From sink to source: Late Quaternary landscape evolution and fluvial dynamics of the Hautapu River catchment, New Zealand

Landscapes are dynamic and respond to a range of internal and external processes that influence the balance between erosion and sedimentary preservation. Documenting past depositional and erosional events is fundamental to understanding processes of landscape formation and unpacking the interplay between stream power and sediment supply. Here, we focus on a paleo-landscape reconstruction of the Hautapu River valley in New Zealand, with the aim of investigating the influence of volcanic activity, uplift, and climate on fluvial and landscape dynamics. The study area is characterized by Pliocene marine sediments superimposed by a suite of Late Pleistocene river terraces and associated cover beds. Volcaniclastic deposits of the Mataroa Formation, derived from partial collapse of the Mt. Ruapehu stratovolcano, overlie the oldest terrace level and are used to reconstruct the paleo-landscape. Depth of incision is obtained by comparing remnant terrace surfaces to present day topography. An early channel switching event after c. 125–150 ka is correlated to increased sediment supply and resultant fluvial aggradation following emplacement of the Mataroa Formation. Remobilization of volcaniclastic material resulting in blockage and channel avulsion is considered to have redirected the flow of the Hautapu 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 and its active source of volcaniclastic sediment. Simultaneous abandonment of the Mangoiwa Stream valley and re-establishment of the Hautapu River into its former channel is attributed to uplift, pre-existing valley morphology and strength contrasts between the valley fill and its underlying marine succession. Accelerated erosion during cold climate phases is considered to have effectively stripped regolith from deforested highlands leading to a reset of the landscape, greatly increased sediment yields and fluvial aggradation. Subsequent landscape stabilization and ongoing regional uplift during climate amelioration is associated with a reduction in sediment supply and excess stream power, leading to incision and knickpoint retreat.