19 October 2018

Local zoological gardens are set to benefit from a major operation that’s removed more than 150 tonnes of wood from the Kawarau River near Queenstown.

A 24 ton excavator was floated up the river on a barge to extract the submerged trees from the river which is now being turned into mulch for the Zoological Gardens at Frankton.

The whole project has been a joint effort funded by Land Information New Zealand (LINZ) and Otago Regional Council, with contractors Boffa Miskell carrying out the work.

“This was a pretty unique project that took an innovative approach to resolve a significant problem”, says Marcus Girvan from Boffa who oversaw the work.

“The wood under the water was like a dense jungle making it very difficult for divers to carry out work to tackle lake-weed, and reduce the risk of it spreading.”

The wood has been removed to allow hessian matting to be laid on the river bed to prevent the spread of the invasive pest plant lagarosiphon.

“This was a very difficult operation, but it’s paid off,” says LINZ Biosecurity Director Dave Mole.

“Removing the dead wood so we can lay biodegradable hessian matting will massively curtail the growth of lagarosiphon. This innovative matting is cost effective and a ‘green’ control method that allows native vegetation to flourish.”

Preventing lagarosiphon from growing in the Kawarau River is part of the wider work being done in Lake Wakatipu.

“Wakatipu is just one of Otago’s pristine lakes and it’s paramount it is protected from the invasion and establishment of lagarosiphon,” says Richard Lord, Team Leader Biosecurity Compliance at Otago Regional Council.

“This joint agency approach and effort in controlling lagarosiphon is a great example of how we can work together in our region.”

An unexpected benefit from the work in the river has been a much-appreciated gift for the Zoological Gardens, with the recovered willows being chipped into mulch.

“This is a fantastic bonus for us,” says Rachel Young from the Gardens.

“The organic mulch will be used in our wildflower garden as plant-starting material, and represents a huge cost saving for us. This will really help us with our programme with primary school children helping them to plant native plants and care for them.”

Work to lay the hessian matting in the Kawarau River is expected to happen early next year and will be part funded by Queenstown Lakes District Council.

Watch the 24 ton excavator in action here.

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