A 2000 year record of palaeofloods in a volcanically-reset catchment: Whanganui River, New Zealand

Abstract

Palaeofloods in the Whanganui River, North Island, New Zealand are investigated using floodplain sedimentary archives at two locations in the lower Whanganui catchment. The ca. AD 232 Taupo volcanic eruption transformed the lower valley of the Whanganui River, emplacing a substantial volume of volcanogenic mass flow material and providing a new starting point for subsequent alluvial sedimentation. At Atene a high–resolution archive of flood sediments is preserved in a valley meander cutoff in the lower reaches of the Whanganui Gorge, where a ~9 m core was extracted. At Crowley House further down valley, two ~5 m cores were also extracted from a terrace-confined floodplain. Organic material from these cores allows the timing of floods at these sites to be constrained using 11 radiocarbon dates (ten from Atene, one from Crowley House). Flood magnitudes are reconstructed using XRF core-scanned geochemistry as a proxy for flood unit grain size. An age-depth model at Atene identifies three distinct phases of sedimentation with above average flood activity recorded at 1450–1125, 950, 650–500, and 400–325 cal. yr BP, which can be linked to the El Nino Southern Oscillation (ENSO) and strengthening of the Southern Hemisphere Westerly Wind circulation. Large floods also cluster in the late 1800s, reflecting a combination of enhanced storminess and land cover change, which also resulted in deeper erosion of regolith in the catchment, revealed by cosmogenic analysis at Crowley House. Climatic and non-climatic drivers are responsible for floods in the Whanganui catchment over the past ~2000 years, with the largest floods occurring during La Nina and positive Southern Annular Mode conditions. The timing of the largest single flood in the Whanganui in this period is consistent with the volcanic-resetting event itself of AD 232. This study demonstrates the close relationship between regional climate variability in the south-western Pacific Ocean and the occurrence of extreme floods in New Zealand, and the importance of using multi-centennial length hydrological series for effective flood risk assessment.