In Illinois, rules for the use of stream water and ground water differ somewhat.
For stream water, Illinois follows the Riparian Doctrine - Reasonable Use Rule:
Each person owning land next to a stream is entitled to use all the stream water
necessary to satisfy natural needs (e.g., to meet household needs), but is limited
to reasonable use of the water to meet artificial needs (e.g., irrigation). Disputes
about what is a reasonable share of the available stream water are resolved in
the courts if they cannot be resolved informally.