On April 29, hundreds of paddlers will race kayaks, canoes, and stand-up paddleboards in the Charles River for the annual Run of the Charles.

That the water quality in the river has dramatically improved over the 36 years since the race first began is a tribute to the non-profit group that organizes the competition, the Charles River Watershed Association, and to its executive director, Robert Zimmerman Jr., who will retire this summer after 28 years of leadership.

The river’s cleanup is a significant accomplishment. It’s a big deal not only to those of us in Eastern Massachusetts who enjoy this particular river. At a moment when the global and national environmental — and, for that matter, political — news headlines often seem apocalyptic, this local success story has the potential to inspire other victories.

Mr. Zimmerman says that when he arrived in 1990 to take over the small nonprofit, the river “was a dump.” Skeptics said it wasn’t worth trying to fix because it had always been that way. On rainy days, stormwater and sewage combined and overflowed into the river.

Nowadays such overflows are more rare and involve much less wastewater. The Charles, which flows 80 miles into Boston Harbor, is now widely recognized as a sparkling gem, sometimes even called the “cleanest urban river in the country.” Day campers swim in it in the summer. The shores teem with turtles and herons.

The story of the Charles departs in significant ways from the stereotypical frameworks often used by activists and journalists. The worst polluter wasn’t some greedy corporate profiteer — instead, it was a regional government agency in charge of sewage for mostly liberal and Democrat-dominated cities, including Boston, Cambridge, Brookline, and Newton.

The solution came in part from Republican leadership and pressure. President Nixon signed the Clean Water Act. The Reagan-era Environmental Protection Agency joined a lawsuit against Massachusetts for illegal sewage discharge into Boston Harbor. Vice President Bush, in the context of his 1988 presidential campaign against Governor Dukakis, described Boston Harbor as “the filthiest harbor in America.” Governor William Weld made a well-publicized 1996 plunge into the river.

Human beings, rather than being just villains trampling on what would be pristine wilderness without them, can be positive environmental forces. The rowers and paddlers who use the Charles for sport and recreation are a constituency for a clean river.

The Charles is cleaner than it was 30 or 40 years ago, but it still has challenges. There’s not enough water in the river, in part because rainwater finds its way to the harbor instead through the sewage system and its leaky pipes. “Pipes don’t leak out, they leak in,” Zimmerman explains. There’s also too much phosphorus, which puts the Charles at risk of eutrophication; last summer a cyanobacteria bloom briefly closed parts of the river to paddlers.

The water flow and phosphorus levels are tracked by the Charles River Watershed Association’s monitoring program. Mr. Zimmerman, the son of a chemist, made the CRWA into not only an advocacy group, but a research science organization.

The cleanups of the Charles River and Boston Harbor have more than one hero. The improvements were spurred largely by litigation. One suit was brought against Massachusetts in 1982 by the mayor of Quincy, a city on Boston’s South Shore, after the city solicitor, William Golden, stepped in untreated sewage while jogging on the beach.

Another lawsuit was brought in 1983 by Peter Shelley, a lawyer representing the Conservation Law Foundation, another New England nonprofit. A federal judge, David Mazzone, famously ruled that “the law secures to the people the right to a clean harbor” and presided over the case for 19 years.

An executive director of what became the Massachusetts Water Resources Authority, Paul Levy, built on Deer Island in Boston Harbor a sewage treatment plant that reduced the overflows. Millions of ratepayers shouldered the costs, which ran into the billions of dollars.

The government and politicians were part of the solution. Had it all been left up to them, though, toilets might still be flushing, in essence, directly into the Charles and on into Boston Harbor. The river is cleaner today thanks to the vision and dedication of individuals like Mr. Zimmerman and mediating institutions like the Charles River Watershed Association.

At the end of an hour long interview in Mr. Zimmerman’s office, I suggest that he must feel good about what he achieved. Meaningful lives come in lots of different varieties. As professional careers go, turning a filthy river into a treasure is something tangible. In improving the water quality, Mr. Zimmerman has demonstrated something else pretty important, too: what can be done if you don’t listen to the people who say it can’t be done.