Could that really be possible? After all, the Charles is "that dirty water" in the famous Standells song .

If you've been desperately trying to stay cool in Boston's broiling heat lately, imagine this: you put on your bathing suit one steaming summer day, you go to the Esplanade, you lay down a beach towel by the Charles River — and you jump in.

The Boston Harbor cleanup is one of the state’s great success stories. But the harbor’s health is directly tied to the Charles River, which flows into it. In the fourth report of our summer series, “ Looking Out: A New View Of Boston Harbor ,” we check up on a long-standing effort to create what may seem unthinkable: a swimmable Charles.

“I’ve heard people say that their eyes used to sting just standing next to the Charles."

"I've heard people say that their eyes used to sting just standing next to the Charles," says Frans Lawaetz.

Lawaetz has gotten used to failure. A few years ago, he started an annual swim in the Charles that's supposed to happen each summer near the Hatch Shell. But the first swim was canceled due to an algae outbreak. Last year it was called off because of too much rain, which can cause bacteria contamination. And this year — yet again — it was canceled because of rain. [This year's swim has been rescheduled to Sunday, July 25.]

Still, Lawaetz has a vision of restoring the Charles to the way it was when parts of the river in Cambridge resembled Coney Island.

"If you look at pictures of Magazine Beach back in the day," he says, "there's whole families that are down there swimming around. It looks just like a beach."

What the Charles shouldn't be, he insists, is just a pretty thing we stare at from the shore.

"I mean, you have people that, they don't even look at the river as being a part of nature because it's something they don't interact with; they just gaze across it," he says. "So to have this big chunk of nature flowing right through our city, and to be able to immerse yourself in it — I think we owe it to ourselves to make it happen."

Some people do swim in the upper Charles, in places such as Dover and Needham. But the lower part of the river, in Boston and Cambridge and Watertown, is a different — and dirtier — story.

Now, the Charles is a lot less filthy than it used to be. That's due to an ambitious cleanup launched in 1995. The goal then was to make the Charles swimmable by 2005. Two-thousand-five. Have you seen many swimmers in the Charles in the last five years? Nope. Because they're still not allowed.

Northeastern University graduate student Xiaodan Ruan dips a sampling bottle into the Charles River. (Jeff Carpenter for WBUR)

Cleaner Water, Higher Grades

One trouble spot is where the Charles River meets Boston Harbor.

Kate Bowditch stands at the dam where the river and harbor converge, right near North Station and the Zakim Bridge. She's a scientist at the Charles River Watershed Association and says this could be a sensational place for swimming.

The water is slow-moving and relatively shallow, so it warms up quickly, like a big lake. But those warm temperatures also encourage toxic algae growth. And when dirty street run-off collects here, there's no constant natural flushing to clean it out. Bowditch says what's bad for the Charles is also bad for Boston Harbor.

"The river discharges its whole flow into the harbor," she points out. "So whatever pollution we have in the Charles, all ends up in Boston Harbor."

When the Environmental Protection Agency first started grading water quality in the Charles 15 years ago, it gave the river a big fat D. Now it gets a B-plus. Bowditch says people actually could safely swim in the Charles on most summer days.

"Nowadays it's clean enough that people are kayaking and canoeing and tipping over and nobody worries about it," she adds. "You don't have to go get a tetanus shot or anything else."

But one heavy rain can change that by sending pollution gushing into the river.

"It's the runoff from the roads and the parklands, geese poop, dog waste, stuff that's in the storm drains," explains Karen Patterson Greene, who work for the Charles River Conservancy as what's called its "swimmable Charles coordinator."

Greene says another problem is that sometimes, like last weekend, when heavy rains overwhelm the sewage system, untreated wastewater is released directly into the river. So better storm water management is badly needed.

Sampling And Seeking Utopia

Until then, it's important to test the levels of all these contaminants to figure out how consistently the Charles is safe enough to swim in.

On a dock near the Esplanade by the Arthur Fiedler statue, a Northeastern University graduate student pops open the lid of an empty water sample bottle. Then, wearing a crinkly, shoulder-length plastic glove, she dips her arm a foot deep into the Charles to scoop up a sample.

Northeastern is working with the Charles River Conservancy to do daily water quality testing this summer at potential swimming sites along the river. Greene says if all this testing shows that the river's water quality is high enough, there could one day be public swimming at Magazine Beach or the MIT Sailing Pavilion or North Point Park in Cambridge or right here near the Hatch Shell.

"This is a beautiful dock, as you can see," Greene says. "It wouldn't take a whole lot to rope off a swimming area, bring out a lifeguard once a week and let people experience that."

If that ever happens, Frans Lawaetz's big dream of bringing back swimming to the Charles might not seem so Quixotic any more.

"I just, I really hope that I get to go across the Longfellow someday and look out and, in addition to seeing everyone sailing and windsurfing and kayaking, that there's also people that are swimming," he says. "Yeah, it would be pretty Utopian to me."

And since a healthier Charles means a healthier Boston Harbor, swimming in the river would be a Utopia to lots of other people, too.