Digging the dirt on sediment

By Anne Gummer

GulfX is all about tackling pollution in the Hauraki Gulf from heavy metals, plastic and sediment. It’s easy to understand how the first two of these - heavy metals and plastics - can have a detrimental effect on the Gulf. We know those nasties shouldn’t be there! But what about sediment? How can a natural substance like soil be such a problem for our waterways?

In a natural environment, free from human intervention, sediment is a normal part of the waterway ecosystem. Problems arise when the changes we make to the natural landscape result in a significant increase in the amount of sediment entering the water. Human activities such as deforestation, mining, harvesting of exotic forestry, converting wetlands to pasture, roading and housing development, and removing trees along stream banks all contribute to increased sediment loss from the land into the water. And over time the Hauraki Gulf catchment has experienced all of these land use changes.

Once sediment enters a waterway it affects water quality, and the plants and animals that live there, in a number of ways:

Problems caused by sediment suspended in the water

  • Sediment reduces water clarity, making the water cloudy and brown. This is no good for fish – they can’t find food and places to live, and no good for plants that need sunlight to grow.
  • Sediment particles also damage fish gills and the filter feeding apparatus of invertebrates such as snails, crayfish, shellfish and crabs.

Problems caused by sediment on the sea floor or river bed

  • Sediment smothers invertebrates’ habitat, and destroys places for fish to find shelter and to lay their eggs.
  • Sediment can significantly change the flow and depth of rivers or streams over time and infill lakes and estuaries, fundamentally changing the environment.
  • Sediment disrupts the natural cleaning process that occurs when water moves across the stony sea floor or river bed.

Other problems caused by sediment

  • Sediment carries other pollutants into the water such as nutrients, bacteria, and toxic chemicals from agriculture, horticulture, transport and industry.

So what can we do to help prevent sediment from entering waterways?

There are many actions that can be taken by individuals, businesses and government to help address sediment pollution of our waterways. Here’s a few examples:

  • Reinstating wetlands which act as filters between the land and water.
  • Reducing hard surfaces so that water can soak into the land rather than flow across it.
  • Using best practice sediment control methods for earthworks and construction.
  • Installing storm water devices, which help prevent pollution from entering our waterways.
  • Regulation – at local, regional and national level – of activities that have the potential to cause sediment loss, and increased oversight and enforcement of these regulations.
  • Riparian planting – planting native plants and trees along waterways which helps to trap sediment.

SBN’s Million Metres initiative is focused on riparian planting. You can support Million Metres’ mission to restore a million metres of New Zealand’s waterways by donating to one of our current projects.

For other ideas on what you can do visit the GulfX Take Action page.


Much of the information in this article comes from niwa.co.nz