When a river is not dredged but has its flow constrained, it tends to carry silt that is deposited on the river bottom, slowly raising it. So river beds tend to rise over time making it gradually easier to flood.
Exactly and this is what has happened in many areas being touted as adversely affected by “global warming “caused” flooding.”
If anything is “man-made” it’s the environmentalists’ push to stop dredging.