For years before the walls and levees, the rivers overflowed their banks and flooded the lands, bringing nutrient and new soil, renewing the fields and farms.
The major rivers suck as the Mississippi have been more and more restricted, the waters kept from flooding the land, and carrying their load to the sea.

The practice is exacerbated because those wanting to live on the flood plains are compensated in many cases by insurance.
If insurance was refused, some of the original purpose of floodplains and marshes could be restored.