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This report provides techniques and procedures for estimating the probable magnitude and frequency of floods at ungaged sites on Iowa streams. Physiographic characteristics were used to define the boundaries of five hydrologic regions. Regional regression equations that relate the size of the drainage area to flood magnitude are defined for estimating peak discharges having specified recurrence intervals of 2, 5, 10, 25, 50, and 100 years. Regional regression equations are applicable to sites on streams that have drainage areas ranging from 0.04to 5,150 square miles provided that the streams are not affected significantly by regulation upstream from the sites and that the drainage areas upstream from the sites are not mostly urban areas. Flood-frequency characteristics for the mains terns of selected rivers are presented in graphs as a function of drainage area. INTRODUCTION This report represents the fourth update in the last 33 years of the magnitude and frequency of floods in Iowa. Flood reports were updated to provide more dependable flood-data and more accurate and reliable methods for estimating the magnitude and frequency of floods. These estimates are needed to implement efficient flood-plain management strategies and for economical design of highway structures, levees, and buildings in the flood plain. Economic design criteria require the availability of adequate data with long periods of record, improved analytical methods, and better understanding of the flood hydrology of Iowa. The first of three previous reports (Schwob, 1953) was limited to an analysis of the magnitude and frequency of floods that was based on data collected in Iowa before 1950. A second report (Schwob, 1963) included updated data and a method for estimating magnitude and frequency of floods at sites on ungaged rural streams that had drainage areas of 10 square miles or more. The third report (Lara, 1973) was prepared using data updated through 1972. This report also included a method for estimating the magnitude and frequency of floods at sites on ungaged rural streams that had drainage areas of two square miles or more. 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 presented in this report, which were defined from an updated data set, should provide flood estimates with increased reliability compared to previous reports. REGIONAL ANALYSIS Methods of estimating flood magnitudes ind frequencies applicable to an entire region rather than to a single gaging station are developed through regional analysis. Many structures ire 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 single station are relatively short-term random samples and may not be representative of the long-term distribution of floods at that station. Combining records for stations in a hydrologically similar area decreases errors associated with relatively short-term nonrepresentative samples. The magnitudes of floods in Iowa vary considerably from one region to another as a function of drainage basin efficiency. River basins with minimal drainage efficiency, such as those in north-central Iowa, are characterized by flat terrain. Streams draining these basins have considerably smaller peak discharges than do streams draining basins having steep terrain and well developed drainage systems, such as the basins in the Paleozoic Plateau Escarpment area of eastern Iowa (Prior, 1976). Typically, the discharge per square mile of a stream in the escarpment area is about 1230 cubic feet per second during a 100-year-flood, whereas the discharge per square mile of a stream in ftorth-central Iowa is about 230 cubic feet per second during a 100-year flood. These two areas are about 100 miles apart and the climatic differences such as temperature, average precipitation, and relative humidity are not significant enough to account for the differences in flood magnitude. However, the physiographic differences between these two areas are significant (Prior, 1976, p.23, fig. 22). The largest floods per unit area occur within 100 miles west of the Des Moines Lobe (north-central Iowa) along the rugged bluffs and steep ridges that border the Missouri River valley., Flood data collected in this area from both Iowa and Nebraska indicate that the discharge of a stream draining a 1-square-mile basin is as much as 1880 cubic feet per second during a 100-year flood. Therefore, it seems reasonable to delineate hydrologic regions based on the landform and physiographic characteristics of the State. Five hydrologic regions have been identified and delineated in Iowa using physiographic regions of Iowa as a guide. Prior (1976) gives a detailed description of the physiographic regions of Iowa, discussing the shape of the land surface, materials that[underlie the land surface, and the geologic history. 2 «*««. l/ Hvdrologic Region 1 Hydrologic region 1 (fig. 1) extends north and south along the bluffs that border the Missouri River valley, with limits approximating those of the physiographic area known as the Western Loess Hills (Prior, 1976). The landscape has a corrugated appearance of alternating waves and troughs. Hills are sharp-featured, with narrow broken ridge-crests, intersecting spurs, and steep-sided slopes; the landscape is conducive to rapid runoff. The western border of the region is well defined and easily distinguished on topographic maps and in the field. The eastern border is more difficult to define and merges gradually with the landscape of hydrologic region 2. Hvdrologic Region 2 The bluff area that borders the Mississippi River valley is typical of the landscape in hydrologic region 2 (fig. 1). The landscape can vary from rugged to rolling topography, where runoff may be rapid, commonly causing flash flooding. Bluff-like areas are not only located in the vicinity of the Missisippi River, they also are present along the divide between the Mississippi River and Missouri River basins; in parts of the Iowa and Cedar River basins, in areas that border the Western Loess Hills, and in the headwater parts of basins of streams in south-central Iowa. Hvdrologic Region 3 Hydrologic region 3 is the largest hydrologic region (fig. 1). Most of the area in this region is typical of landscapes in Iowa. The topography of this region can be described as steeply to gently rolling hills interspersed with areas of more subdued topography. The area has a well-established drainage system. Physiographically, it covers most of the lowan Surface, a large part of the Southern Iowa Drift Plain, and the Northwest Iowa Plains (Prior, 1976). Hvdrologic Re2ion 4 This hydrologic region, which is located in west-central Iowa (fig. 1), is characterized by level terrain and a poorly developed drainage system. The region coincides approximately with the southern two-thirds of the Des Moines Lobe (Prior, 1976). Many clusters of ponds and marshes with no drainage outlets are present in this region. Small streams in level areas are shallow and sluggish. Hvdrologlc Region 5 This hydrologic region in north-central Iowa (fig. 1) coincides approximately with the northern part of the Des Moines Lobe (Prior, 1976). The magnitude of floods in this region are the smallest per unit area in the State. This is due to the flat topography and flood-attenuating effect of abundant bogs, swales, and circular depressions.

Method for estimating the magnitude and frequency of floods at ungaged sites on unregulated rural streams in Iowa