Well the answer to your question is a variable one as species diversity can be unpredictable at times, but usually can be determined locally by native distribution, past management, flood regime, soil texture and drainage, and several other factors.
Common floodplain species include, but is not limited to, silver maple, red maple, boxelder, cottonwood, sycamore, Am. elm, green ash, hackberry, sugarberry, river birch, black willow, black walnut, bur oak, swamp white oak, pin oak, pecan, shellbark hickory, persimmon, honeylocust, KY coffeetree, sweetgum, etc.
Certain species, of course, pop in or drop out as you navigate north-south in Illinois, (i.e., you are not going to find native sweetgum, pecan, sugarberry, and red maple along the Sangamon River here in Champaign).
By the numbers alone, elms (Ulmus spp.) are the most abundant tree species in Illinois.