There's a stretch of the Los Angeles River in the Elysian Valley, roughly midway along its 51-mile route to the sea, where native willows rise from the east bank and arundo—an invasive grass—closes in from the west. If you're lucky enough to be out there in a kayak, and there's no train rushing past on the tracks above, you will hear something very strange in this city of millions: quiet.

On a recent evening, Omar Brownson, the head of the L.A. River Revitalization Corporation, leaned back in his red plastic kayak, closed his eyes, and listened to the soothing sounds of birds singing and water slipping over rocks.

Wait—Los Angeles has a river?

Well, sort of. Once upon a time,