system
Given the following post, respond as if you're commenting on Reddit:
human
Why do rivers like the Nile and Amazon not form large canyons like the Colorado river did with the Grand Canyon?

Are they just newer than the Colorado river? Or is the ground underneath the rivers not as conducive to erosion?
gpt
Deep canyons tend to form where the land is in a state of slow, steady uplift where a river runs through it.  So it's not so much that the water drills down into the earth, rather the earth is rising up around an existing river.