system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 with water's persistent and powerful erosive power, how are all rivers not straight lines from source to ocean by now?
gpt
Because water has momentum. The flow around a bend digs into the far side of the curve while it slows down and deposits sediments on the inside of the curve. 

The result is that curves are exaggerated over time, and can even form circular oxbow lakes. The sediment deposits close off the circle and the river continues around it. 

Over geological time, the actual path of a river can change a LOT, wiggling across the land, carving new paths.