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<br />2 <br /> <br /> <br />Data Sources <br /> <br />A variety of data types can be used in estimating flood quantiles or <br />exceedance probabilities for the American River. These include systematic <br />streamflow and precipitation data, historical and paleoflood data, and regional <br />hydrometeorological information on extreme events. Flood frequency analysis <br />traditionally has been based on systematic streamflow or precipitation records, where <br />use of the latter requires the application of precipitation runoff modeling. Modern <br />statistical innovations have enabled the use of historical and paleohydrologic data in <br />flood frequency analysis. These data can provide infonnation about extreme <br />flooding over much longer time frames than systematic records, and thus could <br />increase the accuracy of the frequency analysis (Cohn and Stedinger, ] 987). <br />Regional studies of maximum precipitation and flood discharges can provide <br />infonnation about extreme floods that can be used to check the accuracy of estimated <br />flood distributions, particularly when these distributions are extrapolated (i.e., applied <br />to flood magnitudes much larger than observed in the systematic record). This <br />chapter begins with a general description of the various sources of data used in flood <br />frequency analysis. It then focuses on specific data relevant to the American River <br />watershed, and potential limitations of these data. <br /> <br />GENERAL DESCRIPTION OF FLOOD FREQUENCY DATA <br /> <br />Systematic Streamflow Data <br /> <br />The US. Geological Survey (USGS) has primary responsibility for <br />operating a streamflow-gaging-station network in the United States. Systematic <br />streamflow gaging by the USGS began in the late 1800s (Mason and Weiger, 1995). <br />As of 1994, the USGS network consisted of 10,240 stations (Gilbert, 1995) and <br />accounted for more than 85 % of the nation's stream-gaging stations. (About 3,000 <br />of these stations employed crest-stage gages that provide only peak flow <br />information). Historical records of daily streamflow and peak flows for almost <br />20,000 USGS stations are available for various periods of record. The data are <br />published in annual USGS water data reports for each state and are available on the <br />Internet (http://water.usgs.gov), through USGS data bases, and on compact disc (CD- <br />ROM) from private vendors. <br />The accuracy of flood discharge data depends on whether large floods are <br /> <br />- <br /> <br />16 <br />