Watersheds of Illinois is a list of basins or catchment areas into which the State of Illinois can be divided based on the place to which water flows.
At the simplest level, in pre-settlement times, Illinois had two watersheds: the Mississippi River and Lake Michigan, with almost the entire State draining to the Mississippi, except for a small area within a few miles of the Lake.
Although it would now be correct to describe Illinois as part of the Mississippi watershed, such classification would not be particularly useful for locating bodies of water within the State.
Any body of water in Illinois can be located within this system.
A list of the small, direct tributaries that have articles in Wikipedia is also shown.