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<br /> <br />than the flow for preceding 20 years, and that a long period of equally <br />low flows must be expected to recur at any time."149 <br /> <br />The Siebert Report would begin a trend that continues today. Almost every new and updated <br />study of the hydrology of the Colorado River reduces or raises additional serious questions about the <br />water supply available from the Colorado River. While I was drafting this paper, the National <br />Research Council of the National Academy of Sciences issued yet another study; "Colorado River <br />Basin Water Management: Evaluating and Adjusting to Hydrologic Variability," February 2007. <br />The "Report in Brief' makes the following conclusion on page 2: <br /> <br />"the preponderance of scientific evidence suggests that warmer future temperatures <br />will reduce future Colorado River stream flow and water supplies.,,150 <br /> <br />Over the years, a number of different studies have estimated natural flow at Lee Ferry and <br />a few studies have estimated the natural flow at the mouth of the river at Yuma. Reconstructing <br />flows for the river at Yuma are much more difficult because the Gila River System is normally dry <br />below Phoenix, making an estimate of natural losses on the Gila as that stream crosses the desert is <br />very difficult. <br /> <br />The following table shows the evolution of the natural flows at both Lee Ferry and where <br />available, Yuma from a number of different sources: <br /> <br />149Further studies using paleohydrology techniques have verified that indeed 1878 to 2006 was a very dry period. The paleohydrology <br />will be discussed later in this chapter. <br /> <br />ISOSee http://books.nap.cdu/catalog/ll857.html. <br /> <br />Page -66- <br />