Hillslope response to climate-modulated river incision in the Waipaoa catchment, East Coast North Island, New Zealand

Abstract

Quantifying how hillslopes respond to river incision and climate change is fundamental to understanding the evolution of uplifting landscapes during glacial-interglacial cycles. We investigated the interplay among uplift, river incision, and hillslope response in the nonglacial Waipaoa River catchment located in the exhumed inner forearc of an active subduction margin on the East Coast of the North Island of New Zealand. New high-resolution topographic data sets (light detection and ranging [lidar] and photogrammetry) combined with field mapping and tephrochronology indicate that hillslopes adjusted to rapid latest Pleistocene and Holocene river incision through the initiation and reactivation of deep-seated landslides. In the erodible marine sedimentary rocks of the Waipaoa sedimentary system, postincision deep-seated landslides can occupy over 30% of the surface area. The ages of tephra cover beds identified by electron microprobe analysis on 80 tephra samples from 173 soil test pits and 64 soil auger sites show that 4000–5000 yr after the initiation of river incision, widespread hillslope adjustment started between the deposition of the ca. 14,000 cal. yr B.P. Waiohau Tephra and the ca. 9420 cal. yr B.P. Rotoma Tephra. Tephrochronology and geomorphic mapping analysis indicate that river incision and deep-seated landslide slope adjustment were synchronous between main-stem rivers and headwater tributaries. Hillslope response in the catchment can include the entire slope, measured from river to ridgeline, and, in some cases, the interfluves between incising subcatchments have been dramatically modified through ridgeline retreat and/or lowering. Using the results of our landform tephrochronology and geomorphic mapping, we derive a conceptual time series of hillslope response to uplift and climate change–induced river incision over the last glacial-interglacial cycle.