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( TECHNIQUES FOR ESTIMATING THE MAGNITUDE AND FREQUENCY OF FLOODS IN MINNESOTA By J. E. Jacques and D. L. Lorenz ABSTRACT Log-Pearson type III flood-frequency analyses were made of annual series peak-flow records from 246 gaging stations on unregulated streams in Minnesota having watersheds ranging in area from 0.08 to 2,520 square miles _ These flood discharges were related to watershed and climatic characteristics by using multiple-regression techniques. On the basis of this preliminary regression analysis of the frequency-analysis results, the data from these stations were grouped into four hydrologically distinct regions for the State. Regression analyses were performed on data from each region relating the 2-, 5-, 10-, 25-, 50-, and 100-year recurrence interval flood discharges to basin characteristics. The resulting regression equations, which may be used to estimate flood flows at ungaged sites, relate basin characteristics (contributing drainage area, main-channel slope, percent of basin covered by water, percent of basin covered by lakes, and mean annual runoff) to estimated flood flows. Different basin characteristics are significant for each of the four regions. Drainage area was found to be most significant and is included in all regional equations. Standard errors of' estimate of the regression equations ranged from 33 to 60 percent. INTRODUCTION BacklZround Knowledge of the magnitude and frequency of floods is essential for regu- lation, planning, and design along Minnesota's rivers and streams. Ideally, discharge information necessary for such projects would be obtained by hydro- logic analysis of nearby long-term flood records from gaging stations on the rivers and streams_ Because such records are rarely available at all sites of interest, particularly in small basins, techniques are needed to estimate the magnitude and frequency of floods at ungaged sites. This report is one of a series of reports prepared in cooperation with the Minnesota Department of Transportation that discuss flood-flow-frequency on small streams. The U. S _ Water Resources Council (198la) indicates that regression equations present a more accurate technique than other methods tested (for example the rational equation and rainfall-runoff models)_ Annual maximum-discharge and basin-characteristic data used in this study are stored, maintained, and updated by the U. S _ Geological Survey in the National Water Data Storage and Retrieval System (WATSTORE). In addition, these data are published by the U.S. Geological Survey as part of cooperative programs wi th various local, State, and Federal agencies. For this study, gaging stations with less than 10 years of record were not used because of the increased probability of time-sampling errors. Stations with more than 3,000 square miles of drainage area and stations influenced by natural and (or) 1