Summary

Auckland Council (Environmental Evaluation and Monitoring Unit) contracted Manaaki Whenua – Landcare Research (MWLR) to analyse data from their event-based sediment monitoring network. The work builds on previous analyses of sediment yields from the network by Curran-Cournane et al. in 2013¹ and by Hicks et al. in 2021.²

The objectives of this study are listed below.

1. Produce long-term specific sediment yield estimates for 11 catchments, comparing specific time periods between 2012 and 2024 and provide related analysis and discussion.

2. Analyse annual, inter-annual, and spatial variability in sediment yields among the monitored catchments.

3. Discuss significant event years including flood or drought years, with specific commentary on the 2023 storm events.

4. Provide commentary of how climate change may influence yields in future.

5. Discuss the feasibility of using monitoring data for identifying current sediment loads and differentiating between natural and anthropogenic components to assist in setting limits for sediment management.

6. Compare load estimates derived from event-based sediment monitoring with estimates derived from discrete river water quality monitoring.

This report compares specific sediment yield estimates for 11 catchments, comparing yields for: the full record for each of the eleven sites, from the start of monitoring to June 2024; July 2012 to June 2024 for 8 sites (longest overlapping period); and July 2020 to June 2024 for all 11 sites (longest overlapping period for new sites). Estimates derived from event-based sediment monitoring are compared with estimates derived from discrete river water quality monitoring at 6 sites where both data sets are available. A list of common abbreviations is provided on p. iv.

The coefficient of determination (𝑅²) values for all event-based sediment rating curves (SRCs) exceeded 0.8, and the value of the concordance correlation coefficient (𝐶𝐶𝐶) was above 0.9, indicating a strong model fit. Sediment rating residuals at four sites (Mangemangeroa River, Te Muri-o-Tarariki Stream, (hereafter ‘Te Muri Stream’) Wairoa River and Kōurawhero Stream (referred to in this report as the ‘Kōurawhero River’) experienced significant temporal changes and required a time-trend adjustment of the sediment load estimates. Continued monitoring is required to identify future drift in the ratings.

Compared to event SRCs, SRCs developed from discrete water quality sampling data at six sites had lower model quality, with an average 𝑅² of 0.6 and a 𝐶𝐶𝐶 of 0.68. At all sites except West Hoe Stream, manual adjustment of the rating extrapolation was required to obtain reasonable 𝑆𝑆𝐶 estimates. Overall, sediment yields estimated using the discrete SRCs were on average 1.66 times higher than those estimated with the event SRC for the same hydrological years with variation across the sites. This overestimation is partly due to limited sample availability from discrete water quality monitoring.

Mean annual suspended sediment yields (𝑆𝑌) ranged from 12.5 to 26,203 t/yr across the monitoring sites from event-based rating curve estimates. Mean annual specific sediment yields (𝑆𝑆𝑌) for the full records range from 23.5 to 221 t/km²/yr, with the lowest at the native forested West Hoe Stream, and highest at Mahurangi River. These sites also had the greatest difference in mean specific sediment yields for the July 2020 to June 2024 period. For the eight sites with data available for July 2012 to June 2024, estimated mean specific sediment yields range from 23.5–107 at West Hoe Stream to Mangemangeroa River, respectively. The hill country catchments of the Mahurangi River, Te Muri Stream, and Mangemangeroa River sites fall into the regionally ‘high’ and ‘very high’ categories, while the mixed terrain and land cover catchments of the Hōteo River, Kōurawhero River, Ōrewa Stream, Kaipara River, and Wairoa River sites are classified as ‘medium’. The remaining sites, Kaukapakapa River, West Hoe Stream, and Lower Vaughan Stream, which are primarily lowland or forested hill country catchments, are in the ‘low’ category. However we note that the arbitrary class definitions are relative to the Auckland region only.

Comparisons of sediment yields estimated from event-based and discrete monitoring programmes highlight the challenges of capturing data across the range of events experienced in these catchments. During the reporting period of 2009–2019 (as covered by the previous reporting of Hicks et al. 2021 (see footnote 2) an average of 70% of the total sediment yield had been sampled, while only 43% of the total yield had been sampled on average during the 2009–2024 period (i.e. the full record presented in the current report). This has contributed to greater uncertainty in total yield estimates, compounded by 2023 containing the largest river flows in most site records relating to large storm events. Continuous monitoring methods, such as turbidity sensors, offer the potential to continuously monitor sediment loads and reduce the uncertainty in long-term sediment yield estimates produced by rating curve methods, provided they are adequately maintained and calibrated.

Changes in catchment hydrology, land cover, and land use through time can lead to changes in the relationship between discharge and sediment yield, requiring adjustment of sediment rating curves. Analysis of the long-term sediment data in the present report identified statistically significant shifts in the SRCs at the Kaipara River, Kaukapakapa River, Mangemangeroa River, Te Muri Stream, and Wairoa River sites. At the Ōrewa and Kōurawhero sites, shifts could be seen, but statistical tests indicated the shifts were not significant. Continuation of monitoring will enable future shifts in sediment ratings to be identified, which may occur at any of the monitoring sites due to future changes in hydrology, land cover, and land use. 

The impact of future climate change on erosion and sediment loads is expected to vary spatially across the Auckland region, driven by divergent trajectories for surface erosion, riverbank erosion, and shallow landslides. The net effect on sediment loads will reflect the relative contribution of these processes to catchment loads. Neverman et al. (2023)³ estimated climate change may increase sediment loads delivered to the coast by between 14% and 75% by late century.

Data from the sediment monitoring programmes is used in combination with other methods, including sediment fingerprinting and sediment budget modelling, to estimate the contribution of erosion sources to sediment loads to support catchment and regional planning efforts.

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1. Curran-Cournane F, Holwerda N, Mitchell F 2013. Quantifying catchment sediment yields in Auckland. Auckland: Auckland Council

2. Hicks DM, Holwerda N, Grant CM 2021. Rural catchment sediment yields from the Auckland region: state of the environment reporting. Auckland: Auckland Council.

3. Neverman AJ, Donovan M, Smith HG, Ausseil A-G, Zammit C 2023. Climate change impacts on erosion and suspended sediment loads in New Zealand. Geomorphology 427:108607. https://doi.org/10.1016/j.geomorph.2023.108607

Auckland Council, Manaaki Whenua – Landcare Research

September 2025

See also

Te oranga o te taiao o Tāmaki Makaurau. The health of Tāmaki Makaurau / Auckland’s natural environment in 2025. A synthesis of Auckland Council’s state of the environment reporting