Rapid post-settlement floodplain accumulation in Northland, New Zealand

Abstract

Many river systems, within New Zealand and globally, have experienced rapid acceleration in floodplain sedimentation in response to anthropogenic catchment disturbance, creating significant issues (i.e., flooding and water quality). Reconstruction of past river responses to environmental- and anthropogenic-driven changes in sedimentation and erosion can provide valuable insights into these river dynamics. This research uses LiDAR data, sedimentology, radiocarbon chronology, XRF analysis and GPR to examine pre- and post-settlement alluviation of the Kaeo River, and floodplain sedimentation in Northland, New Zealand. In Kaeo, rapid rates of post-settlement floodplain aggradation, equating to over 4m of interbedded sand and silt alluvium in a partly-confined valley setting, have created considerable contemporary flooding issues. Radiocarbon dates indicate that terrestrially sourced sedimentation commenced at 7680–7570cal.yrBP, and continued at an average rate of <1mmyr−1. Under conditions of limited accommodation space, the Holocene floodplain has accumulated at a faster average rate (8–13.5mmyr−1) in the last several hundred years in response to anthropogenic catchment disturbance following Māori and European settlement. This response mirrors the general trend for Northland floodplains, where there has been rapid accumulation of floodplains (3–10mmyr−1) in the last 1000yr.