The lower Ica Valley on the hyperarid south coast of Peru is today largely depopulated and bereft of cultivation, yet its extensive archaeological remains attest to substantial prehispanic populations. This paper describes archaeological investigations to retrace changes in geomorphology, ecology, and land-use in Samaca, one of the riparian oasis basins of the lower Río lea, with the aim of investigating when, how, and why such changes took place. Archaeological interpretations of culture change in the region often invoke the impacts of major ENSO perturbations (El Niño). While our investigations confirm that major El Niño events around the end of the Early Intermediate Period likely offer part of the explanation for marked landscape change in the Samaca Basin, we also demonstrate the significance of more gradual, human-induced destruction of Prosopis pallida (huarango) riparian dry-forest. Huarango is a remarkable leguminous hardwood that lives for over a millennium and provides forage, fuel, and food. Moreover, it plays a crucial role in integrating fragile desert ecosystems, enhancing soil fertility and moisture, and accomplishing desalination and microclimatic amelioration. We propose that south coast valleys remained densely forested well into the Early Intermediate Period, attenuating the impact of El Niño events and supporting hitherto underappreciated agroforestry adaptations. Gradual deforestation eventually crossed an environmental threshold: river and wind erosion increased dramatically and precipitated radical desertification, feeding back into cultural changes in the Middle Horizon. Thus we argue Prosopis-human ecological relationships merit proper recognition in our archaeological interpretations of the south coast of Peru.