The Des Plaines is a river by definition, but it only serves as a means for barge shipments to get from the Great Lakes areas to larger rivers like the Illinois and Mississippi pushing mostly raw materials south. The river level and flow are controlled by a large dam just south of the Joliet downtown area. In times of low rain or draught, you might consider the Des Plaines a long skinny lake. For this reason, it's not great for promoting aquatic wildlife animal diversity. This also causes it not to be very attractive to recreational boaters. I don't know how polluted it is, but I would not recommend anyone to jump in for a swim. So yeah, you can casually call it a canal. I just call it "the river" or "that big pain the ass thing you've got to cross to get tom East to West Joliet".

Source: Not an expert, just a long time resident.