For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/.Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.Suggested citation: England, J.F., Jr., Cohn, T.A., Faber, B.A., Stedinger, J.R., Thomas, W.O., Jr., Veilleux, A.G., Kiang, J.E., and Mason, R.R., Jr., 2018, Guidelines for determining flood flow frequency-Bulletin 17C: U.S. Geological Survey Techniques and Methods, book 4, chap. B5, 148 p., https://doi.org/10.3133/tm4B5. PrefaceThis series of manuals on Techniques and Methods (TM) describes approved scientific and data-collection procedures and standard methods for planning and executing studies and laboratory analyses. The material is grouped under major subject headings called "books" and further subdivided into sections and chapters. Book 4 is on hydrologic analysis and interpretation and section B is on surface water.The unit of publication, the chapter, is limited to a narrow field of subject matter. These publications are subject to revision because of experience in use or because of advancement in knowledge, techniques, or equipment, and this format permits flexibility in revision and publication as the need arises. Chapter B5 of book 4 (TM 4-B5) deals with flood flow frequency analysis at gaged sites using the Expected Moments Algorithm. The use of extreme flood data represented by interval and censored data types, including historical, paleoflood, botanical evidence, is emphasized.This revision is adopted with the knowledge and understanding that review of these procedures will be ongoing. Updated methods will be adopted when warranted by experience and by examination and testing of new techniques. AcknowledgmentsThese revised Guidelines were written by a team assembled from the Hydrologic Frequency Analysis Workgroup (HFAWG), under the auspices of the Subcommittee on Hydrology (SOH) of the Advisory Committee on Water Information (ACWI). Input from other members of HFAWG and of the SOH is gratefully acknowledged. HFAWG and SOH Work Group Members and participants in this revision are listed in appendix 2. The authors would also like to acknowledge the contributions of the U.S. Geological Survey (USGS) colleague reviewers, whose comments and suggestions materially improved this report. We also appreciate the technical advice and assistance from Dr. William H. Asquith (USGS).Lastly, our colleague and friend, Dr. Timothy A. Cohn, authored or coauthored many of the papers that form the foundation of this update to the flood ...

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Graph showing annual exceedance-probability curves for Bloody Run Tributary near Sherrill, Iowa (05414605), showing the difference between expected moments algorithm (EMA/MGB) and Bulletin 17B/GB annual exceedance-probability analyses for a crest-stage gage (CSG) with four annual peak discharges below the minimum recording threshold ..

Methods for estimating the magnitude and frequency of floods in California that are not substantially affected by regulation or diversions have been updated. Annual peak-flow data through water year 2006 were analyzed for 771 streamflow-gaging stations (streamgages) in California having 10 or more years of data. Flood-frequency estimates were computed for the streamgages by using the expected moments algorithm to fit a Pearson Type III distribution to logarithms of annual peak flows for each streamgage. Lowoutlier and historic information were incorporated into the flood-frequency analysis, and a generalized Grubbs-Beck test was used to detect multiple potentially influential low outliers. Special methods for fitting the distribution were developed for streamgages in the desert region in southeastern California. Additionally, basin characteristics for the streamgages were computed by using a geographical information system. Regional regression analysis, using generalized least squares regression, was used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged basins in California that are outside of the southeastern desert region. Flood-frequency estimates and basin characteristics for 630 streamgages were combined to form the final database used in the regional regression analysis. Five hydrologic regions were developed for the area of California outside of the desert region. The final regional regression equations are functions of drainage area and mean annual precipitation for four of the five regions. In one region, the Sierra Nevada region, the final equations are functions of drainage area, mean basin elevation, and mean annual precipitation. Average

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