Today, in the deepest part of the Grand Canyon, the Colorado River flows past rocks that are 1.7 billion years old. As described in this video segment adapted from NOVA, the erosive action of water in the river is responsible for deepening the main channel, but other forces, in particular debris flows, caused the widening of the canyon. Gathering speed and volume, piles of loose rock and soil that originate in a river's tributaries merge to form powerfully erosive debris flows, which are capable of dramatically affecting the surrounding terrain.
© 2014 PBS & WGBH Educational Foundation. All rights reserved.