You’re talking about the Mississippi though. You simply couldn’t do here what your talking about, it’s a mile wide in some places here in SE Iowa. They do keep a channel dredged in the middle of it though for the barges.
They used to dredge many of the smaller rivers for steamboat and barge travel in the 1800’s. Problem is it has to be constantly done - all those rivers are filled in and went back to their undredged state decades ago.
BTW, the Hawkeye out of Burlington has good flood coverage of SE Iowa. Lots of pics:
On the plus side, the dredge matter can be used to build increasingly large levees. Preferably used with wired rock barriers to increase volume and strength.
At that point, the biggest problem would man made restrictions on the river, which unless raised to match the levees would be submerged.