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thi6 report are applicable to unregulated rural streams located within the State. The drainage-basin flood-estimation method is limited to streams with drainage areas less than 1,060 mi2. The channel-geometry flood- estimation method is applicable to stabilized stream channels in Iowa. Acknowledgments The U.S. Army Corps of Engineers, Omaha and Rock Island Di6tricts, contributed to the funding of the sediment-sample analyses. James J. Majure, formerly with the U.S. Geological Survey, Iowa City, Iowa, and now with the Iowa State University, GIS Support and Research Facility, Ames, 'Iowa, developed the computer software used to quantify the drainage-basin characteristics and the software used to integrate the overall GIS procedure. FLOOD-FREQUENCY ANALYSES OF STREAMFLOW-GAGING STATIONS IN IOWA , Flood-frequency curves were developed for 188 streamflow-gaging stations operated in Iowa by the U.S. Geological Survey. They were developed according to procedures outlined in Bulletin 17B of the Interagency Advisory Committee on Water Data (IACWD, 1982, p. 1-28). These flood-frequency curves include data collected through the 1990 water year for both active and discontinued continuous-record and crest-stage gaging stations having at least 10 years of gaged annual-peak discharges. A water year is the 12-month period from October 1 through September 30 and is designated by the calendar year in which it ends. The locations of the 164 gaging stations studied using the drainage-basin flood-estimation method are shown in figure 1, and the locations of the 157 gaging stations studied using the channel- geometry flood-estimation method are shown in figure 2. Map numbers for the gaging stations shown in figures 1 and 2 are referenced to gaging-station numbers and names in tables 8 and 9 (at end of this report). The observed annual-peak discharge record at each site includes water years during which the gaging station was operated, which is termed the systematic period of record. The observed annual-peak discharge record also may include historic-peak discharges that occurred during water years outside the systematic period of record. A flood-frequency curve relates observed annual-peak discharges to annual exceedance probability or recurrence interval. Annual exceedance probability is expressed as the chance that a given flood magnitude will be exceeded in any 1 year. Recurrence interval, which is the reciprocal of the annual exceedance probability, is the average number of years between exceedances of a given flood magnitude. For example, a flood with a magnitude that is expected to be exceeded once on the average during any 100-year period (recurrence interval) has a I-percent chance (annual exceedance probability = 0.01) of being exceeded during any 1 year. This flood, commonly termed the lO'O-year flood, is generally used as a standard against which flood peaks are measured. Although the recurrence interval represents the long-term average period between floods of a specific magnitude, rare floods could occur at shorter intervals or even within the same year. Flood-frequency curves were developed by fitting the logarithms (base 10) of the observed annual-peak discharges to a Pearson Type-III distribution using U.S. Geological Survey WATSTORE flood-frequency analysis programs (Kirby, 1981, p. CI-C57). Extremely small discharge. values (low outliers) were censored, adjustments were made for extremely large discharge values (high outliers), and the coefficient of 6kew was weighted for each gaging station with skew values obtained from a generalized skew-coefficient map (IACWD, 1982). Whenever possible, historically adjusted flood-frequency curves were developed to extend the flood record for gaging stations with historic peak-flood information. The recommended equation (IACWD, 1982, p. 9) for fitting a Pearson Type-III distribution to the logarithms of observed annual-peak discharges of a gaging station is log(QT(g)) =x+ks, (1) where QT(g) is the design-flood discharge for a gage, in cubic feet per second, for a selected T-year recurrence FLOOD.FREQUENCY ANALYSES OF STREAMFLOW.GAGING STATIONS IN IOWA 3