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estimates of the probable maximum precipitation (PMP), which is the estimated upper limit of precipitation for a basin. In the United States, the estimation ofPMP is based on data contained in the previously mentioned extreme storm catalog. The standard approach for estimating PMF includes (I) estimating the PMP for the basin; (2) deducting appropriate precipitation losses to estimate the excess rainfall available for runoff; (3) converting rainfall excess into a flood hydrograph; and (4) adding interf10w and snowmelt hydro graph components to obtain the fmal PMF hydrograph (Cudworth, 1987). The PMF method is widely used for assessing maximum flood potential at a site. Although the concept of maximum limits for floods is widely accepted, methods used to estimate these limits are subject to large uncertainties. Over the years, estimates ofPMP and PMF have typically increased. Furthermore, it is possible for a computed PMF to be exceeded at a given site. In a study of 61 watersheds, Bullard (1986) found that there had been nine rain-flood events that had produced peaks greater than or equal to 80% of the PMF peaks estimated by methods of the U.S. Bureau of Reclamation (USBR), and two events that produced peaks greater than or equal to 90% of the USBR-estimated PMF. AMERICAN RIVER DATA 24 Improving American River Flood Frequency Analyses The USACE based its American River flood frequency analysis solely on daily discharge data from the USGS gaging station at Fair Oaks (USGS station #11446500, drainage area 1,888 square miles) corrected for storage in upstream reservoirs. The validity of these data, particularly the data collected since the completion of Folsom Dam, has been questioned by various observers. In addition, critics have suggested that the frequency analysis should include the use of historical and paleoflood data and have argued that the results of the USACE flood frequency analysis are inconsistent with an existing flood envelope curve for California and with current estimates of PMF. These issues are explored in the remainder of this chapter. Homogeneity of the Systematic Flood Record The most important data set for use in flood frequency analysis for the American River is the set of annual maximum rain-flood discharges for various durations. We focus on the three-day rain-flood discharges, because three days is the most critical duration for designing and evaluating flood mitigation strategies for Sacramento. The systematic maximum three-day rain-flood series covers the period 1905-1998 (Figure 2.4), and is based on the USGS station at Fair Oaks corrected for storage in upstream reservoirs. Figure 2.4 shows that the five largest three-day discharges in the series occur after 1950, as well as 10 of the top 13 discharges. Completion of the Folsom Dam in 1956 raises the question of whether the apparent increase in the frequency of large flood discharges is an artifact of the corrections for Folsom storage. The USACE-estimated unregulated discharge on a daily basis using a simple mass balance is as follows. If Qu,t is estimated unregulated discharge for day t; Qg,t -