The grand canyon, pictured, was created over millions of years as the colorado river cut through the land. what is this process called?

The process in question is called erosion. It happens when water and other natural things like wind cause this to happen.

erosion

Explanation:

The erosion is process by which the river acts as a tool to cut between the lands and it creates a landscape that are formed by the millions of years. The Colorado rover is one of such river system that has developed and formed the great grand canyons in the state of Arizona and is a masterpiece of the works of rover processes.