Sometime at or before 1050 A.D. the level of Lago Wiñaymarka dropped to 7-12 m below overflow level, or 12-18 m below its modern average of 3810 m.a.s.l. ([Abbott et al. 1997a]: 179) and seems to have remained low until the arrival of the Inka empire - and the Little Ice Age - in the Titicaca Basin around 1450 A.D. Kolata and his colleagues have argued that this drop coincides with a period of reduced rainfall which caused the collapse of the Tiwanaku agricultural base - principally raised field production in their view ([Binford et al. 1997,Kolata and Ortloff 1996a,Ortloff and Kolata 1992]).
Janusek summarizes this position as follows:
Recent paleo-ecological research strongly suggests that changing climatic conditions triggered a profound crisis in Tiwanaku political economy. Sediment cores from Late Titicaca and paleo-environmental data from the Quelccaya ice cap of southern Peru point to a substantial decrease in yearly precipitation around A.D. 1000, possibly leading to drought conditions in the altiplano and the collapse of intricate raised field agricultural systems. The timing of the ecological data corresponds well with the full scale ... urban depopulation [documented by] our excavations at Tiwanaku and Lukurmata. ([Janusek 1994]: 384)Janusek suggests that the late Tiwanaku period was characterized by increasing tensions between a centralizing tendency of the state and autonomous local sources of authority and power. Stress on the agricultural base only served to exacerbate existing tensions between state and local power sources, leading to a generalized crisis of the political economy and to state collapse.
Whether or not the Tiwanaku state collapsed as a result of a massive, climatically-induced agricultural failure - and this is by no means certain - collapse it did, and the event took place sometime between 1000 and 1100 A.D. The collapse was perhaps more properly a process than an event - though what is an event but a rapid process? - and seems to have taken place over the course of as much as a century. It was certainly complete by 1100 A.D., and it is this date which I use here for the Tiwanaku/Early Pacajes transition. What followed the Tiwanaku period was an entirely different social order.
After the decline of Tiwanaku, cities and urban civilizations disappeared in the Lake Titicaca Basin for nearly 400 years. Central organization and management of intensive agriculture, craft production, long-distance trade, and other sources of wealth broke down. Across the south-central Andes, human populations dispersed across the landscape, and settled into smaller, defensible settlements. The demise of the Tiwanaku empire brought with it widespread political instability. ([Kolata 1993]: 299)Kolata's catastrophic vision of Tiwanaku state collapse certainly seems to be borne out by the Taraco Peninsula settlement data. In addition, however, the Taraco data suggest that the reorganization of the settlement system was accompanied by a general demographic collapse on the peninsula. The end of the Middle Horizon brought about profound changes in virtually all aspects of Titicaca Basin life.