“I still have my doubts,” he said, watching as a freight train thundered overhead.

Residents remain cautious, too. To protest water pollution in February, a University of Richmond student floated the “James River UFO,” a disc made of feathers, downriver. And in April, the public utilities’ Twitter account warned: “Green water in the James tomorrow is a V.C.U. art student project. Don’t panic. Don’t call. Don’t worry.”

Bathers needed reassurance as well when shad were seen floating on the surface of the river, looking like so many textured brushstrokes of paint. As it turned out, biologists from Virginia Commonwealth University and state agencies were sending electric currents into the water, momentarily stunning but not harming the fish so they could measure the shad’s resurgence.

After almost 20 years of restocking and a fishing moratorium, the fish are a food source for the herons, now some 200 strong, with a rookery on an island in the river. Observant visitors — a record one million are expected this year — can spot the rookery by parking under the elevated train tracks and walking along a narrow gangway built atop a city water pipeline.

Ralph White, the manager of the James River Park System for more than three decades, has watched the river’s transformation from a dammed and divided symbol of Richmond’s Confederate past to a point of modern union. Because of mercury, the biggest catfish — some run to 50 pounds — are still not yet fit to eat, he concedes.