FLOODFLOW FREQUENCY OF STREAMS IN THE ALLUVIAL PLAIN OF THE LOWER MISSISSIPPI RIVER IN MISSISSIPPI, ARKANSAS, AND LOUISIANA by M.N. Landers Prepared by the U.S. Geological Survey in cooperation with the Mississippi State Highway Department 1985 WATER-RESOURCES INVESTIGATIONS REPORT 85-4150 TABLE OF CONTENTS Abstract Introduction Flood records Flood-frequency relations at gaging stations Regional flood-frequency relations     Floodflow frequency regression equations Weighted flood-frequency estimates     Comparison of log-Pearson and regression estimates Extrapolated flood-frequency estimates Summary Selected references FIGURES 1. Map showing location of study area and stream stations used in analysis 2-5. Graphs showing: 2. Distribution of drainage area magnitude 3. Basin charcteristics plotted against 2-year flood magnitude 4. Comparison of 2-year flood-frequency estimates 5. Comparison of 50-year flood-frequency estimates TABLES 1. Basin characteristics and station, regional, and weighted T-year flood estimates 2. Standard error and equivalent years of record for regional T-year flood estimates ABSTRACT The magnitude and frequency of floods are key factors in the design of bridges, highway embankments , culverts, levees, dams, and other structures near streams. Effective flood-plain manage- ment and the determination of flood insurance rates also require information on the magnitude and frequency of floods. Flood information is of particular importance in the alluvial plain of the lower Mississippi River due to the hydrologic ef- fects of the unique regional geography (fig. 1). The topographic features of the lower Mississippi region are primarily the re- sult of aggradation and deposition from streamflow. The topo- graphy is a series of abandoned meander belts, oxbow lakes, swamps, and flat-sloped watersheds. Regional drainage charac- teristics are broad, widely meandering stream courses with low channel slopes, and large amounts of depression and channel storage. The topography and hydrology of the lower Mississippi region also lead to greater flood damage as waters cover a larger area for a greater length of time for a given magnitude flood than in surrounding regions. The hydrology of the lower Mississippi region differs significantly from that of surround- ing regions, which suggests a separate analysis for this region. Wilson and Trotter (1961) presented this region within for the State of Mississippi as a separate hydrologic area with a unique flood-frequency relation, as did Patterson (1964) for the en- tire lower Mississippi River basin. This report provides techniques for estimating the magnitude of floods with recurrence intervals from 2 to 100 years for streams in the lower Mississippi alluvial plain having drainage areas between 0.11 and 1,170 square miles. Statistical estimates of flood magnitude are presented for 30 streams in the region having at least 10 years of annual peak streamflow record. Results from a linear regression analysis of these data are presented for estimating flood-frequency relations on ungaged streams using watershed characteristics. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.