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CONTENTS Page Abstract....................................................................................................................... ................... 1 In trod uction........................................................................................................................ ............ 1 Purpose and scope.... .... ...... ... ... ..... .... .... ........ ... ...... ...... ...... ...... ... ..... .......... ........ .... ...... ..... 2 Acknowledgments .... .... ... ... ...... ... ... ... ................. ..... ....... .... ...... ... ..... .......... ............. ..... ..... 3 Flood-frequency analyses of streamflow-gaging stations in Iowa...............................................3 Development of multiple-regression equations. ............. ........... .................. ......... ................ ........ 6 Ordinary least-squares regression .... .... ............. ........... ............. ..... .... ................ .... ......... 6 Weighted least-squares regression.................................................................................. 8 Estimating design-flood discharges using drainage-basin characteristics................................. 9 Geographic- information-system procedure ................................................................... 10 Verification of drainage-basin characteristics............................................................... 13 Drainage-basin characteristic equations ... ......... .... ........................ ......... ............... ....... 16 Example of equation use--example 1 .............................................................................18 Estimating design-flood discharges using channel-geometry characteristics ..........................19 , I ~ Channel-geometry data collection.................................................................................. 19 Channel -geometry characteristic equations.................................................................. 22 Analysis of channel-geometry data on a statewide ba6is................................. 22 Analysis of channel-geometry data by selected regions................................... 22 Comparison of regional and statewide channel-geometry equations.............. 24 Examples of equation use--examples 2-4.......................................................................27 Application and reliability of flood-estimation methods............................................................ 32 Limitations and accuracy of equations ..........................................................................32 Weighting design-flood discharge estimates .................................................................34 C8lculation of estim8tes .................................................................................... 35 Example ofweighting--example 5.....................................................................35 Weighting design-flood discharge estimates for gaged sites ........................................35 Calculation of estimates ....................................................................................36 Examples of weighting--examples 6-7............................................................... 36 Estimating design-flood discharges for an ungaged site on a gaged stream............... 37 Calculation of estimates .................................................................................... 37 Example of estimation method--example 8 ......................................................38 Summary and conclusions........... ..... ...... .......... .......... .................... ............ ...... ....... ...... ...... ........41 References.................................................................................................................... .................42 CONTENTS iii