As the Colorado River flows across the Colorado Plateau high country into northern Arizona, the serpentine bends of the channel carve a 277-mile canyon that is more than a mile deep, and, at its widest point, 18 miles across. Established as a national park by President Woodrow Wilson in 1919, Grand Canyon remains one of the most popular parks in America and has been named one of the Seven Natural Wonders of the World.

With more than nine million annual visitors, The Great Smoky Mountains National Park is America's busiest. It features a forested landscape brimming with countless waterfalls, mountain streams and Blue Ridge views. The Appalachian Trail winds along the spine of the park, passing Clingmans Dome and the observation tower where tourists take in panoramas. Travelers often take a welcome break in the nearby town of Gatlinburg, TN.

Yellowstone, the world's first national park in 1872, is 3,500 square miles – most in Wyoming, with some acreage in Idaho and Montana. Created by President Ulysses S. Grant in 1872, this geothermal wilderness site is home to bison, elk, gray wolves, grizzly bears and countless species of flora. Old Faithful, the highlight geyser, erupts every 91 minutes and is visited by more than three million people a year.