This is a fantastic illustration of geological processes. Sometimes I look at hillsides and geological layers and wonder how they could ever be built up so much. How could hundreds of feet of rock be deposited little by little, to make such thick layers? Well, here it is. A boat that sank in a river is 49 feet below a cornfield only a few human generations later. Amazing.
No geology here, just geomorphology. These are surface features, and the movement of water and soil happens at a rapid clip when humanity doesn't lock it in irons.
Natural rivers are usually not static features. The Mississippi is full of sediment, and strongly inclined towards major floods. In a natural landscape, those floods deposit mud every time the banks are breached, the banks are constantly degrading, the surrounding areas are swampy, flattened areas that get reshaped on a regular basis. A natural floodplain-river exists in a dynamic equilibrium, constantly shifting a tiny to medium-sized trickle of water around a vast, deep, soft bed of mud it deposited, only occasionally bumping into the hills (geological or aeolian features usually) bordering the floodplain.
What's strange and unnatural is that we could draw a line on a map, expect a river to be confined to this line, and reinforce that line with rock walls and soil levies; That we could dam major portions of the continental watershed in order to regularize water distribution; That we could drain swamps the size of states and turn them into cropland, or fill prairie or desert with canals to do the same.
We have cut off this area from the natural cycles of sedimentation, all but the most extreme "natural disasters" are prevented through intensive engineering began sometime in the 20th century. Because we needed to make spring planting. We replaced the silty mud that made the region such great cropland with chemical fertilizers & pesticides, and specialized farm equipment.
Either we feed our species, or we die. Granted, we as Americans eat too much and waste too much, but even if we all stuck to 2000 calories a day and cleaned our plates doing it, 19th century and prior farming techniques would simply not support our current population. And I disagree with your last sentence. Fertilizers and pesticides don't replace dirt, they make it more effective at growing crops.
I just feel the need to push back against the people who think about agriculture as some kind of natural ecological outgrowth. Ag changes the ecology of a landscape enormously, and the accomodations to ag end up reshaping the physical character of things so much we forget what they originally looked like or how they originally functioned.
