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streamflow-gaging stations in Iowa (fig. 1). Drainage-basin characteristics were quantified using a GIS procedure to process topographic maps and digital cartographic data. An overview of the GIS procedure is provided in the following section. Geographic- Information-System Procedure The GIS procedure developed by the U.S. Geological Survey (USGS) quantifies for each drainage basin the 26 basin characteristics listed in Appendix A (at end of this report). These characteristics were selected for the GIS procedure on the basis of their hypothesized applicability in flood-estimation analysis and their general acceptability as measurements of drainage-basin morphology and climate. Techniques for making manual measurements of selected drainage-basin characteristics from topographic maps are outlined in Appendix B (at end of this report). The GIS procedure uses ARC/INFO computer software and other software developed specifically to integrate with ARC/INFO (Majure and Soenksen, 1991; Eash, 1993). The GIS procedure entails four main steps: (1) creation of four GIS digital maps (ARC/INFO coverages) from three cartographic data sources, (2) assignment of attribute information to three of the four GIS digital maps, (3) quantification of 24 morphologic basin characteristics from the four GIS digital maps, and (4) quantification of two climatic basin characteristics from two precipitation data sources. The first step creates four GIS digital maps representing selected aspects of a drainage basin. Examples of these maps are shown in figure 4, The drainage-divide digital map (fig. 4A) is created by delineating the surface-water drainage-divide boundary for a streamflow- gaging station on 1:250,000-scale U,S. Defense Mapping Agency (oMA) topographic maps, Tbis drainage-divide delineation ]S manually digitized into a polygon digital map using GIS software. If noncontributing drainage areas are identified within the drainage-divide boundary, then each noncontributing drainage area also is delineated and digitized. The drainage-network digital map (fig. 4B) is created by extracting the drainage network for the basin from I:IOO,OOO-scale USGS digital line graph (DLG) data. The extraction process uses GIS software to select and append together the DLG data contained within the drainage-divide polygon. The elevation-contour digital map (fig. 4C) is created from I:250,000-scale DMA digital elevation model (OEM) data that are referenced to sea level (in meters). GIS software is used to convert the OEM data to a lattice file of point elevations for an area slightly larger than the drainage-divide polygon. This lattice file of point elevations is contoured with a 12-meter (39.372-ft) or smaller contour interval using ARC/INFO software. The contour interval is chosen to provide at least five contours for each drainage basin. GIS software selects the contours contained within the drainage-divide polygon to create the elevation-contour digital map. Elevation contours then are converted to units of feet. The basin-length digital map (fig. 4D) is created by delineating and digitizing the basin length from 1 :250,000-scale DMA topographic maps. The basin length characteristic is delineated by first identifying the main channel for the drainage basin on 1:IOO,OOO-scale topographic maps. The main channel is identified by starting at the basin outlet and proceeding upstream, repetitively selecting the channel that drains the greater area at each stream junction. The most upstream channel is extended to the drainage-divide boundary defined for the drainage-divide digital map. This main channel identified on I:IOO,OOO-scale topographic maps is used to define the main channel on 1:250,000-scale topographic maps. The basin length is centered along the main-channel, flood-plain valley from basin outlet to basin divide and digitized with as straight a line as possible from the 1:250,000-scale maps. When comparing the basin length shown in figure 4D to those stream segments corresponding to the main channel in figure 4B, it can be seen that the basin length does not include all the sinuosity of the stream segments. ~ The specific second polygon, step assigns attributes to line-segment, and point 10 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA ~