Unconfined resedimentation in the Rangitaiki catchment, North Island, New Zealand: precursors to river re-establishment following the 1.8 >ka Taupo eruption
The potential for generation of dangerous and damaging lahars and floods in response to tephra deposition from volcanic eruptions is now widely appreciated. The style and tempo of this response varies both from eruption to eruption and from catchment to catchment for single eruptions, and an understanding of factors controlling this variation is needed for precise hazard assessment. For the 1.8 ka Taupo eruption, various studies have already addressed the intracaldera response, including the refilling of Lake Taupo, and the extra-caldera re-establishment of the Waikato River, the natural outlet, which involved the catastrophic release of 20 km3 of water. Far-field effects of the eruption have also been investigated in mountainous catchments draining eastwards into Hawke's Bay. The topography of the upper Rangitaiki catchment, the subject of this study, differs in that the Taupo ignimbrite buried an essentially flat land surface inherited from a ~ 320 ka welded ignimbrite sheet. Previous researchers have inferred that the Rangitaiki River was immediately re-established as a meandering stream, implying sediment loading and transport rates similar to those of the present. Our mapping in contrast reveals that a low gradient area of hundreds of square kilometres was initially flooded and washed by hyperconcentrated and sheet flows, resulting in shallow reworking. In higher gradient areas, re-integration of drainage systems was accompanied by incision of deep channels and gullies, interspersed with ephemeral lakelets formed in response to localised damming by pyroclastic material. Ephemeral braided streams and meandering rivers eventually developed as stable rill and gully systems were established and overland flow across the remaining unconsolidated deposits diminished. This pattern of landscape response has implications for the timing and nature of the remobilised pyroclast flux to downstream depocentres and the Bay of Plenty coast more than 100 km to the north.
Eos Trans. AGU, 83 (22), Western Pacific Geophysics Meeting Supplement, Abst., SE32C-03, 2002.