"All of my field work was for this one picture."
Joe Riis, wildlife photographer
Of the roughly 900,000 Rocky Mountain elk in North America, some 20,000 migrate through the Greater Yellowstone Ecosystem.
As snows begin to fall each autumn, pronghorn gather and begin their trek back to their Upper Green River valley winter range. They must reach and cross the hydrographic divide between the Green and Gros Ventre rivers before snow blocks their way.
Their migration route, now known as the "Path of the Pronghorn," has been protected as a wildlife corridor.
People didn't realize that mule deer migrated 300 miles round trip, until scientists in Wyoming discovered the "Red Desert to Hoback" migration route.
Elk are built with stout, muscular bodies and durable hooves, so they can migrate. This bull elk was photographed moving through Eagle Pass, on the south boundary of Yellowstone National Park, as fall turned to winter.
The natural world presents many challenges along these hooved mammal's migration routes. Here, a cow elk swims through the South Fork of the Shoshone River while it is pulsing from spring snowmelt.
In addition to natural challenges, animals migrating through the Greater Yellowstone region also face manmade obstacles.
Fences are one of the key barriers to migrating pronghorn as they approach towns and cities along their route, including Cody, Pinedale, and Jackson, Wyoming.