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features in the first three of the four GIS digital maps shown in figure 4. As a prerequisite, the digital maps are edited to ensure that drainage-divide boundaries, stream segments, and the basin-length line segments are connected properly. If noncontributing drainage areas are identified, they are assigned attributes with separate polygon designations so that the basin-characteristic programs can distinguish between contributing and noncontributing areas. Each line segment in the drainage-network digital map is assigned a Strahler stream-order number I Strahler, 1952) and a code indicating whether tbe line segment represeQ.ts part of the' main channel or a secondary channel. Specific GIS programs have been developed to assign the proper stream- order number to each line segment and to code those line segments representing the main channel. Figure 4B shows the Strahler stream-order numbers for streams in the Black Hawk Creek at Grundy Center I station number 05463090; map number 73, fig. 1) drainage basin. A description on how to order streams using Strahler's method is included in Appendix B (at end of this report). The line segments in the elevation-contour digital map were assigned elevations from the processing of the OEM data. Line segments overlain by noncontributing drainage-area polygons are assigned attribw:es designating noncontributing contour segments. Two point attributes are added to the elevation-contour digital map to represent the maximum and minimum elevations of the drainage basin. The maximum basin elevation is defined from the highest OEM-generated con'cour elevation within the contributing drainage area. The minimum basin elevation is defined at the basin outlet as an interpolated value between the first elevation contour crossing the main channel upstream of the basin outlet and the first elevation contour crossing the main channel downstream of the basin outlet. The third step uses the four GIS digital maps shown in figure 4 and a set of programs developed by the USGS (MajurE and Soenksen, 1991) to quantify the 24 morphologic basin characteristics listed in Appendix A (at end of this report). These basin characteristics include selected measurements of area, length, shape, and topographic relief that define selected aspects of basin morphology, and several channel characteristics. The programs access the information automatically maintained by the GIS for each of the four digital maps, such as the length of line segments and the area of polygons, as well as the previously described attribute information assigned to the polygon, line-segment, and point features of three of the four GIS digital maps. The GIS programs then use this information to automatically quantify the 24 morphologic basin characteristics. The fourth step uses a software program developed to quantify the remaining two basin characteristics listed in Appendix A (at end of this report). These two climatic characteristics are quantified using GIS digital maps representing the distributions of mean annual precipitation and 2-year, 24-hour precipitation intensity for the area contriouting to all surface-water drainage in Iowa. This area includes a portion of southern Minnesota. The mean annual precipitation digital map was digitized from a contour map for Iowa, supplied by the Iowa Department of Agriculture and Land Stewardship, State Climatology Office (Des Moines), and from a contour map for Minnesota (Baker and Kuehnast, 1978). The 2-year, 24-hour precipitation intensity digital map was digitized from a contour map for Iowa (Waite, 1988, p. 31) and interpolated contours for southern Minnesota that were digitized from a United States contour map (Hershfield, 1961, p. 95). The digital map representing the distribution of 2-year, 24-hour precipitation intensity for Iowa and southern Minnesota is shown in figure 5. The weighted average for each climatic characteristic is computed for a drainage basin by calculating the mean of the area-weighted precipitation values that are within the drainage-divide polygon. Of the 26 drainage-basin characteristics listed in Appendix A, 12 are referred to as primary drainage-basin characteristics because they constitute specific GIS procedure or manual topographic-map measurements. They are listed under headings containing the word "measurement." The remaining characteristics are calculated from the primary drainage-basin characteristics; they are listed in Appendix A under headings containing the word "computation." Each drainage-basin character- istic listed in Appendix A is footnoted with a I ~ ! 12 ESTlMATING DESIGN.FLOOD DISCHARGES FOR STREAMS IN IOWA