value. That's really important for kind of the overall health of the river.

[Recording ends]

Herwick: So generally speaking, her overall assessment — Here's what she said:

[Previously recorded]

Laura Jasinski: We are definitely much better off than we were a couple of decades ago. But we also have new challenges with climate change that we need to address.

[Recording ends]

Siegel: Well, what are the challenges that climate change does pose?

Herwick: There are numerous, but there are two main ones. One is runoff from the ground level during storms. So if you think about the lower Charles in particular, you have Memorial Drive, you have these roadways right near it. When we have a big rain, what happens is water from the ground level can't get absorbed by the ground and it ends up flowing all the way down into the river. And it carries with it all of the pollutants that we're putting at the street level. So there's all kinds of stuff that is getting into the river. So as we know, climate change means big rain events are happening more frequently. And so they're trying to figure out ways to keep that water from running off down into the river. So that's one big one.

The other one is with rising temperatures comes increased likelihood of outbreaks of cyanobacteria or algae bloom. We see this, many years there'll be an outbreak of algae bloom in the river. And that stuff, not always, but it can sometimes be dangerous for pets, dangerous for people. And so trying to figure out ways to, as the temperatures rise with that happening more often, make the river a little bit more strong against that happening. So those are the two main sort of things happening right now in terms of climate change.

Alston: So as you mentioned earlier, one of the measures of whether the Charles is healthy is whether it's swimmable. And so, can you swim in it?

Herwick: So technically, no, it is prohibited. It remains prohibited unless you get a proper okay from the state.

Siegel: But is that because it's dirty or no?

Herwick: Right. And that's the thing. No. So in the past, like in the '50s, when they said no swimming in the Charles, it was mainly because of pollution. Now, most days, it is technically safe to swim in it. But the problem is we don't necessarily have the infrastructure for it, right? So you need a place to be able to get into the water safely. You don't want to hit boats, all of that stuff.

Gary Liversidge, of Boston, swims in the Charles River during the "City Splash" event, Tuesday, July 18, 2017, in Boston.
Elise Amendola/AP Photo AP

Alston: Yeah. Not jumping off of a burning MBTA train. Well, that wasn't the Charles.

Herwick: It was the Mystic. But The Charles River Conservancy does do, once a year for the last seven years, they do something called City Splash. And they've just announced that this year, that is happening on June 17. And that is a day where they get all the permissions from the state and then they let you swim in the Charles in a safe spot. So they sort of mark out an area and you can go swimming in the Charles on June 17. One of their goals, as we look at the next 20 years, is they want to try to figure out a way that people can swim in the Charles. They're looking for ways to find, through the summer, a place where they can establish that says, here's where you can swim. And hopefully in the future we will be able to swim in the Charles most days all summer.

Siegel: For now, June 17, if you don't hear the three of us on the air, you can figure out why.

Herwick: You can guess where we are.

Siegel: Yeah. So, Edgar, we've made a kind of biweekly tradition of having a little trivia time with you as our question master when you're in here. I think we probably have time for one question.

Herwick: Well, since we're talking about the Charles, let's stay with the Charles. So your question is: I think we, most of us, know that the Charles empties out into the Boston Harbor, and that's where it goes. But where does it begin? Your question is, do you know where the Charles River begins?

Siegel: Okay, so rivers usually begin from mountains or something like that. Wachusett Mountain?

Herwick: No. Do you want a hint?

Alston: Yes.

Herwick: You could describe, if we think about where it begins, you could describe its journey to Boston as a marathon.

Alston: Hopkinton?

Herwick: Hopkinton, Massachusetts, is where the Charles River begins. And unlike the marathon, which goes 26.2 miles, it goes for 80 miles. That's how meandering the path is. And it goes through 23 different communities on its way to Boston Harbor. But yes, just like the marathon, it begins in Hopkinton.

Alston: Well, look at you bringing all the knowledge we did not know that we needed. I'm so glad.