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The purpOSE' of this report is to present a simple and effective method to estimate the magnitude and frequency of floods at ungaged sites on unregulated rural streams in Iowa. The regional flood-frequency equations and techniques presente~ in this repor~, which ~ere ~efined from an.UPdated. data set, should provide flood estimates with increased reliability compared to previous reports. REGIONAL ANALYSIS Methods of estimating flood magnitudes and frequencies applicable to an entire region rather than to a single gaging station are developed through regional analysis. Many structures are built across or adjacent to streams at sites where there is no record of streamflow. For this reason, methods are needed to extend information pertaining to flood magnitude and frequency based on gaging-station data from gaged to ungaged sites. Flood data for a sing;le station are relatively short-term random samples and uy not be representative of the long-term distribution of floods at t:h&t station. Combining records for stations in a hydrologically similar ar_ decreases errors associated with relatively short-term nonrepresentatiVe ,,, ~ -~ -.. ,'.s;' samples. ;Y1.~.:' . .~ . The magnitudes of floods in Iowa vary considerably from one regio another as a function of drainage basin efficiency. River basins.l minimal drainagE' efficiency, such as those in north-central Iowau characterized by flat terrain. Streams draining these basins ;'" considerably sma.ller peak discharges than do streams draining basins.., .' steep terrain and well developed drainage systems, such as the basina'. the Paleozoic Plateau Escarpment area of eastern Iowa (Prior. 1 Typically, the discharge per square mile of a stream in the escarpmen~~ is about 1230 cL~ic feet per second during a lOO-year-flood, wher.~~ discharge per square mile of a stream in north-central Iowa is a _Wft;' cubic feet per second during a lOa-year flood. These two areas ar.e.t:" 100 miles apart and the climatic differences such as temperature", . precipitation, and relative humidity are not significant enough to ; for the differences in flood magnitude. However, the physi!.. differences bet"een these two areas are significant (Prior. 197~, ". fig. 22). 2 '{ ,;, The largest: floods per unit area occur within lOa miles wes~"ili' Des Moines Lobe (north-central Iowa) along the rugged bluff~",.. ridges that border the Missouri River valley. Flood data collee area from both Iowa and Nebraska indicate that the discharge of I' . draining a I-square-mile basin is as much as 1880 cubic fee~o' during a lOa-year flood. Therefore, it seems reasonable C9., hydrologic regions based on the landform and physiographic ehar of the State. .... Five hydrologic regions have been identified and delineat. using physiographic regions of Iowa as a guide. Prior <;, detailed description of the physiographic regions of Iowa. dil! ,_ shape of the land surface, materials that underlie the land the geologic history.