Floodplain Development, El Niño, and Cultural Consequences in a Hyperarid Andean Environment

Abstract

Abstract Using field-mapping, remote-sensing (ASTER, aerial photography), and GIS/GPS, we identified and analyzed reach-scale geomorphic responses to contemporary floods along a 20-km valley length in hyperarid southern Peru. We combined these data with alluvial stratigraphy of remnant terrace patches recording late Holocene aggradation and incision. In this manner we ascertained how climate change, especially the occurrence of the El Niño–Southern Oscillation (ENSO), affects the magnitude and frequency of large floods at mid-elevations of southern Peru's Atacama Desert, and we documented the geomorphic effects of large floods on stream channel properties, especially the large-scale floodplain erosion and channel enlargement occurring during these large ENSO-related floods. Reach-averaged channel widening of ∼30 m occurs during large ENSO-related floods, with floodplain erosion rates of 2 ha/river km commonly occurring. Recovery occurs in two stages: channel narrowing requires several decades of lateral bar development which can ultimately aggrade sufficiently for complete floodplain development to occur. There was a similar response to ENSO floods during the late Holocene, and our radiocarbon dating of alluvial surfaces, combined with an ASTER-DEM of the floodplain, indicates that ∼80 percent of the floodplain is younger than at least 550 14C years. We conclude that the lateral erosion and recovery cycle related to ENSO floods could have been a critical factor for prehistoric societies dependent on floodplain agriculture. If cultural systems can maintain pace with geomorphic change in these irrigated floodplain systems, the loss of arable land may not be critical. However, severe social stresses may result if episodes of accelerated floodplain loss outpace both natural recovery and social adaptation.