Audrey L. Ishii Jennifer B. Sharpe Charles F. Avery David T. Soong 2004 Flood-peak discharge magnitudes and frequencies at streamflow-gaging sites were developed with the annual maximum series (AMS) and the partial duration series (PDS) in this study. Regional equations for both flood series were developed for estimating flood-peak discharge magnitudes at specified recurrence intervals of rural Illinois streams. The regional equations are techniques for estimating flood quantiles at ungaged sites or for improving estimated flood quantiles at gaged sites with short records or unrepresentative data. Besides updating at-site floodfrequency estimates using flood data up to water year 1999, this study updated the generalized skew coefficients for Illinois to be used with the Log-Pearson III probability distribution for analyzing the AMS, developed a program for analyzing the partial duration series with the Generalized Pareto probability distribution, and applied the BASINSOFT program with digital datasets in soil, topography, land cover, and precipitation to develop a set of basin characteristics. The multiple regression analysis was used to develop the regional equations with subsets of the basin characteristics and the updated at-site flood frequencies. Seven hydrologic regions were delineated using physiographic and hydrologic characteristics of drainage basins of Illinois. The seven hydrologic regions were used for both the AMS and PDS analyses. Examples are presented to illustrate the use of the AMS regional equations to estimate flood quantiles at an ungaged site and to improve flood-quantile estimates at and near a gaged site. Flood-quantile estimates in four regulated channel reaches of Illinois also are approximated by linear interpolation. Documentation of the flood data preparation and evaluation, procedures for determining the flood quantiles, basin characteristics, generalized skew coefficients, hydrologic region delineations, and the multiple regression analyses used to determine the regional equations are presented in the main text and appendixes. application/pdf 10.3133/sir20045103 en U.S. Geological Survey Estimating flood-peak discharge magnitudes and frequencies for rural streams in Illinois reports