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Published: 2007-08-14 00:00:00

Trout numbers in the Waipa River have bounced back thanks to an Environment Waikato work programme to repair damage caused by the Tunawaea slip near Otorohanga.

The slip happened in 1991, when a hillside in the Rangitoto Ranges collapsed, dumping a whopping 900,000 cubic metres of gravel, clay and pumice into the Tunawaea Stream. This mass of material created a 70m high dam that burst during a minor flood event in 1992, sending earth, gravel and water surging into the stream, and down into the Waipa River.

The water rushed through the system quickly, but the gravel and sediment is taking years to move down the river. As it has slowly worked its way through it has raised the riverbed, causing significant environmental damage. Gravel has been deposited on the banks, trout pools have been filled in, and water has been driven outwards to chew away large areas of land previously untouched by the current.

Five years ago Environment Waikato launched the Tunawaea project, funded through Project Watershed. More than $1 million was committed by Environment Waikato over a 10-year period to stabilise the slip and river, and to protect farm land. The project, which has been supported by Fish and Game New Zealand, the Department of Conservation and local land owners, is now entering its sixth year.

Fish and Game Auckland/Waikato fisheries manager Ben Wilson said trout habitat had been “pretty much wiped out after the natural dam broke” and the river had struggled as a fishery for nearly 10 years.

“But now the works have stabilised the river and it’s back to a single channel, things are rapidly recovering,” he said.

“We’ve had very good reports from anglers that the trout population has improved considerably. Certainly the fishing in the upper Waipa last season was superb and we put that down to the work the Environment Waikato’s done in the area.”

Farm land lost

Fishing enthusiasts weren’t the only people affected by the slip damage.

Waipa farmer John Oliver, whose land flanks the river, lost acres of his farm to erosion.

The altered river cut into cliffs below his paddocks, ripping off huge sheets of pumice and grazing land and sending it plummeting into the gorge below.

“It wiped out all my grazing flats on the Waipa River – it just tore the banks out,” he said.

“Every time we had a big rain event another 20-30 metres would be chewed out. You used to stand on Toa bridge and see these great dollops of pumice coming down the river. It was just eating the land away and it was unstoppable; you used to see it eroding away before your eyes.”

There were other environmental impacts too. Water quality deteriorated significantly because of the continuing erosion, and because clays and silts continued to wash off the landslide area during heavy rain. The gravel also caused the channel to braid into shallow streams in some places.

Repairing the damage

Environment Waikato’s Tunawaea project has focused on re-contouring the river, stablising the slip area and river banks, reducing erosion and improving water quality.

Diggers and engineering solutions have been used to train the river back into a single channel, and channel maintenance is ongoing. Forty thousand plants (flaxes, natives and matsudana willow hybrids) have been planted to knit together loose gravel on the bed and banks of the Waipa River. The plants have helped to trap sediment and filter run-off from the land, resulting in better water quality.

The slip has been fenced to exclude stock and an ongoing pest control programme has been carried out in conjunction with the Department of Conservation to stop goats and other animals from damaging plantings on the gravel flats and areas where land has been retired. Two thousand goats have been removed from the area to date.

Environment Waikato Waipa zone manager Michael Duffy said the long term benefits of the project would be less erosion of valuable farm land, less pasture damage from silt, cleaner water, healthier bush areas and better trout and eel habitat.

The work completed in the upper catchment will have positive effects on water quality in the the Waipa River downstream. It will also enable the transport of gravel into the lower reaches of the Waipa River above Otorohanga that were historically mined, which will assist in restoring the hydrological balance of the river.

The retirement and planting of land will also help to absorb more rainfall and reduce run-off into the river, lessening flood risks in Otorohanga.

The river recovers

Mr Oliver, who donated what was left of his river flats to the council to be planted, believes Environment Waikato’s work programme is having a positive impact.

“They’ve now got the river at the upper end controlled into a singe lane and planted the banks to support the sides of the river,” he said.

“It’s quite a sight to see the willows holding hands together on each side of the bank. They’ve done a good job of putting it back. There’s still a lot of work to be done but the back of it’s broken.”

This spring the council will complete more channel training work several kilometres downstream of the landslide area, where a large volume of gravel is threatening to wipe out new plantings.

“We’ve generally got control of the river alignment now – the trick is to hold it in that alignment and that’s what we’re focussing on now,” Mr Duffy said.

The $1.15 million Tunawaea project is being funded through Environment Waikato’s Project Watershed rating scheme over a 10-year period.