When George Washington chose to build his Mount Vernon estate along the Potomac River, he declared the then-pristine body of water “the nation’s river.” At the time, even dolphins were a common sight. In fact, as Karin Bruillard at the Washington Post reports, the porpoises were seen as far upriver as Alexandria, Virginia, in the 1840s.

But by the 1960s, the river that flows through the nation’s capital had lost its luster. Bald eagles—the national bird—struggled for survival on its shores. Dolphins had long since disappeared from its waters. The Potomac became overrun with algae, trash, human waste and pollutants. The nation's river became a “national disgrace,” as President Lyndon Johnson called it at the time.

Now, after almost 50 years of pollution control, clean-up and restoration efforts, researchers have catalogued well over 1,000 bottlenose dolphins living, mating, and even giving birth in the lower reaches of the river.

“People actually forgot that there were dolphins in the river because they hadn’t been seen since the 1880s and because the river was in poor condition, people weren't seeing them,” Melissa Diemand, spokesperson for Potomac Conservancy tells NBC4.

Over the past four years, researchers from Georgetown University’s Potomac-Chesapeake Dolphin Project have been cataloguing dolphins in the lake-like area where the Potomac joins the Chesapeake Bay, reports Bruillard for the Post. In 2015, they counted only 200 individuals. Now, population has reached more than 1,000 individuals in the area, with several small groups of 200 dolphins hanging out in the river. Some have even swam upstream within 50 miles of Washington, D.C.

But the most exciting development came in August, when researchers witnessed a dolphin giving birth in the river. It was one of only three times scientists have seen a bottlenose dolphin giving birth in the wild anywhere, reports Briullard.

Ann-Marie Jacoby, the assistant director of the Potomac-Chesapeake Dolphin Project, was following a group of 50 dolphins near Lewisetta, Virginia, when a cloud of blood floated to the surface of the river, according to a Potomac Conservancy blog post. Then she saw a “slightly bent and wobbly fin” surface near an adult dolphin. Without underwater video, Jacoby is cautious to claim she witnessed a birth right then and there. What she saw was evidence of a birth that certainly happened recently, given the size of the baby as well as its location in the river.

“We have been trying to understand why dolphins come into the Potomac River and the Chesapeake Bay,” Georgetown biologist Janet Mann, director of the project, tells Whitney Pipkin at the Chesapeake Bay Journal. “We see some very young calves and we see lots of mating behavior, but this is the most definitive evidence we have that they have their calves here.”

The team named the mother dolphin after Patsy Mink, the former Hawaiian congresswoman and co-author of the Title IX Amendment of the Higher Education Act, which requires equal access to educational employment and activities for women. The dolphin baby was named after Mink’s daughter, Gwendolyn.

Earlier this year, the public was invited by the Conservancy to name two other Potomac dolphins, reports Cory Smith and Christian Paz at NBC4 in Washington. The names Mac and Chessie were ultimately chosen. (Yes, Dolphin McDolphinFace was in the running.) About 600 dolphins in the area have also been named after political figures, including George, Lyndon, Pelosi and Zachary Taylor. The team has used up so many political names they are now moving on to other historical figures, including abolitionists.

The Chesapeake Bay Journal’s Pipkin reports that Helen Bailey at the University of Maryland Center for Environmental Science runs a phone app that lets the public log dolphin sightings from the area. Photographs are the most useful because the team uses sophisticated fin matching software to indentify individual dolphins and track their movements throughout the year. So far, citizen scientists have logged 2,700 dolphin sightings.

While bottlenose dolphins are one of the most widely studied marine mammals, researchers still don’t know everything about them. Pipkin reports that the team doesn’t know if the dolphins have been there all along, but went unnoticed, or if the cleaner water has drawn them back. The team hopes to interview local fishers to find out how long they’ve seen the dolphins in the area and also gather family stories or anecdotes from their parents and grandparents.

The team also wants to figure out where the dolphins come from. Currently, they believe there is a resident group of dolphins in the lower Potomac, along with two migratory groups that come to the area in the summer. Biologists hope that as the river becomes even cleaner and fish populations increase, the dolphins and other marine mammals will move farther upriver.

Due to pollution and sewage, people have been warned for several generations not to swim in the Potomac and Anacostia Rivers, the District’s main waterways. But Jacob Fenston at NPR reports that things have improved so much that D.C. may have swimming platforms in the Potomac by 2025. Currently, the city is working on a 2.6 billion Clean Rivers Project, which will separate its sewage system and storm run-off systems—and hopefully make the river even cleaner.