The origin of the water would be the upper three Great Lakes draining into the Chicago River by way of Lake Michigan.
Doesn’t matter.
It is not a matter of available water volume, it is a question of flow rate. You just can’t get enough cubic feet per second coming down that piddly Chicago River into the Canal and on southwestward to bother the Mississippi River much. It’s like trying to overflow an average well-drained ditch with a soda straw: How much water you have supplying the straw is irrelevant.
This neglects the effects of any buildings that might fall into the Chicago River, further congesting its flow. (Well, you did suggest a “megaquake”.)
Now... You get that earthquake to create a 1/4 mile wide 60 ft. deep crevasse from the Chicago Lakefront to Central IL, and we might be talkin’. Same for some sort of tremendous uplift of the Great Lakes, or (more likely) a large subsidence in roughly the same area as that crevasse I speculated about.
(The crevasse could likely be readily filled in at some point to block the flow, assuming the earth-moving equipment was still around.) (”Details”.)