I'm creating a bronze age setting and one of the civilizations gets their powers from a river whose irrigation produces a vast food supply. So, I decided to figure out where the most powerful city states would be placed based around where the earth would be most fertile and I figured it would be at the delta, close to the mouth.
However, doing some research I found that the Mississippi delta is not considered at New Orleans (where the mouth is) but further up stream where it merges with the Yazoo river. Apparently this area was a huge, bustling cotton mecca and I was just wondering how this works. I understand that forks irrigates a wider area of land, but how is this a delta? And how can it be better for agriculture than the river's mouth?
I'm not American and don't have a very good grasp of rivers, so if someone could give a better explanation of a river delta to me (so that it encompasses the case of Mississippi) I would greatly appreciate it.