Final answer:
Water falling in Illinois is part of the Mississippi River Basin, leading it to ultimately flow into the Gulf of Mexico.
Step-by-step explanation:
Water falling in Illinois would ultimately end up in the Gulf of Mexico. This is because the waterways in Illinois are part of the Mississippi River Basin.
As indicated by references, including historical agreements allowing travel along the full length of the Mississippi River and evidence of species movement from the Great Lakes through the Mississippi Basin, the flow of water in this region is directed toward the Gulf. In fact, the city of Chicago notably reversed the flow of the Chicago River in the late 1800s, specifically to direct its water away from Lake Michigan and towards the Mississippi River Basin. This engineering feat further reinforced the natural flow of water from the Illinois region into the massive Mississippi River system, which ultimately discharges into the Gulf of Mexico.