Technique for estimating flood-peak discharges and frequencies on rural streams in Illinois

Flood-peak discharges and frequencies are presented for 394 gaged sites in Illinois, Indiana, and Wisconsin for recurrence intervals ranging from 2 to 100 years. A technique is presented for estimating flood-peak discharges at recurrence intervals ranging from 2 to 500 years for nonregulated streams in Illinois with drainage areas ranging from 0.02 to 10,000 square miles. Multiple-regression analyses, using basin characteristics and peak streamflow data from 268 of the 394 gaged sites, were used to define the flood-frequency relation. The most significant independent variables for estimating flood-peak discharge are drainage area, slope, rainfall intensity and a regional factor. Examples are given to show a step-by-step procedure in calculating a 50-year flood for a site on an ungaged stream, a site at a gaged location, and a site near a gaged location. (USGS)