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<br />agricultural practices (Anderson and Moratto, 1996). In the late 1950s, after more <br />than a half century of active fIre suppression, greater emphasis was placed on <br />prescribed burning to reduce the buildup of fuelwood and hence decrease the <br />potential of catastrophic fIres (Weise and Martin, 1995). It is not known whether the <br />hydrologic effects of prescribed bums are the same as those of wildfires. <br />Levees were built in the Sacramento area to aid in draining wetlands for <br />agriculture and for protection from floods on the American and Sacramento Rivers. <br />As noted in Chapter 1, the first levees were built following the flood of 1850. These <br />levees failed in the 1852 flood, and were subsequently rebuilt to higher levels <br />(Woodward and Smith, 1977). Following the disastrous flooding in 1861-1862, <br />substantial efforts were directed towards major levee projects. Unfortunately, as a <br />result of aggradation from mining sediments, the height of flood waters for a given <br />discharge progressively increased. This led to levee failures during moderate floods, <br />requiring additional levee improvements. <br />Most rivers in the Sierra Nevada have surface water impoundments for <br />multi-use purposes to help support the rapid population growth in California, <br />particularly after World War II. These impoundments can dramatically affect <br />streamflows, reducing flood flows and increasing low flows. As a result of the <br />substantial impact of impoundments on flood flows it is necessary to correct <br />rneasured streamflows to establish unregulated conditions, as discussed previously in <br />this chapter. <br />What are the implications of these various human activities with respect to <br />the use of historical and paleotlood data for flood frequency estimation on the <br />American River? The most obvious implication is that the enormous amount of <br />rnining sediment in the American River during the latter part of the 19th century <br />makes it very difficult to accurately estimate historical flood discharges during that <br />period, precisely the period when historical information is available. There is also <br />the possibility that the net effect of human activity has been to increase the flood <br />response of the American River. With the available information it is not possible to <br />quantifY this potential effect. <br /> <br />32 <br /> <br />Improving American River Flood Frequency Analyses <br /> <br /> <br />HistorieaI Data <br /> <br />Reliable observations of historical floods on the American River began in <br />1848 with the discovery of gold at Sutter's Mill. Major floods damaged Sacramento <br />in 1850, 1862, 1867, 1881, 1891, and 1907 (the systematic flood record begins in <br />1905). Of these, the flood of 1862 clearly had the largest peak discharge, although <br />the maximum stage of the 1867 flood on the lower American River may have been <br />higher as a result of channel aggradation (McGlashan and Briggs, 1939). <br />The winter of 1861-1862 was extremely wet with few interruptions of the <br />heavy rains from early November 1861 to mid-January 1862. The culminating event <br />was a warm storm in January that had a three-day precipitation of 12.2 inches at <br />Nevada City, the only station in the upper American River basin having records <br />(Weaver, 1962). This was exceeded at this site by only the February 1986 storm (15 <br />inches) and the January 1997 storm (12.7 inches). Flooding was extreme on all rivers <br /> <br />- <br />