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Yet the average virgin flow of the River from 122-1985 at <br />Lee's Ferry, where the Upper Basin relinquishes water to the <br />Lower, was only 14.3 m.a.f.47 For substantial periods of <br />time during recent decades it has been less. From 1930 to <br />1985 the flow averaged 13.9 m.a.f. per annum, 48 and from <br />1953 to 1964 it averaged only 11.6 m.i.f. per ann .49 <br />The volume of water emptying into the Colorado River may <br />decline still further in the future. Analyses y the Tree <br />Ring Laboratory at the University of Arizona yielded an <br />estimate that the average long-term flow of t e River at <br />Lee's Ferry "was 13.5 m.a.f. per annum between 1564 and <br />1960.50 The lowest ten-year flow during those four centuries <br />was 9.7 m.a.f. per annum between 1584 to 1593,51 and it is to <br />be expected that at some point in the future the flow will <br />again subside to that level or less. Indeed, the worst <br />drought of which researchers are now aware occurred before <br />1564. Tree-ring studies completed in 1979 indicate that the <br />driest period in the Colorado Basin was from 1130 o 1180 and <br />was probably what drove the Anasazi off the Col rado plat- <br />eau.52 Furthermore, the "greenhouse effect" m y diminish <br />River flows even below projections based on these historical <br />studies. In 1983 a report by the Carbon Dioxide Assessment <br />Committee of the National Academy of Science warned that <br />increased levels of carbon dioxide in the atmosphere could <br />reduce the water supply on the Upper Colorado by 3 .6 percent <br />and on the Lower Colorado by 56.5 percent.53 <br />-20-