"It is not entirely clear why the Ancestral Puebloans migrated from their established homes in the 12th and 13th centuries. Factors examined and discussed include global or regional climate change (cf. Little Ice Age), prolonged periods of drought, cyclical periods of topsoil erosion, environmental degradation, de-forestation, hostility from new arrivals, religious or cultural change, and even influence from Mesoamerican cultures. Many of these possibilities are supported by archaeological evidence. Current opinion holds that the Ancestral Puebloans responded to pressure from Numic-speaking peoples moving onto the Colorado Plateau as well as climate change which resulted in agricultural failures. The archaeological record indicates that it was not unusual for ancient Pueblo peoples to adapt to climatic change by changing residences and locations. Early Pueblo I sites may have housed up to 600 individuals in a few separate but closely spaced settlement clusters. However, they were generally occupied for a mere 30 years or less. Archaeologist Timothy A. Kohler excavated large Pueblo I sites near Dolores, Colorado, and discovered that they were established during periods of above-average rainfall. This would allow crops to be grown without benefit of irrigation. At the same time, nearby areas experiencing significantly drier patterns were abandoned. The ancient Pueblos attained a cultural "Golden Age" between about 900 and 1130. During this time, generally classed as Pueblo II, the climate was relatively warm and rainfall mostly adequate. Communities grew larger and were inhabited for longer periods of time. Highly specific local traditions in architecture and pottery emerged, and trade over long distances appears to have been common. Domesticated turkeys appear. After approximately 1150, North America experienced significant climatic change in the form of a 300 year drought called the Great Drought, which also led to the collapse of the Tiwanaku civilization around Lake Titicaca. The contemporary Mississippian culture also collapsed during this period. Confirming evidence is found in excavations of the western regions of the Mississippi Valley between 1150 and 1350, which show long-lasting patterns of warmer, wetter winters and cooler, drier summers. In this later period, the Pueblo II became more self-contained, decreasing trade and interaction with more distant communities. Southwest farmers developed irrigation techniques appropriate to seasonal rainfall, including soil and water control features such as check dams and terraces. However, the population of the region continued to be mobile, abandoning settlements and fields under adverse conditions. Along with this change in precipitation patterns, there was a drop in water table levels due to a different cycle unrelated to rainfall. This forced the abandonment of settlements in the more arid or over-farmed locations. Evidence also suggests a profound change in religion in this period. Chacoan and other structures constructed originally along astronomical alignments, and thought to have served important ceremonial purposes to the culture, were systematically dismantled. Doorways were sealed with rock and mortar. Kiva walls show marks from great fires set within them, which probably required removal of the massive roof - a task which would require significant effort. Habitations were abandoned, tribes split and divided and resettled far elsewhere. This evidence suggests that the religious structures were deliberately abandoned slowly over time. Puebloan tradition holds that the ancestors had achieved great spiritual power and control over natural forces, and used their power in ways that caused nature to change, and caused changes that were never meant to occur. Possibly, the dismantling of their religious structures was an effort to symbolically undo the changes they felt they caused due to their abuse of their spiritual power, and thus make amends with nature. Most modern Pueblo peoples (whether Keresans, Hopi, or Tanoans) and historians such as James W. Loewen, in his book Lies Across America: What Our Historic Markers and Monuments Get Wrong(1999), assert the ancient Pueblo did not "vanish" as is commonly portrayed in media presentations or popular books, but migrated to areas in the southwest with more favorable rainfall and dependable streams. They merged into the various Pueblo peoples whose descendants still live in Arizona and New Mexico. This perspective is not new and was also presented in reports from early 20th century anthropologists, including Frank Hamilton Cushing, J. Walter Fewkes and Alfred V. Kidder. Many modern Pueblo tribes trace their lineage from settlements. Evidence also suggests that a profound change took place in the Anasazi area and areas inhabited by their cultural neighbors, the Mogollon. For example, the San Ildefonso Pueblo people believe that their ancestors lived in both the Mesa Verde and the Bandelier areas." Source and further information: http://en.wikipedia.org/wiki/Anasazi