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A flowchart Section 4 of the report describes techniques in this report guides the user to the appropriate for estimating peak flows for ungaged sites on estimates and (or) estimating techniques for a site gaged, unregulated streams in rural drainage on a specific stream. basins. Section 5, “Estimating peak flows for Section 1, “Estimates of peak flows and ungaged, unregulated streams in urbanized drain- maximum recorded flows at USGS streamflow- age basins,” describes regression equations for use gaging stations,” contains peak-flow estimates and when a drainage basin is urbanized. These urban the maximum recorded flows at 98 U.S. Geologi- regression equations come from a previous USGS cal Survey (USGS) streamflow-gaging stations. In nationwide study. As stated in section 6, because the development of the peak-flow estimates at peak flows on regulated streams are dependent on gaging stations, a new generalized skew coeffi- variable human actions, estimating peak flows at cient was calculated for Maine. This single state- ungaged sites on regulated streams is beyond the wide value of 0.029 (with a standard error of scope of this report. prediction of 0.297) is more accurate for Maine than the national skew isoline map in Bulletin 17B of the Interagency Advisory Committee on Water Data. PART 1: PURPOSE AND USE OF THIS REPORT Two techniques are presented to estimate the peak flows for ungaged, unregulated streams in rural drainage basins. These two techniques were INTRODUCTION developed using generalized least squares regres- sion procedures at 70 USGS gaging stations in Estimates of the magnitude of peak streamflows (such as the 50-year-recurrence-interval peak flow) are Maine and eastern New Hampshire. Section 2, necessary to safely and economically design bridges, “Estimating peak flows for ungaged, unregulated culverts, and other structures that are in or near streams in rural drainage basins,” uses the final streams. These estimates are also needed by Federal, explanatory variables of drainage area and basin State, regional, and local officials for effective flood- wetlands. The average standard error of prediction plain management. This report, prepared by the U.S. for the 100-year peak flow regression equation in Geological Survey (USGS) in cooperation with the section 2 was 48.6 percent to -32.7 percent. Drain- Maine Department of Transportation (MDOT), will age area was the only explanatory variable used in help MDOT and many others better estimate the mag- section 3, “Estimating peak flows for ungaged, nitude of peak flows for streams in Maine. Estimating the Magnitude of Peak Flows for Streams in Maine for Selected Recurrence Intervals—USGS WRIR 99-4008 1 This report gives estimates of, and presents tech- The reciprocal of the recurrence interval is called niques for estimating, the magnitude of peak flows for the annual exceedance probability; that is, the probabil- streams in Maine for recurrence intervals of 2, 5, 10, ity that a given peak flow will be equaled or exceeded 25, 50, 100, and 500 years. Peak flows and the maxi- in any given year. For example, the annual exceedance mum recorded flows are listed for USGS streamflow- probability of the 50-year peak flow would be 0.02. In gaging stations with 10 years or more of recorded other words, there is a 2 percent chance that the 50-year flows. Two techniques are presented for estimating the peak flow will be equaled or exceeded in any given peak flows for ungaged, unregulated streams in rural year. drainage basins. Techniques also are described for esti- To obtain estimated peak flows for streams in mating peak flows at ungaged sites on gaged streams Maine, information on the site (site refers to a location (for unregulated sites in rural drainage basins) and for on a stream) of interest is needed, including whether estimating peak flows on ungaged, unregulated streams the site is at or near (and on the same stream as) a U.S. in urbanized drainage basins. A technique for estimat- Geological Survey (USGS) streamflow-gaging station ing peak flows for ungaged sites on regulated streams and whether the site drains an urbanized or regulated is beyond the scope of this report, although a possible drainage basin. The different peak-flow estimates and approach is mentioned and cautions about inappropri- ate approaches are given. estimating techniques in this report are appropriate to various combinations of these site characteristics. Many peak-flow studies have been published for Maine and New England since the 1940’s, including The flowchart in figure 1 should be used to Morrill (1975) and