Stream bank erosion as a source of sediment within New Zealand catchments

ABSTRACT

Stream bank erosion has been anecdotally identified as an important source of sediment in New Zealand catchments, however, there have been few attempts to quantify its contribution. Here we use a radionuclide-based sediment tracing approach to determine the relative contribution of stream bank- and hillslope-derived sediment within three catchments in the upper North Island of New Zealand. Both lithogenic (radium-226 and radium-228) and fallout radionuclides (caesium-137 and excess lead-210) were used to differentiate sediment derived from stream bank and hillslope sources. The relative contribution of stream banks and hillslopes to fluvially transported suspended sediment were predicted using a mixing model approach. Our results indicate that both fallout and lithogenic radionuclides provide good source differentiation. We demonstrate that stream bank erosion can contribute very high proportions of sediment within New Zealand catchments. We used independent assessments of bank erosion from each of the study catchments to support the sediment source fingerprinting results. Further work is required to determine the spatial and temporal variability of the contribution of sediment from stream banks. Information on the importance of different sediment sources is needed to target limited catchment rehabilitation resources where they will have the most impact.

KEYWORDS:

• Stream bank erosion
• catchment sediment sources
• fallout radionuclides
• sediment fingerprinting
• lithogenic radionuclides
• caesium-137
• excess lead-210

Acknowledgements

The Mangaotama catchment data were collected as a component of MH’s MSc thesis research. Gamma spectrometry analysis was carried out at the Institute of Environmental Science and Research’s (ESR) National Centre for Radiation Science in Christchurch. Reviews by two anonymous referees, Sebastian Naeher and Rob Davies-Colley improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The Hoteo catchment data were collected for a project within NIWA’s Strategic Science Investment Fund (SSIF) funded Managing Mud Programme. Data analysis for the Mangaotama catchment data was funded by NIWA’s SSIF funded Long-term Stream Response to Sustainable Land Management Project. The Kopurererua Stream data were collected as part of a project funded by the Bay of Plenty Regional Council (Contract Number: 2014 0053). The Manaaki Whenua - Landcare Research-led Smarter Targeting of Erosion Control Programme funded the drafting of this manuscript.