As I mentioned, there are a lot to rivers. You can simply draw a squiggly line from point A to point B and call it a day, but if you really want to understand the how and why of a river, especially if you are seeking an accurate story, then we need to go into finer detail of rivers.
In reading this, you may ask why do we need to go into so much detail about rivers. Often times in narrative, rivers are treated as water inside a ditch. It’s more complicated that that. In knowing how rivers work, may change elements of your story.