Storm-generated Holocene and historical floods in the Manawatu River, New Zealand

This paper reports the first reconstruction of storm-generated late Holocene and historical river floods in the North Island of New Zealand. The sedimentary infills of nine palaeochannels were studied in the lower alluvial reaches of the Manawatu River. Floods in these palaeochannels were recorded as a series of sand-rich units set within finer-grained fills. Flood chronologies were constrained using a combination of radiocarbon dating, documentary sources, geochemical markers, and palynological information. Flood units were sedimentologically and geochemically characterised using high resolution ITRAX™ X-Ray Fluorescence (XRF) core scanning and laser diffraction grain-size analysis. The longest palaeoflood record extends back ca.3000 years. The temporal resolution and length of the Manawatu record reflects accommodation space for fluvial deposits, channel dynamics and mobility, and high sediment supply. Floods that occurred in the Manawatu during the mid-1800s at the time of European land clearance and in the first decade of the twentieth century appear to be among the largest recorded in the last 3000 years.