National Geographic: 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...