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107.4 percent based on the 50-year recurrence interval information determined for the gaged site upstream of this ungaged site. SUMMARY AND CONCLUSIONS Drainage-basin and channel-geometry equations are presented in this report for estimating design-flood discharges having recurrence intervals of 2, 5, 10, 25, 50, and 100 years at stream sites on rural, unregulated streams in Iowa. The equations were developed using ordinary least-squares and weighted least-squares multiple-regression techniques. Statewide equations were developed for the dratnage-basin flood-estimation method and statewide and regional equations were developed for the channel-geometry flood- estimation method. The drainage-basin equations are applicable to stream sites with drainage areas less than 1,060 mi2, and the channel-geometry equations are applicable to stabilized stream channels in Iowa. Flood-frequency curves were developed for 188 continuous-record and crest-stage gaging stations on unregulated rural streams in Iowa. Pearson Type-III estimates of design-flood discharges are reported for these gaging stations. Regression analyses of Pearson Type-III design-flood discharges and selected drainage- basin characteristics, quantified using a geographic-information-system (GIS) proce- dure, were used to develop the statewide drainage-basin flood-estimation equations. The significant characteristics identified for the drainage-basin equations included contributing drainage area; relative relief; drainage frequency; and 2-year, 24-hour precipitation intensity. The regression coefficients for these equations indicated an increase in design-flood discharges with increasing magnitude in the values of each drainage-basin characteristic. The average standard errors of prediction for the drainage-basin equations ranged from 38.6 to 50.2 percent. Techniques on how to make manual measurements from topographic maps for the primary drainage-basin characteristics used in the regression equations are presented along with examples. Several of the primary drainage-basin characteristics used in the regression equations are map-scale dependent. Use of maps of scales other than the scales used to develop the equations may produce results that do not conform to the range of estimation accuracies listed for the equations. Regression analyses of Pearson Type-III design-flood discharges and selected channel-geometry characteristics were used to develop both statewide and regional channel- geometry equations. On the basis of a geographic bias identified from the statewide regression residuals, two channel-geometry hydrologic regions were defined for Iowa relative to the Des Moines Lobe landform region. The significant channel-geometry characteristics identified for the statewide and regional regression equations included bankfull width and bankfull depth for natural channels unaffected by channelization, and active- channel width for stabilized channels affected by channelization. The regression coefficients for the statewide and regional channel- geometry equations indicated an increase in design-flood discharges with increasing magnitude in the values of each channel- geometry characteristic. The average standard errors of prediction for the statewide regression equations ranged from 41.0 to 68.4 percent and for the regional regression equations from 30.3 to 70.0 percent. The regional channel-geometry regression equations provided an improved estimation accuracy compared to the statewide regression equations, with the exception of the Region II active-channel regression equations developed for design floods having recurrence intervals of 25, 50, and 100 years. Guidelines for measuring the channel-geometry character- istics used in the statewide and regional regression equations are presented along with examples. Procedures for applying the drainage-basin and channel-geometry regression equations vary and depend on whether the design-flood discharge estimate is for a site on an ungaged stream, an ungaged site on a gaged stream, or a gaged site. When both a drainage-basin and a channel-geometry regression-equation estimate are available for a stream site, a procedure is presented for determining a weighted average of the two flood estimates. The procedure for estimating a design-flood discharge for an SUMMARY AND CONCLUSIONS 41