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Omission of these smaller streams, however, has its rationale. Detailed mapping is costly and time consuming, and time and funds are curve drawn through the two smaller events can be extrapolated to esti- mate the discharge for a rarer flood frequency. If the log Pearson Type III distribution recommended by the Water Resources Council (10) is used, an estimate of the skew is needed as well. The basic equation is illusion, especially among local authorities and citizens, that the un- mapped areas are not subject to much damage. limited. Consequently, the Flood Insurance Administration (8) permits approximate flood plain mapping if time and funds are not available for studying the entire community in detail. Furthermore, during the waiting period before detailed flood hazard information becomes available, a city must regulate development in flood hazard areas as best it can. Here a definite need exists for an approximate method of estimating flood risks until thorough studies can be made. The proposed a~proximate method begins from the fact that flows or stages for floods of relatively small magnitude can be estimated more quickly and with more confidence than can those for rarer floods (3, 5). Many engineers and municipal engineering departments have had most of their experience in estimating floods of small magnitude for such purposes as drainage structures. The U. S. Geological Survey has done considerable work in defining flood magnitudes by frequency and published their results in reports that present regression equations and graphical procedures for est:l,.mating flood magnitudes. For many small baains, this is the only hydrologic data that is available; however, many of these reports only log Q" X + K s ........................(1) where Q is the discharge in cfs, x is the mean of the logs of recorded annual flood peaks. s is the standard deviation of the logs of annual flood peaks, and K is a frequency factor which for the log Pearson dis- tribution varies with the skew(G) of the logs of recorded annual flood peaks. Eq. I can be used to extrapolate flood peaks for rarer floods from flood peaks for more ordinary floods by introducing a "disCharge index slope" (018) defined as the difference between the log of the 2S-year flood and the log of the lO-year flood discharges. From this definition, log Q25 - log QlO = x + K25 s - 0l + KlO 8) DIS .. s (K2S - KlO) ...................... (2) which shows that the "discharge index slope" relates to the standard de- viation and the frequency factor which depends on the skew. Similarly, cover floods up to about s 50-year return period. In comparison, the lOO-year flood is legislatively mandated for most flood insurance and regulatory purposes. log Qf - log Q10 .. s (Kf - KlO) ......................... (3) The standard deviation can be eliminated by dividing. Thus, Normalized DischarRe Curves If discharges for two flood frequencies are known and a log normal distribution is assumed for the annual flood peaks, a flood frequency log Qf - log QIO Kf - KlO ..,..",..,.. ..(4) ND log Q25 - log QlO K25 - KlO 2 where ND is defined as the normalized discharge. Any two frequencies could 3