Simulating post-LGM riverine fluxes to the coastal zone: The Waipaoa River System, New Zealand

HydroTrend, a climate-driven hydrologic transport model, is used to simulate the suspended sediment discharge of the Waipaoa River System (WRS) over the last 5.5 kyr. We constrain the precipitation input with a paleo-rainfall index derived from the high-resolution Lake Tutira storm sediment record. The simulation is extended to 22 ka using a lower resolution version of the model, constrained by terrestrial and marine paleoenvironment indicators and a simulated model of northeast New Zealandʹs climate at the Last Glacial Maximum (LGM). Comparison of the 5.5 kyr simulation with the shelf sediment core MD97-2122 suggests that the sediment flux variations observed on the shelf primarily reflect changes in rainfall associated with wetter and drier periods of centuries to millennia duration. Storage of sediment on the Waipaoa River floodplain (Poverty Bay Flats) moderates the signal by reducing the sediment flux reaching the coast. During the LGM conditions were more erosive than the Holocene with tussock and grass dominated vegetation. For erodibility four times the Holoceneʹs and half todayʹs, the LGM Waipaoa River System would have generated approximately half the current sediment yield and about 3 times the amount generated when the catchment was fully forested.