Sediment

Sediment (e.g. mud and silt) accumulates on the bottom of our rivers, lakes and estuaries. It can make the water murky, block fish gills, smother the habitat that fish live in and promote slime algae growth. Sediment quality is also affected by the amount of organic matter and nutrients it carries.

Sediment can come from runoff after heavy rainfall, disturbance of the riverbed or bank by heavy machinery or through direct discharges.

It’s one of the key contaminants affecting water quality - alongside too much nutrients (nitrogen and phosphorus), and high levels of bacteria (e.g. E. coli).

Sediment science

There are three key indicators we measure when it comes to sediment: suspended sediment (suspended materials, usually dirt particles), visual clarity, and sedimentation rate in estuaries. Deposited fine sediment is also an attribute we’re required to measure as part of the National Bottom Lines, but it isn’t able to be modelled so isn’t presented as part of this work.

Several modelling studies have been undertaken to predict (estimate) the size of sediment load reductions needed to achieve objectives related to sediment.

So, what have we learned so far about sediment loads to achieve objectives in rivers, lakes and estuaries in Murihiku Southland?

In summary, the total regional reductions in sediment load needed to achieve objectives are: 24% - to achieve the suspended sediment objective in rivers; and 32% - to achieve the visual clarity objective in rivers.

The studies also show us that these reductions will help to reduce the average sedimentation rates in our estuaries, but further reductions may be needed to achieve the objective in all parts of all estuaries. Other location-specific mitigations could also be investigated to help with particular problem areas, such as physical works to enhance tidal flushing of sediment for example.

It’s important to note these load reductions vary across and within catchments – and are not limits. They do not relate to individual properties.

Where is sediment coming from?

A recent modelling study made estimates of where suspended sediment is coming from in the Waiau, Ōreti, Aparima and Matāura catchments, but did not include Fiordland and Islands catchments. The study found that:

• Mean annual suspended sediment loads were dominated by surface erosion in the headwaters of main catchments. Bank erosion becomes the dominant source of local erosion in larger rivers.
• The highest rates of erosion were estimated in the headwaters and along the main channel on the middle to lower reaches of the main catchments. Of suspended sediment load in the modelled area, 8% was derived from bank erosion, with 92% from surficial erosion.
• In the main catchments, surface erosion was estimated to contribute 75% of the load to the Matāura, 66% of the load to the Aparima, 83% of the load to the Ōreti, and 95% of the load to the Waiau.
• The Indigenous Forest and Conservation land use class was estimated to be the largest contributor of sediment loads in the Waiau, Ōreti, and Aparima catchments. Sheep and beef farms contributed close to 50% of the sediment load in the Matāura catchments and 36%, 25% and 4% in the Oreti, Aparima and Waiau catchments respectively. Dairy farms contributed 7%, 3%, 11% and 1% in the Matāura, Oreti, Aparima and Waiau catchments respectively..
• The modelling suggested that sediment associated with winter forage cropping was having a larger impact on the smaller lowland rivers and streams, likely to be much closer to the cropped paddocks.
• Mean annual suspended sediment loads at the end of our catchments suggested that winter forage cropping made up a very small proportion (5%) of the total load.
• It’s important to note that this study estimated total mean annual suspended sediment loads and did not consider the quality or different types of sediment. There may also be highs and lows throughout the year.

What can we manage?

While land uses such as sheep and beef, dairy and winter forage cropping contribute only part of the sediment load to our larger rivers compared to the dominant source from more natural indigenous forest and conservation land, we can take action to manage the sediment loss from these agricultural uses, as well as managing sediment from urban and industrial sources.

Read the full report here

Find out more about this work and our programme to achieve the improvements in water quality needed

There are a number of things you can do to take action. Find out more about what you can do