I ask myself the same question every year when i have to wait till late summer to wade my favorite stretches.
I believe the grade is the primary river characteristic that causes the big difference. I also believe the river doesn't see as much water after ag fields are well into the growing season. The crops along with drain tile lower the water table. So when we get a good rain, the fields act like a sponge and soak up more than it allows to run off. From my understanding, some of the best producing ag grounds fall within the watershed. Having worked in water management for a few years, I can assure you, there are miles and miles of tile in the watershed. Once spring rains are drained, the subsurface can be pretty dry down to about 3 ft.
Another characteristic i wonder about is the presence of the LaSalle anticline that runs along it for most of its course. How does that affect the river levels? I believe that is why there is enough grade to allow up to class three rapids in the Lowell to the mouth stretch. It also exposed bedrock.
Just to be clear, most of my experience wading is from Lowell down. This is the section that has the most grade. It is dangerous if you don't have experience reading rivers. If asked, i will be conservative with my opinions in the name of safety. That is not to say it cant be fished. I just don’t because I can just as easily fish other more wadable rivers near by.
If the original question was about floating, we would be having a different conversation.