5-year median mud content per cent at monitored estuary and mudflat environment sites in 2023/24 compared to 2024/25.
| 5-year median % mud content |
| Site name |
2023/24 |
2024/25 |
| Brooklands Lagoon/Te Riu-o-Te-Aika-Kawa |
44 |
44 |
| Governors Bay/Ōhinetahi |
66 |
68 |
| Head of the Bay |
52 |
51 |
| Port Levy/Koukourarata |
81 |
81 |
| Okains Bay/Kawatea |
11 |
11 |
| Robinsons Bay/Kākakaiau |
12 |
12 |
| Children’s Bay/Ōtāhuahua |
16 |
17 |
Why are we measuring this?
Muddiness refers to fine silt and clay particles less than 0.063mm in diameter. Often these fine sediments are generated through urban and rural land use, such as earthworks and deforestation. The fine sediments are then washed into aquatic systems, including estuaries, which are particularly vulnerable to fine sediment accumulation over time. Muddiness is characterised by that gloopy, sinking feeling you might feel underfoot when walking through an estuary.
Muddiness is one of the main factors that determines where plants, such as seagrass, and animals, such as shellfish, can live within an estuary, as well as the overall health of the estuary. A small amount of mud can be beneficial because the fine particles contain organic matter that some animals feed on and estuary ecosystems are most resilient when mud content is less than 10 per cent. When mud content is between 10 and 25 per cent, we see a noticeable decline in resilience. When the mud content exceeds 30 per cent, estuary communities become unbalanced. Once mud content is over 60 per cent, the communities are degraded. Five mud content grades are described on the Land, Air, Water Aotearoa website (LAWA).
How do we measure the result?
We collect sediment samples from seven estuary monitoring sites between February and April each year in accordance with the National Estuarine Monitoring Protocol (2002). These estuaries are:
- Brooklands Lagoon/Te Riu-o-Te-Aika-Kawa
- Governors Bay/Ōhinetahi
- Head of the Bay (Whakaraupō/Lyttelton Harbour)
- Port Levy/Koukourarata
- Okains Bay/Kawatea
- Robinsons Bay/Kākakaiau
- Children's Bay/Ōtāhuahua.
These estuaries are considered representative of the region.
Sediment samples are analysed at a certified laboratory to determine the mud content (percentage of silt/clay) at each site. Annual mean mud content values are calculated from the samples, and a 5-year median is used to report the percentage muddiness grade of each site.
What work have we undertaken to contribute to this outcome?
As the regional council for Waitaha Canterbury, we work with ngā Papatipu Rūnanga, Canterbury’s city and district councils, and our communities, to set and implement environmental policies and rules that establish how to look after our coastal environment.
As directed under the New Zealand Coastal Policy Statement (NZCPS), and Canterbury’s Regional Coastal Environment Plan, we have responsibilities to manage activities and development to avoid any significant adverse effects on coastal ecosystems and processes. The Regional Coastal Environment Plan directs the sustainable management of the natural and physical resources of our coastal environment and guides councils in their day-to-day management of the coastal environment.
Measuring mud content in monitored estuary and mudflat sites informs us of the current state of health of these environments and if activities are being appropriately managed. It also helps in our policy evaluation and future planning work.
Who else plays a role?
No single agency is responsible for the management of Canterbury’s coastal environment, so we work together with others to manage its protection and use.
Central government sets policy direction for local authorities to implement through the NZCPS.
Resource users including consent holders such as land developers, farmers, stormwater network operators and industries, are required to follow rules set in regional plans to meet standards and conditions set to protect coastal environments.
Other agencies that undertake monitoring in estuaries include Fisheries NZ/Tini a Tangaroa (fish and shellfish focus) and the Department of Conservation/Te Papa Atawhai (marine reserves focus), alongside citizen science across the country. District councils play an important role when developing stormwater management plans that discharge to the coast.
Where can I find more information?
Read about estuary health on the Land, Air, Water Aotearoa website.
View Ministry for the Environment/Manatū Mō Te Taiao and Stats NZ/Tatauranga Aotearoa publications that use our data for legislated national environmental reporting (i.e. 3-yearly Our marine environment reporting series and national environmental indicators).
