Late Quaternary Landscape Evolution and Fluvial Dynamics of the Hautapu River Catchment, New Zealand

Quantifying erosion and sediment yield is fundamental to understanding processes of landscape formation. Here, we focus on a paleo-landscape reconstruction for the Hautapu River Valley, New Zealand, with the aim of investigating the influence of climate, sediment supply and uplift on fluvial and landscape dynamics. The study area is characterised by Pliocene marine sediments and a suite of late Pleistocene river terraces overlain by coverbed deposits that provide a regional chronostratigraphy. Channelized mass-flow deposits of the Mataroa Formation, derived from collapse of Mt Ruapehu andesite stratovolcano, overlie the oldest-preserved terrace. The remnant surface of these mass-flow deposits is used to reconstruct the paleogeomorphology of a 180-km 2 area of the Hautapu River catchment. The depth of incision is constrained by the reference surface and is obtained by comparing the reconstructed surface with present day topography. An early channel switching event at c. 125 ka is correlated to an increase in sediment supply and resultant fluvial aggradation following emplacement of the Mataroa Formation. Remobilisation of volcaniclastic material resulting in blockage and channel avulsion is thought to have redirected flow of the Haupatu River into the Mangoiwa Stream valley, evidenced by the gentle gradient, wide and more deeply incised valley geomorphology. A more recent channel abandonment event at c. 15 ka is correlated to subsidence of the Ruapehu Graben and piracy by the Whangaehu River, effectively isolating the Hautapu catchment from Mt Ruapehu. Simultaneous abandonment of the Mangoiwa Stream valley and re-establishment of the Hautapu River into its former channel is attributed to uplift and strength contrasts between valley fill and the underlying Latest Miocene-Early Pliocene marine succession. Accelerated erosion during glacial and stadial periods, evidenced by fluvial aggradation, is considered to have effectively stripped regolith from deforested highlands leading to greatly increased sediment yields. Subsequent landscape stabilisation and soil formation during the Holocene is associated with a reduction in sediment supply and enhanced fluvial incision.