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<br />period of record. This can lead to inconsistent data, which may appear to indicate <br />climatic non-stationarity (potter, 1979). <br /> <br />18 <br /> <br />Improving American River Flood Frequency Analyses <br /> <br /> <br />Historical Flood Data <br /> <br />Historical data are episodic observations of flood stage or conditions that <br />were made before systematic data were collected (Jarrett, 1991). In the United <br />States, historical data typically are available for 100 to 200 years (Thomas, 1987). In <br />Egypt and China, historical data are available for several thousands of years (Baker, <br />1987; Pang, 1987). Historical data are obtained from a variety of sources, including <br />newspapers, human observers, diaries, historical museums, and libraries. Historical <br />descriptions of storms and floods are typically qualitative, and sometimes <br />exaggerated and contradictory; hence, they require careful review (Engstrom, 1996; <br />Pruess, 1996). Historical floods were generally recorded because they disrupted <br />people's lives. The threshold of perception typically depends on the location of <br />people, buildings, and economic activity, and may change in time and with observers <br />(Stedinger and Cohn, 1986). <br />In statistical terms, historical (and paleoflood) data are usually treated as <br />censored samples. An important type of historical data is knowledge of a level that <br />has not been exceeded at a given location over a known period oftime, which USGS <br />has compiled and annotated at many gaging stations. <br />There are three potential problems with the use of historical data in flood <br />frequency analysis. First, estimates of peak flood discharges associated with <br />historical stage information are subject to error; such errors can be reduced, however, <br />by careful hydraulic analysis, and their impact on flood frequency analysis can be <br />minimized by explicitly accounting for them in the analysis. Second, the most <br />serious error (and one that cannot be statistically accounted for) is an erroneous <br />conclusion that a given level has not been exceeded over a knOv.TI period of time. <br />Such an error is less likely to happen in heavily populated areas. Finally, as in the <br />case of systematic data, the use of historical data is conditioned on the assumption of <br />stationarity. This can be problematic because of the hydrologic impacts which <br />OCCUlTed during the early history of the United States and because of the scarcity of <br />systematic data with which to assess these impacts. <br /> <br />Paleoflood Data <br /> <br />Paleoflood hydrology is the study of ancient flood events, which OCCUlTed <br />prior to the time of human observation or direct measurement (Baker, 1987). <br />Paleoflood data provide a perspective on long-term hydrologic and climatic <br />variability that can be useful in flood project design and management Paleoflood <br />data complement short-term systematic and historical records. They provide <br />information at ungaged locations, provide likely upper limits of the largest floods that <br />have OCCUlTed in a river basin, and potentially decrease the uncertainty in estimates of <br />the magnitude and frequency of large floods (Baker, 1987; Costa, 1987; Enzel et aI., <br />1993; Jarrett, 1991; Kochel and Baker, 1982; Patton, 1987; Stedinger and Baker, <br /> <br />- <br />