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77 <br />Fullmer Drought <br />Seventy Index <br />DRY _ WET <br />-6-4 -2 0 2 4 6 <br />Fic. 7. Comparison of duration and extent of two severe droughts, the 1950s drought (top) and the drought centered on. 1820 (bot- <br />tom), both reconstructed from tree -ring chronologies (Cook et al. 1996; Cook et al. 1998). <br />tribution of populations, and widespread societal re- <br />organization (Douglass 1935; Dean 1994). This pe- <br />riod of drought is also reflected in unprecedentedly <br />low lake levels as reconstructed through the dating of <br />tree stumps rooted in what are today bottoms of sev- <br />eral streams and Ickes in the Sierra Nevada of eastern <br />California (Stine 1994). <br />Several tree -ring reconstructions allow a temporal <br />evaluation of the thirteenth and sixteenth - century mega- <br />droughts relative to more recent droughts (Dean 1994; <br />Grissino -Mayer 1996; Hughes and Graumlich 1996). <br />Of the reconstructions that reflect both sixteenth- and <br />thirteenth - century droughts, the sixteenth - century <br />drought appears to have been the most severe and per- <br />sistent drought in the Southwest in the past 1000 -2000 <br />years, whereas the thirteenth - century drought was the <br />most persistent and severe drought in the California <br />mountain ranges and, likely, the Great Plains (Weakly <br />1965). It is more difficult to evaluate the spatial extent <br />of the two major paleodroughts. At a minimum, both <br />droughts appear to have impacted the Great Plains, <br />Southwest, southern and western Great Basin, and <br />Sierra Nevada (Figs. 9a,b). A survey of other tree -ring <br />chronologies for the northwestern Great Basin and <br />northeastern Utah (from the International Tree -Ring <br />Data Base, World Data Center -A, Paleoclimatology, <br />Boulder, Colorado) shows marked periods of low <br />growth in the latter part of the thirteenth century in <br />Bulletin of the American Meteorological Society 2703 <br />