Holocene river dynamics in Northland, New Zealand: The influence of valley floor confinement on floodplain development

Valley floor mapping, sedimentology, and 14C-dating have been used to reconstruct the fluvial history at eight floodplain sites spread throughout Northland, a region removed from the main areas of tectonic and volcanic activity in New Zealand. We present a probability-based record of Holocene river behaviour for Northland using 14C-dated Holocene fluvial deposits and compare this with independent palaeoclimate proxy records from the North Island. Holocene floodplain evolution and fluvial behaviour have been conditioned by the degree of valley-floor confinement. In the most and least confined valley settings, Holocene floodplain evolution has involved the development of a single floodplain surface. At partly confined sites, the river terrace and floodplain geomorphology are more complex. Region-wide progressive floodplain alluviation through the mid to late Holocene and a period of increased river activity between 3500 and 2800 cal. YBP in response to climatically driven increases in sediment supply was followed by a period of valley floor incision and terrace formation beginning after 1900 cal. YBP. In partly confined valley settings, this was followed by the aggradation of a lower Holocene floodplain surface, with rapid rates of vertical accretion in response to post-settlement catchment disturbance. The results of this study indicate that valley floor confinement has played a major role in controlling Northland Holocene river floodplain development, producing a continuum of floodplain and river terrace landforms in response to climatically and anthropogenically driven variations in sediment flux.