It isn’t expansion that it would do it is a return to wandering. When a river wanders, typically people die. It is not a good thing to be near a wandering river.
The river wandering is part of this. Since the silt can only be deposited where the water goes, for the delta to grow (or just maintain itself) the river has to go here and there all over it.
And yes, people die when this happens. But if you stop the river wandering, the silt isn't going to be deposited so as to maintain the delta.
Maybe it's not a good location for permanent human settlement.