Friday, 13 September 2019 10:55

Clearer understanding of water needed

Written by  Jacqueline Rowarth, CNZM CRSNZ HFNZIAHS, PhD in soil science
Jacqueline Rowarth. Jacqueline Rowarth.

OPINION: New Zealanders like their rivers to be clear, to be able to see through the water to the bottom — but this is not always possible.

A NZ Institute of Economic Research (NZIER) survey last year asked people to choose between 20% of rivers being swimmable and clear or 40% being swimmable but muddy. Most people chose clear. 

NZIER mused that the result might reflect an idealised view, perhaps stretching back into their childhood, that seeing the rocks/stones signified what a river should look like. 

NIWA scientist Dr John Quinn (died November 2018) suggested as much in 2017. In the June NIWA magazine he wrote that ‘people think they remember what water looked like in their childhood, but they do so through rose-coloured spectacles’. He reminded readers that water quality in most rivers is now considerably better than during the 1960s.

His comment was about water quality in general, not sediment in particular, but it is sediment that is increasingly of concern. 

An investigation by Waikato Regional Council last year ranked sediment management as of most importance for rivers, followed by riparian, instream and then nutrient management. 

Fish and Game river ecologists agree. The problem with sediment is that it gets in between stones, fills up the gaps that could otherwise be hiding places for small organisms and smothers plants which provide food.  But sediment is natural. It is part of the formation of rivers as well as the landscape. Over time, rivers create gorges, waterfalls, banks and plains as they flow from source to sea. 

Sir David Attenborough has described the process of rivers in The Living Planet, explaining that ‘young rivers are by nature vigorous and dangerous: they flow fast and form rapids, thick with mud and sediment’. 

In New Zealand, with regular and frequent earthquakes, plus volcanic activity, and heavy rainfall in some areas, sediment is part of our heritage.

But the legacy of settlement and development, whether hill country, housing or highways… is increased sediment. Soils without their original vegetation are vulnerable. 

Measurements in Auckland in the 1990s indicated sediment load from urbanising areas was 50 times greater than from pasture. Techniques of building have improved since then, but anybody now passing by a motorway construction site after heavy rain may see that problems still exist, new technologies or not.

NIWA has estimated that sediment has increased from less than 1mm per year prior to human settlement, to 2-5mm per year. 

The challenge is for present-day producers of food and fibre to mitigate the actions of their forefathers, using the best available research.

Dr Mike Dodd and co-authors from AgResearch and NIWA, have reviewed the options. In a paper for the Grassland Association Hill Country Symposium in 2016, Dodd concluded that culverts and bridges had a relatively high effect on reducing sediment, whereas riparian fencing had a low effect. Both were estimated to cost thousands of dollars per hectare.

And, as indicated by Sir David Attenborough, at least some sediment is part of nature. 

Indeed, Professor James Brasington, Waikato Regional Council chair of river science at the University of Waikato, has stated publicly that ‘New Zealanders are in danger of creating zombie rivers, not because of nutrient overloading but because we’re locking our waterways into positions between stop-banks and impounding the headwaters.’ 

Brasington, suggested we should be allowing rivers to erode their corridors and flood naturally. 

Erosion on river banks is sediment in waterways. Clearly the issues are complex.

Public concern could be questioned given the result from NZIER that 60% of the general public would not be prepared to pay a levy of $50 a year to improve flora and fauna affected by sediment. However, farmers are doing what they can to reduce the current impact of past action. 

The LAWA 2018 report indicated that they are having an effect: fewer than 10% of rivers are deteriorating whereas 25% are improving. As new technologies become available, more improvements will occur. But rivers without sediment would not be natural.

• Jacqueline Rowarth, CNZM CRSNZ HFNZIAHS, has a PhD in soil science.

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