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Understanding of Central Canterbury’s Water – the Selwyn Te Waihora Water Zone – It’s all connected


A NZFFA executive member questioned whether the Selwyn River is “a naturally intermittent river” after reading a NIWA scientist’s article suggesting Dry rivers offer a preview of climate change.

This prompted my attempt to explain the Central Plains hydrology and pollution issues based on local knowledge & observations, and from attending/viewing several presentations by Ecan’s chief scientist Tim Davies.

Formation of the Central Canterbury Plains

The plains were formed by outwash gravels from the Rakaia River to the south and the Waimakariri River to the north.

In the (geologically) recent past the Waimakariri River has been directed either north or south by the Port Hills. The Avon, Heathcote, Hallswell and LII Rivers are all old water courses of the Waimakariri River.

The land surface is subtly marked by old water courses. The soils reflect their river origin alternating from light stony soils to heavy silt loams of varying depth.

In some places on the plains the soil is so porous that when the shallow surface soil is removed, a small stock water race will simply disappear into the ground.

Beneath the surface, ground water continues to “flow” in braided patterns similar to the two main rivers of their birth.

The unconfined aquifer between the parent rivers is separated into connected but distinct layers at variable depths.

The Selwyn River lies between the two major gravel flows being fed by foothills rain and surface recharge. It contributes to the common aquifer beneath its bed.

There is a transition line where the main road and rail line cross the plains. To the west the fall is greater and to the east the contour is flatter.

The main road formed the demarcation between wetlands and swamps to the east and dry land to the west.

A flat contour and the sea or lake water barrier causes water from the shallow aquifer(s) to emerge as springs and swamps near the coast.

The Selwyn River has a permanent flow as it travels over more impervious soils where it emerges from the foothills and then water is lost to the aquifer beneath porous soils of the plain before emerging again at Chamberlain’s Ford.

This is common for all the rivers in this zone including the Rakaia and Waimakariri Rivers.

The Rakaia River loses up to 40 m3 from the gorge to the sea.

Since European Settlement

For over 150 years most of the wetlands and swamps have been cleared and converted into highly productive arable land.

Until quite recently the Central Canterbury Plains were farmed in sympathy with the dry East Coast climate. Crops of lambs were finished with early crops of rape by Christmas. The region was the country‘s grain basket taking advantage of dry summers to help ripen and harvest cereal crops.


The Selwyn District was a late adopter of irrigation initially relying on groundwater which rapidly became over allocated as the early boom in dairying on previously dry land farms began in the 1980s.

Spring fed creeks and streams such as the Irwell River near Lake Ellesmere regularly began to dry up in times of drought from this time.

Since 2012 the Central Plains Water CPW irrigation company gained consents to abstract 40 m3 of water from the Rakaia River which is now piped throughout the western side of Central Canterbury.

The water take made possible by “constitutionally repugnant legislation”; the ECan Act 2010, passed under urgency, replaced an elected regional council with government appointees and gave the non-statutory Canterbury Water Management Strategy primacy over the RMA.

The Rakaia River water enabled CPW to supply 30,000 ha of new irrigation and 30,000 ha of existing irrigation previously supplied by groundwater in the Selwyn water zone.

Canterbury’s Wicked Water

The CPW scheme has massively increased intensive dairy farming on the once dry plains

Over allocation of groundwater continues to plague farmers and rural residents in the Selwyn zone. Wells are re-drilled to connect with deeper layers in the aquifer as the shallow aquifers are depleted or polluted.

The once pristine aquifer beneath the Canterbury Plains is now polluted with nitrate and pathogens so both aquatic ecosystems and public health are compromised

Water pollution – nitrate and pathogens from cow urine and faeces percolates into the aquifer beneath the dairy farms to emerge in the “permanent” lower Selwyn River.

Nitrate sampling show consistent results between 7 – 8 mg/L NO3-N, levels toxic to trout eggs and fry.

Low flows, high water temperature, and nutrient enrichment are destroying the natural and recreational values of this section of the river.

A lowered water table resulting from over abstraction increases both the length of river and length of time the intermittent section of the Selwyn River dries up – the aquatic “bottleneck” is increased.

The promotors, politicians, and paid Hearing participates have all taken their short term gains knowing they have no accountability for their decision(s).

Agribusiness and farming interests have so successfully captured ECan and the “Press” that we are told this unacceptable outcome is warranted because of the resultant increase in GDP.

Farmers who committed to the scheme and disenfranchised Canterbury ratepayers are left to pay for the environmental consequences.

GDP has increased by $300 million resulting from an irrigation scheme made possible by a $200 million subsidy using water from a river protected? by a NWCO.

At the farm level profitability is marginal at best.

A study commissioned by ECan found it would cost $300 million to restore Lake Ellesmere/Te Waihora alone. This high cost of remediation was used by ECan to avoid their statutory task of cleaning up the lake.

The much vaunted farm environment plans mandated by Ecan address about 1/3 of the excess nitrate loading. Any further mitigation is unlikely to be accepted by polluters.


Yes the Selwyn is a “naturally intermittent river” and it was in the past.

Thanks to profit focused decision making a once prolific although vulnerable recreational fishery has been lost and a community’s drinking water put at risk.

The short term gains from dairying made possible by CPW will be far less than the costs faced by future generations to reduce nitrate in the Selwyn River and Selwyn District groundwater.

Lake Te Waihora is fed by the polluted Selwyn River together with numerous springs and streams sourced from nitrate loaded groundwater seeping down from under the recently created dairy platform.

The Selwyn Water Zone story is repeated in other water zones across greater Canterbury.

This NIWA scientist new to New Zealand since 2001, is wrong to suggest the present state of the Selwyn River is due to global warming.

Peter Trolove