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Whanganui/Westhaven Inlet

Whanganui Inlet is a relatively unmodified, large-sized (2741 ha, third-largest of its type in the South Island), macrotidal (3.1 m spring tidal range), shallow (mean depth ~1-2 m at high water), well-flushed (residence time <1 day), seawater-dominated, tidal lagoon type estuary. It has a single wide tidal opening (1.5 km), a large well flushed central basin, several islands, and two main arms (5 km and 7 km long, 1-2 km wide), each with many small arms where streams enter the estuary, and which are often flow constricted by road causeways. The subtidal zone comprised 769 ha (28%) of the estuary area.

The catchment is 91% native forest, and the estuary is perceived to be near-pristine.

There has been some historical loss of high value saltmarsh habitat due to reclamation and drainage around margin areas (~60 ha), with resulting shoreline modification (e.g. seawalls, bunds, roads) now restricting the capacity of saltmarsh to migrate inland in response to predicted sea-level rise.

Broad-scale mapping using the National Estuary Monitoring protocol was conducted in 2016 and a fine-scale survey in 2016-17.

Estuary summary

What makes my estuary unique?

Explore the characteristics of this estuary

Overview

Estuary characteristics

  • Total area
    2741 hectares
  • Tide
    3.1 m spring tidal range
  • Flushing time
    <1 day

What's happening upstream?

See results from monitored river quality sites influencing this estuary

River quality

What's happening upstream?

The physical characteristics and health of estuaries are influenced by the rivers and streams flowing into them. For instance, when it rains the mud and contaminants generated on land can be washed into rivers and eventually flow into the estuary. The health of our rivers and streams can therefore be very important for Estuary Health, and understanding the upstream pressures can help with interpreting estuary monitoring data. Monitoring is undertaken for a range of river health indicators (e.g., water quality and ecology) in many catchments across the region. Where there are monitored river catchments that influence this estuary, these are shown below. You can click through to view monitoring results from these River Quality sites to see current state and how health has changed over time.

What surrounds my estuary?

See land cover information from monitored catchments that surround this estuary

Land cover

What surrounds my estuary?

The physical characteristics and health of estuaries are influenced by local geography and the way we use our land. This is because estuaries are the receiving environments for many of our land use activities. Land cover information can be used as an indicator of land use, therefore knowing the surrounding land cover can help us understand which pressures might be affecting Estuary Health. For example, the sandflats of estuaries surrounded by rural areas will typically contain contaminants related to rural activities (e.g., cadmium from crop fertilisers and copper from fungicides), whereas those surrounded by urban areas are more likely to contain contaminants associated with cities (e.g., zinc and lead from roads and building materials). Where there is land cover information available for nearby catchments, these are listed below. These figures show the types of vegetation and built or natural features that surround the estuary margins and the rivers that flow into this estuary. You can click through to the Land Cover topic to see these land cover classes broken down into further detail, and view changes over time.

Monitored sites 3

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