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The differences in peak-discharge estimation between regional and statewide active-channel width (ACW) equations are shown in figures 88 and 98 for the 2- and 100-year recurrence intervals, respectively. Figures 8B and 9B illustrate the higher estimated peak discharges obtained from the Region I equations relative to those obtained from the Region II equations for a specified active-channel width. The slopes of the Region I regression lines are parallel to those of the statewide regression lines at a higher estimated discharge. The Region II regression lines have steeper slopes relative to the Region I and statewide regression lines but at a lower estimated discharge. Figures SA and 9A illustrate the relation of the Region I, bankfull regression equations for 2- and 100-year recurrence-interval discharges, respectively. Tests performed using STATIT procedure REGGRP (Statware, Inc., 1990, p. 6-32 - 6-36) indicated that there were statistically significant differences in the slopes and intercepts of the Region I and Region II regression lines for both the bankfull and active-channel equations. The paired-t test was used to test whether design-flood discharge estimates obtained by both the bankfull and active-channel regression equations for the same gaging station were significantly different at the 95-percent level of significance. The paired-t test was applied using STATIT procedure HYPOTH (Statware, Inc., 1990, p. 3-21 - 3-23). For table 3, discharge estimates for 111 stations were not significantly different for all design-flood recurrence intervals. For table 4, discharge estimates for 78 stations were significantly different for the 2-year recurrence interval, but estimates were not significantly different for the 5-year to 100-year recurrence intervals. For table 5, discharge estimates for 33 stations were not significantly different for all design-flood recurrence intervals. The application of the channel-geometry regression equations listed in tables 4 and 5 for a stream site are determined by two factors, and the application of the channel-geometry equations listed in table 3 are determined only by the second factor. First, the stream site is located in figure 2 to determine whether Region I or Region II equations apply. The user may be faced with a dilemma if design-flood discharges are to be estimated for a stream site located within the shaded transitional zone or for a stream that crosses regional boundaries. The discharges could be estimated using both the Region I and II equations and hydrologic judgment used to select the most reasonable design-flood estimate, or a weighted average based on the proportion of drainage area within each region could be applied. The most reasonable alternative to resolving this dilemma may be to use the statewide equations listed in table 3 because they preclude regional subjectivity and the majority of statewide design-flood estimates calculate between Region I and Region II estimates. Second, the stream site is inspected to determine whether the stream was channelized. If evidence of channelization is not found, then the bankfull equations are applicable (the first set of equations listed in tables 3, 4, and 5); if evidence of channelization is found, then the active-channel equations may be applicable for stabilized channels (the second set of equations listed in tables 3, 4, and 5). Appendix C (at end of this report) outlines a procedure for identifying channelized streams and describes the stabilization conditions for which channel- geometry measurements of channelized streams are applicable. Examples of Equation UsenExamples 2.4 Examole 2.--Use a regional, channel- geometry equation to estimate the 100-year peak discharge for the discontinued Black Hawk Creek at Grundy Center crest-stage gaging station (station number 05463090; map number 73, fig. 2), located in Grundy County, at a bridge crossing on State Highway 14, at the north edge of Grundy Center, in the NWl/4 sec. 7, T. 87 N., R. 16 W. Step 1. The appropriate regional equation is determined on the basis of which hydrologic region the stream site is located in and whether the stream has been channelized. This gaging station is located in Region I, and an inspection of the USGS 1:100,000-scale Grundy County map and a visit to the site show no evidence of channelization. Therefore the 100-year bankfull equation for Region I, listed in the first set of ESTIMATING DESIGN-FLOOD DISCHARGES USING CHANNEL-GEOMETRY CHARACTERISTICS 27