This public pressure, combined with new technological cleanup advances, is changing some cities’ waterways so quickly that they may soon be unrecognizable. In fact, this transitional moment might be the most intriguing time to explore such areas, especially for anyone who loves the hidden and ignored corners of cities where few dare to venture — those weedy, quiet, eerily beautiful abandoned spots that, just a few years from now, might be jam-packed with backstrokers and jet skiers. “It’s a very urban experience,” says Jim Burns.

Burns is a fisherman who lives in Los Angeles and “can be at a mountain stream within 25 minutes.” But more often than not, he chooses to fish in the L.A. River, a brutalist, concrete-lined waterway that offers an unusual — and unusually peaceful — refuge from the city’s throb. “I’d lived in Los Angeles for 34 years and didn’t even know there was a river,” says Burns. Three years ago, he and his son heard about it and decided to find it. “It was an incredible hassle. L.A. is lots of concrete and freeways. We would spot the river and try to find access to it, and you’d end up behind a Target or something, and the parking lot would be right up against the river with barbed wire and No Trespassing signs.” {…}