The final stretch of the McCloud River before it empties into the state’s largest reservoir is a place of raw beauty. On a recent morning, the river’s icy water, flanked by flowering dogwood trees and jagged rock formations, flowed fast and clean. This part of the McCloud is off limits to almost everyone except a few Native Americans and some well-heeled fly fishermen. Its gatekeeper is an unlikely one, an organization that also happens to be a hugely controversial player in California water politics.