America's Most Incredible National Parks. The sheer size of Arizona’s Grand Canyon can overwhelm the senses: It’s 277 river miles long, up to 18 miles wide and a mile deep. Upon visiting the area in 1903, President Theodore Roosevelt, an avid outdoorsman, advocated for its protection, saying, “The ages have been at work on it, and man can only mar it. What you can do is to keep it for your children, your children's children, and for all who come after you, as one of the great sights which every American should see.” Grand Canyon National Park was established on Feb. 26, 1919.