So when Muhlfeld, the fish expert, looked at where bull and cutthroat trout were spawning eggs, it was always on gravel-bed streams where underground seeps brought fresh oxygen to the egg redds and flushed away the waste that might suffocate the embryos.

When Proctor checked his grizzly satellite collars in spring, the locations were always in floodplains where bears were grazing on the first plant shoots.

When Hebblewhite plotted his wolf den locations, they all fell within short distances of river bottoms. So did most of the kill sites where they took down elk. Three weeks ago on a trip to Yellowstone National Park’s Lamar River Valley, he and Locke watched a wolf take an antelope fawn to a den near Slough Creek, just off the gravel-bed floodplain.

“There are other flat places they could be, but they’re not,” Hauer said. “They’re on the floodplains. The floodplain is the arena. It’s not just a corridor where animals go up and down.”