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Rising Sea levels

Braided River Status

All braided river estuaries and coastal lagoons/hapua throughout New Zealand will be affected by rising sea levels.


Sea levels have been rising since the early 1900s. The rate is accelerating, doubling approximately every 10 years. The initial rise was due primarily to thermal expansion as the seas warmed. Today, glaciers and ice-caps are melting, decanting into the ocean faster than at any time in the last 10,000 years. NIWA currently estimates a rise in sea levels up to 2m by 2100. It will rise at least 30cm before 2050, a rate that is ‘locked in’, ie it will happen regardless of what actions we take (see video below) because of lag between the amount of CO2 already in the atmosphere and the effect on rising temperatures. When ice caps have melted in the past, sea levels have abruptly jumped as much as 2m in 50 years, so the minimum rate may need to be revised upwards as the rate that ice shelves holding back glaciers collapse accelerates (see video below).

As seas rise, river mouths migrate inland as the land is inundated. Because braided rivers mouths form complex estuaries and hapua, the morphology of these areas will undergo physical re-arrangement as they move inland. Estuaries will become harbours, hapua will become estuaries, the locations of which will vary or vanish altogether depending on landform, land use, and flood control measures used by local and regional governments to prevent this inundation.

Where river-mouths can freely retreat inland, new hapau will readily form in low-lying areas behind historic and extant dune systems. Where engineering prevents this, hapua will disappear.

The following was released Dec 11, 2014 by the Parliamentary Commissioner for the Environment, Dr Jan Wright, warning that flooding and erosion from rising sea levels will have a large impact on many New Zealanders in their lifetimes.

Why is it a problem for braided rivers?

  • Short term – saltwater influx into existing brackish or freshwater hapua, leading to degradation of existing ecology
  • Mid-term – increasing frequency of inundation leading to permanent inundation of existing hapua (see Table 3.2 below)
  • Progressive – coastal squeeze in some areas will prevent the natural inland migration of estuaries and hapua, with flood control measures built or reinforced to protect existing land, built structures and critical infrastructure. Hapua and lagoons will disappear in these circumstances, reducing the habits of species dependent upon them
  • See discussion in relation to the Ashley estuary

Conservation Activities

  • Reduction of greenhouse gasses (GHG). Regardless of this, a minimum 30cm sea-level rise over the next 35 years is now ‘locked in’, that is, nothing can be done to prevent or reverse this.

More information


The inundation map above is set to the 2m for the Ashley River estuary:,-187.284238&z=11&e=2 Note: this is an inundation map based on topography; it does not factor in coastal erosion or how the river may change its path. Nor is engineering to prevent inundation or drainage of low-lying areas factored into the image.


Research papers

This area of research is moving at such a rapid pace, we have opted not to include specific links here. If you are looking for the latest peer-reviewed research on the topic, you might like to start with the journal Nature Climate Change.