Since approximately 5200 cal yrs BP, five sets of eight to nine beach ridges were built and preserved along the northwestern Peruvian coastal desert (3°30′S–9°S). Potential ridge-building mechanisms in the hyper-arid environment of northwest Peru include El Niño floods and storms, seismic activity, and sea-level change, as well as more gradual climate changes that affected coastal morphology. Image processing and Geographic Information System (GIS) methods were used to analyze aerial photographs and measure historic coastal patterns along three beach-ridge plains over a 37-year time period. Coastal features were digitized from image mosaics of each ridge plain at different time intervals from 1946 up to 1983. Progradation rates were examined at ridge locations north of the Chira River and Piura River, as well as at the base of ephemeral stream valleys in Colán. The total change in beach area was measured from historic aerial photographs taken at different time intervals. The resulting measurements showed that sediment distributed by El Niño storms was redeposited along the shoreline within a few years following each event. The difference between the frequency of El Niño events (currently 2–7 years) and the rate of ridge preservation (1 per 500 years average) suggests that individual ridges may be composites of multiple depositional events, or that ridges result from the rare convergence of multiple processes and conditions. A change in style of ridge formation in all studied beach-ridge sets correlates with, and may be explained by, change in the frequency of El Niño events at about 3000 cal yrs BP.
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