Data from 177 U.S. Geological Survey gaging sites in Illinois were analyzed by multiple-regression techniques. Equations to predict depth of water above channel bottom for various frequency floods were developed. The 2-year-flood discharge was used as the descriptive variable.

The depth-frequency equations assume and should be used only for Illinois streams flowing under natural conditions. They do not apply to streams affected by backwater, or significant artificial controls, or in situations where ice or debris jams cause peak stages.