Guest essay by Samuel I Outcalt
The Data: The major tourist attractions at Mesa Verde National Park (near Cortez, Colorado) are the large cliff top pueblos inhabited by the Anasazi between 1128 and 1273 AD. To explore the relationship between the Anasazi occupancy of these large pueblos and climate change a 101 year center weighted moving average filter was applied to the Annual Treeflow Discharge Estimate of the Colorado River at Lees Ferry, Arizona (180 miles due west). These discharge estimates were calculated using the parameters derived from the linear regression of annual modern stream discharge [dependent variable] and modern regional tree ring data sets [independent variables].
The regression parameters were then used to estimate stream discharge at the gage site for length of the tree ring records. The explained variance of the tree ring data sets used in this reconstruction varied between 57% and 77%. The reconstruction is therefore noisy. The application of the century length filter attenuated high frequency noise in the annual data. The normalized record is displayed below as Figure 1.
Figure 1 indicates that the large pueblo occupancy was limited to a single wet regime near the end of the Medieval Warm Period. When the Colorado River discharge estimate declined to the approximate level of the large pueblo construction in 1128 AD the Anasazi abandon Mesa Verde in 1273 AD.
The Conclusion: The Treeflow Discharge estimate from Lees Ferry supports the hypothesis that evacuation of Mesa Verde and other Anasazi sites in the Colorado Plateau were a response to a regional drought, which reduced the discharge of the Colorado River to the west. It is interesting to note that the Colorado River discharge increased during the Medieval Warm Period and declined during the Little Ice Age.
Treeflow Data for many sites in the contiguous United States are posted at http://treeflow.info/.