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Panorama of Coporaque and Yanque
Panorama of Coporaque (foreground) and Yanque (background-right), showing the Colca river gorge in the center.

 
The Colca Valley region lies on the western slope of the western branch of the southern Peruvian cordillera, in the Province of Caylloma, Department of Arequipa (see map). The region is dominated geographically by great expanses of especially high, cold puna lands, punctuated by volcanic peaks that reach altitudes of over 5000 and 6000 meters above sea level (masl). The 50 km-long central section of the Colca River Valley was formed by fluvial incision into this plateau during the Pleistocene. The valley is characterized by a series of seven mesa-like alluvial benches of volcanic parent material, divided by higher-angle slopes from the puna lands above 4000 masl to the inner river gorge around 3300 masl (Sandor 1992). These bench surfaces, denominated Qal 1-7 (Quaternary alluvium 1-7) increase in age with altitude, with the most recent surface (Qal 1) at the modern entrenched river channel (Sandor 1992:233-234; Treacy 1989:65-67). Glaciated peaks lie above the puna on both sides of the valley. The Nevados Mismi, Huillcaya, and Quehuisha supply the glacial meltwater which constitute the main source of water for the irrigation systems on the north side of the Colca river. The Nevados Huarancate, Sabancaya, and Ampato fulfill the same function for the south side of the valley. Canal systems vary in length from a few kilometers to over thirty. Most of the present valley population of 21,000 inhabitants is distributed in twelve villages, originally constructed between 1571-1574 under the reducción policy of Viceroy Toledo (Málaga 1977:116-123; Denevan et al. 1986:47).
 
The project survey area is approximately 90 square kilometers in size, and surrounds the villages of Yanque and Coporaque (see map). This specific area was chosen for a number of reasons. First, the area chosen includes the kichwa and suni ecological zones of the valley, as well as broad expanses of surrounding puna where camelid pastoralism was likely to have been most intensive (see Figure 2). Also, the greatest amount of archaeological research has been conducted in this area of the valley, providing the best possible preliminary information. Third, as observed in the my preliminary reconnaissance (Wernke 1996, 1997), there is a relatively high density and quality of preservation of sites in the chosen area. Finally, this area provides the greatest spatial overlap with the area inspected in a series of sixteenth and seventeenth century census inspections, which constitutes the analytical centerpiece of the historical portion of the project. An all-weather road frequently traveled by buses and taxis runs through the puna and between villages, providing relatively easy access throughout the survey area.
 
The Colca Valley is one of the most intensively terraced regions in the Andes, making it an ideal location for the investigation of prehispanic Andean production and political dynamics. Approximately 11,000 hectares of agricultural terraces cover both sides of the valley, from the Qal 1 terrace of the inner river gorge to abandoned terraces between approximately 3600 and 4000 masl (Denevan 1986b, 1988b). Agricultural lands in the central section of the valley thus extend above the 3500-3600 masl upper limit of maize. The more frost-tolerant quinoa and potato crops can be grown up to approximately 4000 masl (ibid:24). Modern farmers plant maize, quinoa, and potaoes along with Eurasian cultigens in the valley, concentrating maize in the lower reaches of the valley, preferring steep terraced slopes for their cold air drainage (frost prevention) characteristics (Treacy 1989:81-82, 297-298). Approximately 42% of the terraces are presently under cultivation. The presently cultivated terraces tend to cluster below ca. 3650 masl (ibid:10; Denevan and Hartwig 1986:103). Surrounding this irrigated "core" of presently-cultivated terraces is a periphery of higher irrigated and unirrigated terraces of generally earlier construction date (Treacy 1989; see below).
 
Soils in the study area are very fertile Mollisols of volcanic alluvium and colluvium parent material with high quantities of organic matter (Sandor 1986:242, 1992: 232-236). The high organic content, good tilth, and high water capacity of these soils compare favorably in overall fertility to the Mollisols of the midwestern U.S. (Sandor 1986:249).
 
The climate of the Colca valley is cool, semiarid, and characterized by strong seasonality in precipitation. Due to the high altitude of the area, mean annual temperatures are low (10 degrees Celsius at about 3600 masl), and diurnal temperature fluctuations are much greater than seasonal ones. Frosts can occur during any month of the year (ONERN 1973:57). Water is without question a major limiting factor for agricultural production in the Colca Valley (Denevan 1986b, Gelles 1990; Guillet 1992). The average annual precipitation of 387.7 mm is insufficient for unirrigated maize or potato production. Precipitation also widely varies annually, and is highly seasonal, with about 65 per cent falling between the months of January and March, making cultivation highly precarious or impossible without some kind of water augmentation regime.