“Well, there are plenty of inland river deltas and plains and agriculture is just as predominant inland as on the coast.”
Inland flood plains are usually narrower, and don’t offer as much land area for farming, which is why all the major ancient civilizations that developed agriculture were centered around deltas and flood plains near coasts (India, China, Mesopotamia, Egypt).
“We’ve had knowledge of fertilizer and crop rotation since the Old Testament.”
Rudimentary knowledge perhaps, but not the kind of knowledge we have today. Just throwing manure on stuff is not enough to compensate for depletion of a specific mineral, unless the manure happens to be rich in that mineral. Changing crops by season was done, yes, but it was done because different crops grew better in different seasons, not because we had systematic knowledge of which crops depleted which minerals, and how to counteract that by using crop rotation.
Cahokia... inland floodplain, major civilization with trading networks that extended to the Great Lakes, the Gulf Of Mexico, the Atlantic and the Rocky Mountains.
There are also lots of flood plains and deltas around the Nile river in Egypt and the Tigris and Euphrates in Mesopotamia, the Ohio river valley and the Amazon that are nowhere near the coast.
I think you’re a little bit confused.
The Egyptian civilization in a flood plain up to 800 air miles from the coast, at least via the Nile. The Two Lands in Egypt were upper and lower Egypt. Upper Egypt was the river’s narrow floodplain, Lower Egypt was the delta.
Mesopotamian civilization developed in areas up to 600+ air miles from the sea.
The first Chinese civilization was centered over 400 miles from the coast.