Towards understanding river sediment dynamics as a basis for improved catchment, channel, and coastal management: the case of the Motueka catchment, Nelson, New Zealand

ABSTRACT

This paper brings together work in the Motueka catchment that has focused on both suspended sediment data and bedload transfers to provide a more holistic understanding of sediment dynamics in the catchment to inform effective river management. The annual suspended sediment load averages 349,000 t and shows considerable temporal variability (49,000 t to 1.7 Mt). Event yields may increase by an order of magnitude in response to single high magnitude storm events. Much of the sediment is generated from high rainfall areas of the catchment under indigenous forest and grassland. Short-term studies show pasture areas have a higher specific sediment yield than production forest, but that forest harvesting leads to a short-term increase in yield. Bedload transfers assessed via morphological budgeting from digital elevation model (DEM) differencing in selected reaches of the upper Motueka reveal similarly highly variable transfers at an annual scale, reflecting the magnitude and frequency of competent flow events. Longer term mean bed-level (MBL) changes demonstrate a high degree of spatial variability in the upper Motueka. Overall, DEMs of difference and longer term MBL changes both reveal a net channel degradation and export of bedload in the mainstem of the upper Motueka. Suspended sediment data also suggest an overall reduction in sediment yield from the catchment, suggesting a catchment-wide limitation of sediment supply, or a period of lower flows reducing sediment mobilization. This understanding has informed on issues such as the role of river channel management and catchment land use on in-stream ecosystems, coastal erosion, and shelf water quality and fisheries. Future river management, if it is to be effective, needs to recognize the history of this system, its likely longer term trajectory, and its linkages with the coast.

Keywords:

• Suspended sediment
• bedload
• sediment yield
• DEM
• gravel extraction

Acknowledgements

The following Massey University students assisted with fieldwork and data analysis in the Motueka: Jane Richardson, Rob Dykes and Simon Vale. Field assistance was also provided by David Feek, Erin Hutchinson, Alastair Clement, Emma Phillips, Arved Schwendel (Massey), Christine Daly, George Knights, Stephanie Merrill, Maria Borlinghaus, Diarmuid Neilan and Claire Butty (Landcare Internships) and Brenda Rosser (Landcare Research).

Funding

This work was funded as part of the Motueka Integrated Catchment Management Programme by the Foundation for Research, Science and Technology under contract C09X0305. We thank the Tasman District Council for providing the long-term cross-section data.