Yes, parts probably should be filled . .
. . but destroying everything and building a theme park on top of important history is a kinda crude idea.
The cost of that fill would be pretty high and you'd have to get it somewhere. Problem with a delta river is it keeps getting higher and higher above the surrounding land until it overflows and cuts a new course, but here it's restrained so it just keeps getting higher and higher, so the obvious source of fill is to dredge the river.
Of course, given a bit of time, the river will become higher than that fill and you'll be back to containing it.
On the other hand, preserving the historical areas using current methods will probably be cheaper than fill and rebuild.
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