Gravel extraction damaging ecology at Ohau River

Stone extraction from the Ohau River.

Extraction of gravel from the Ohau river is not only causing issues at Levin water treatment facility but is also having detrimental effects on the river's ecology, a freshwater ecologist says.

Horizons Regional Council has issued contractor Higgins with a consent to extract gravel from the river, something that freshwater ecologist Dr Mike Joy believes should not be allowed.

Dr Joy is a senior lecturer in ecology and environmental science at Massey University.
He said that extraction of gravel from the river and surrounding banks is rough on the river and its ecology.

"Under most consents they are not allowed to take from the wetted area, they have to take it away from where the water is," he said.

"You think that is good because it won't be affecting the stream, but what happens is because they make a lower spot somewhere else in the river bed, the next time a flood comes down, the whole river shifts across to where they have dug it out.

"All the fish that lived in the old stream bed are history because the stream has disappeared on them overnight."

Horizons river management group manager Ramon Strong says part of Higgins' consent is ensuring that they contact Horizons' river management engineers to ensure the extraction of gravel is done in a way that achieves the best alignment of the river.

"If gravel beaches get too big they can cause the river to change direction and create disturbance downstream," says Mr Strong. "Gravel extraction has been done in this section of the river for many years. The contractor is only allowed to be extracting gravel that is above the water."

Dr Joy said that the mitigation of digging away from the water channel doesn't actually work, and has a really big impact on the life in the stream.

However, he says sometimes you have to take gravel out because of bad land management up stream.

"There is so much sediment coming down because you have cleared all the forests up stream, so the only way you can stop flooding and bridges being washed out, is to take gravel out downstream."

Horizons Regional Council regulatory manager Greg Bevin says that Higgins has consent to extract gravel on the upper reaches of the Ohau River.

"Their consent includes conditions such as times of the year they can extract and how much, supplying the council with relevant extraction data, and not operating during the dotterel season," says Mr Bevin.

Mr Bevin says Higgins complies with its consent.

Dr Joy said contractors either dig it out of the wetted area so sediment gets into the river, or the channel shifts across into the hole that's been dug so there is all the soft sediment that gets moved out of there, and down the river.

"You nearly always get sediment effects downstream," he said.

This statement was given weight by Horowhenua District Council water services manager Paul Gaydon.

He said that it seemed Higgins's activity up stream had stirred up a lot of sediment and was blocking the inlet gallery for the town's water supply.

"We are having to then scarify the inlet gallery rock a lot quicker and we are having to make a head that has compressed air to clean that out properly," he said. "It's really worrying when you have legal activities really close to your water works.

We have to make sure we have a clean catchment, if [Horizons] can allow a consent upstream that affects us ... without consulting us ... that is just not clever and really stupid catchment management."

My Gaydon said he felt Horizons should be completing settling tests on the sediment and see how far it travels.

"Do a proper environmental impact assessment. [Higgins] seem to be going in [the river] from what I have heard."

The National Policy Statement for Freshwater Management 2011 sets out objectives and policies that direct local government to manage water in an integrated and sustainable way while providing for economic growth.

It states policy should "safeguard the life-supporting capacity, ecosystem processes and indigenous species including their associated ecosystems of fresh water, in sustainably managing the use and development of land, and of discharges of contaminants".