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Sources of eroded soils and their contribution to long-term sedimentation in the Firth of Thames

TR 2016/32

Report: TR 2016/32

Author: A Swales, M Gibbs, G Olsen, R Ovenden, K Costley (NIWA), T Stephens (Dairy NZ)

About this report

Waikato Regional Council, Dairy NZ, and NIWA co-funded a study to determine current and historical sources of fine sediment accumulating in the Firth of Thames. Sources were identified using carbon isotopic analyses of catchment soils, river sediments, and dated estuarine sediment cores. The cores, collected from the southern Firth of Thames, preserve records of environmental change over the last 1,800 years, including the effects of large-scale catchment deforestation and subsequent conversion to land uses including pastoral agriculture and production forestry since the mid-1800s.

Results from the study showed that the amount of sediment deposited into the Firth of Thames by each of the Waihou and Piako Rivers differed markedly; the Waihou River deposited about four times more sediment than the Piako River. Overall, the study showed that about 50% of the sediment being deposited from the rivers was from pine forestry. Dairy and Sheep and Beef land contributed about 14% and 13%, respectively.

Over the past century, sedimentation rates in the Firth of Thames have increased about 10-times and the dominant source has shifted from marine organic sediment to terrestrial subsoil. 

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Sources of eroded soils and their contribution to long-term sedimentation in the Firth of Thames [PDF, 2.4 MB]


  Executive summary
1 Introduction
1.1 Sediment source tracking
1.2 Land use change and estuary sedimentation
2 Methods
2.1 Present-day land use and soil erosion sources
2.2 Soil sampling
2.3 Estuarine sediment sampling
2.4 Sediment composition
2.5 Radioisotope data and sediment accumulation rates
2.6 Sediment sources
3 Results
3.1 Sources of sediments by river tributary
3.2 Contemporary sources of catchment sediments
3.3 Sediment core geochronology and sedimentation
3.4 Historical changes in sources of sediments accumulating in the Firth of Thames system
4 Discussion
4.1 Contemporary sources of eroded soils in the Waihou and Piako Rivers
4.2 Changes in sediment accumulation rates
4.3 Changes in sources of eroded soils accumulating in the Firth of Thames
5 Conclusions
6 Acknowledgements
7 References
  Appendix A: Bulk carbon and fatty acid delta-13C data for catchment soils and sediments, and estuarine sediment cores
  Appendix B: Radioisotrope dating
  Appendix C: Summary of CSSI method
  Appendix D: Statistical analysis of sources and tracers