Fact 1: Most canyons are formed by flowing rivers through erosion over long periods of time.

Fact 2: Most river erosion occurs when the river is in flood.

Given the two facts above, answer the question "what are most canyons formed by over long periods of time?" with the following options: 
- hurricanes
 - wind tunnels
 - dangerous
 - forces
 - kalenchoes
 - flooding
 - high populations
 - Sediment


 

      flooding   