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Seasonally snow-covered areas of Earth’s mountain ranges are important components of the global hydrologic cycle. Although their area is limited, the snowpacks of these areas are a major source of the water supply for runoff and ground water recharge over wide areas of the mid-latitudes. They are also sensitive indicators of climatic change. The release of ions from the snowpack is an important component in the biogeochemistry of alpine areas and may also function as a sensitive indicator of changes in atmospheric chemistry. The demand for water in the semiarid areas of the western United States is reflected in extensive systems of reservoirs, canals, and flow diversions that have been constructed over the past century. Most of the water resources tapped by these systems derives from the mountain environments of the Rocky Mountains, where contributions of the alpine have long been recognized (Martinelli 1975). In Colorado, 9000 km2 of alpine terrain, less than 4% of the state’s area, provide more than 20% of the state’s streamflow and is especially important in maintaining late-summer flows (Martinelli 1975). Lakes in the Rocky Mountains are relatively uncontaminated compared with many other high-elevation lakes in the world, with the median value of NO-3 concentrations less than 1 μeq L-1 (Psenner 1989). However, in comparison with downstream ecosystems, these high-elevation ecosystems are relatively sensitive to changes in the flux of energy, chemicals, and water because of extensive areas of exposed and unreactive bedrock, rapid hydrologic flushing rates during snowmelt, limited extent of vegetation and soils, and short growing seasons (Williams 1993). Hence, even small changes in atmospheric deposition have the potential to result in large changes in ecosystem dynamics and water quality (Williams et al. 1996a). Furthermore, these ecosystem changes may occur in alpine areas before they occur in downstream ecosystems (Williams et al. 1996b). Apart from its use in municipal supply, agriculture, recreation, and power generation, this water also mediates transfers of geomorphic and biological materials. For this reason, the drainage basin, or catchment, has long been recognized as a basic geomorphic unit in environmental research (e.g., Chorley 1967; Bormann and Likens 1969).

This chapter investigates the role of environmental and political disruption on migration patterns in the South-Central Andes during the Middle Horizon (c. A.D. 500–1100) and Late Intermediate Period (c. A.D. 1100–1400) in the San Pedro de Atacama oases of northern Chile. During the centuries covered by this study, life in the oases was impacted by the decline of the large, expansive Tiwanaku polity in neighboring Bolivia and a severe drought. This study presents bioarchaeological and biogeochemical, including isotopic, data to identify migration in the oases. These data suggest that what had been potentially regular and visible migration and interregional movements in the Middle Horizon was greatly reduced in the subsequent Late Intermediate Period, a period characterized by political and environmental disruption. This chapter illustrates that human responses to environmental disruptions vary, and may or may not include migration from one environmental zone to another, presumably less affected, zone.