A deep valley with cliffs on both sides is called a canyon. Sometimes a large river may run through a canyon. The Grand Canyon in Arizona, USA is the world’s largest canyon. It was formed by the Colorado River. Millions of years ago, this region was covered by sea. Slowly, a part of the sea floor was pushed up to form a plateau. Over the years, rainwater collected to form a river. This river cut into the rocks on the plateau to form the canyon.