How can a river delta not be at the mouth? (Mississippi example)

Question

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.

Answer 1

You've fallen into a naming trap here.

There is The Mississippi River Delta and the Mississippi Delta. They are different things.

The Mississippi Delta is actually an Aluvial Plain not a delta at all. The Mississippi River Delta is a conventional delta and is found at the sea by New Orleans.

You can get a River Delta inland, although at the sea is the most common location.

As Wikipedia says:

River deltas form when a river carrying sediment reaches either (1) a body of standing water, such as a lake, ocean, or reservoir, (2) another river that cannot remove the sediment quickly enough to stop delta formation, or (3) an inland region where the water spreads out and deposits sediments.

Answer 2

This exists.

In Botswana, the Okavango River branches out to create a wide, swampy area called, appropriately enough, the Okavango Delta.

To quote the first line from the Wikipedia entry (emphasis added):

The Okavango Delta (or Okavango Grassland) (formerly spelled Okovango or Okovanggo) in Botswana is a very large, swampy inland delta formed where the Okavango River reaches a tectonic trough in the central part of the endorheic basin of the Kalahari.

Also, be sure to check it out on a map or satellite view.