Mt. St. Helens

One thing that hasn't been mentioned (and faith24 does not seem to know) is that the walls of "canyons" carved in mud or soft sediment form relatively shallow-angle walls (on the order of 45 degrees), while canyons carved on solidified rock have relative steep walls (often on the order of 90 degrees from horizontal).The Toutle River at Mt. Saint Helens in 1983: In 1985: (both from No webpage found at provided URL: Mount St. Helens, Washington: 1980 Debris Avalanche: 1980-Current, part of No webpage found at provided URL: Mount St. Helens, Washington: Photo Archives.)The Grand Canyon walls are generally pretty vertical (except for landslides) as shown by the photos already presented, but just to be complete here's the Vishnu Temple: Another important point is that meanders are only formed by relatively slow-moving water. There are no meanders in the Toutle River "canyon", but just upstream of the Grand Canyon, the Goosenecks: and Horseshoe Bend: Note the near-vertical walls.So we conclude that the Grand Canyon was carved in hard rock by a river moving much slower than the Mt. Saint Helens devbris avalanche.