Case Studies:Case study 14

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Human and environment interactions in northern Peru during the late Holocene
Dillehay et al.'s (2004) paper studies the responses to climatic changes of the Moche, Chimu and Inca societies (c. AD250-1553), in the Jequetepeque and Zana valleys on the north coast desert of Peru. Periods of extreme aridity are suggested by extensive deposits of aeolian sand and abandonment of large irrigation canals. This is seen in stratigraphic records from excavated canals in the Jequetepeque Valley during Late Moche and Lambayeque periods (AD650-1100). Intermittent debris-flow and fine grained aeolian deposits characterise the profiles of most gullies. Thin sheet deposits (1-3mm thick) of low energy sedimentation probably associated with normal precipitation and runoff. Clear evidence is also present for erosional truncation of gully walls by periodic vertical aggradation and channel cutting and widening indicative of high energy debris-flow events. These erosional outwash events are characterised by 30 to 90cm thick, graded debris-flow layers of unconsolidated coarse grained sediment and rock fragments that suggest rapid aggradation and water level fluctuations most likely associated with El Nino events. Graded beds 30-85cm thick were also recorded in several natural drainage cuts in archaeological sites throughout the lower valley contained Late Moche and Chimu ceramics indicative of high magnitude outwash events that radiocarbon date to AD415-650, 560-780, 1185-1295, 1180-1420 and 1640-1950.

Stratigraphic evidence in the non-elite domestic structures and elite platform mounds of many Late Moche sites reveal erosional truncation of floors and debris caused by high-energy debris flows. Human responses to El Nino floods included defensive overflow weirs designed to reduce the pressure of excessive flow rates and erosion to the aqueduct bed. Also, aqueduct foundations were constructed of large fieldstones that incorporated small diversion channels to further inhibit structural damage from high-flow precipitation events. Communities avoided severe damage by locating key settlements and agriculture infrastructure in areas less susceptible to rapid runoff.

Prolonged drought may have been more detrimental to sustained agriculture than episodic El NIno flooding as multistratified palaeodunes that are inter bedded with major outwash deposits dating between AD1000 and 1450. Human responses to the droughts included periodic abandonment of buildings and collapse of production systems. The rich alluvium deposited by the floods, which also recharged depleted groundwater tables, enabled rural populations to sustain themselves in the low-lying desert areas where the water accumulates after flooding.

The full article is Dillehay, T., A.L. Kolata and Mario Pino Q (2004) Pre-industrial human and environment interactions in northern Peru during the late Holocene The Holocene 14, (2) 272-281. This can be found at http://hol.sagepub.com/cgi/reprint/14/2/272

Keywords: Holocene, Peru, aluvium, climate change, irrigation canal

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