Quantifying temporal variations in landslide‐driven sediment production by reconstructing paleolandscapes using tephrochronology and lidar: W aipaoa R iver, N ew Z ealand

Abstract Hillslope response to climate‐driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (∼18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low‐relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km 2 tributary principally composed of weak mélange—pervasive post‐LGM landslides responded to >50 m of fluvial incision by sculpting and denuding >99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar‐derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide‐prone terrain to post‐LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post‐LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km 3 , which equates to a subcatchment averaged erosion rate of ∼1.6 mm yr −1 . This is double the previous hillslope volume when normalized by study area, demonstrating that landslide‐prone catchments disproportionately contribute to the terrestrial post‐LGM sediment budget. Finally, we observe particularly rapid post‐Waimihia erosion rates, likely impacted by human settlement.