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<br />20 <br /> <br />Improving American River Flood Frequency Analyses <br /> <br /> <br /> <br />FIGURE 2.1 Diagrammatic section across a Stream channel showing a flood stage and various <br />flood features. SOURCE: Jarrett, 1991. <br /> <br />determined from laboratory measurements of the ratio of radioactive carbon-14 <br />to stable carbon-12 in a samples of organic carbon. Typical sources of organic <br />carbon include wood, charcoal, leaves, humus in soils, and bone. A recent <br />advance in radiocarbon dating based on the use of tandem accelerator mass <br />spectrometry has resulted in more accurate age estimates and requires a smaller <br />sample of organic carbon (Kochel and Baker, 1988). Using this approach, <br />samples having an age of 10,000 years or less generally can be dated with an <br />uncertainty of less than 100 years. When flood-scarred or -damaged trees are <br />present, dendrochronological methods can be used to date floods. In some cases, <br />these dates are accurate to the year and even the season. <br />The use of paleoflood information in flood frequency analysis is subject to <br />errors in the estimation of discharge peak and age, errors in field interpretations, and <br />questions of hydrologic stationarity. Errors in the estimation of peak discharges and <br />ages can be controlled by careful hydraulic and laboratory analysis. Furthermore, <br />these errors can be quantified and incorporated into the flood frequency analysis. <br />Qualified paleohydrologists can avoid errors in field interpretations by collecting <br />information at several sites to provide internal checks. Note, however, that there are <br />no universally accepted methods for quality assurance and control in the practice of <br />paleohydrology. <br />The most problematic issue regarding the use of paleoflood information in <br /> <br />- <br />