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

•Floodplain evolution is conditioned by the degree of valley-floor confinement.•Climatically and anthropogenically driven variations in sediment flux•Region-wide progressive floodplain alluviation through the mid to late Holocene•Increased river activity between 3500 and 2800 cal. YBP•Incision and terrace formation beginning after 1900 cal. YBP

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.