Downcutting

Downcutting, also called erosional downcutting, downward erosion or vertical erosion, is a geological process by hydraulic action that deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor.

A concurrent process called lateral erosion refers to the widening of a stream channel or valley.

This happened in the case of the Colorado River in the western United States, resulting in the process that created the Grand Canyon.

Lake bed downcutting is the erosion of cohesive material such as clay or glacial till from a shoreline by wave action.

Unlike sand, cohesive material cannot be replenished by natural events such as bluff erosion.