Every year in the early summer, a group of Wyoming ranchers drive their cattle high into the Rocky Mountains, to the largest federal grazing area in the United States. The Green River Drift is an old-fashioned horseback cattle drive - in some cases, the livestock travel 100 miles - but the threats it faces are modern: roads, drilling rigs, government-protected grizzly bears, vacation homes, and the waning interest of a younger generation in this arduous and communal enterprise.