Ferrying fish and 40,000 trees: the ecology of the Waimea Dam

Carrying fish and eels in a tank up and down the Waimea Community Dam is just one method that will be employed to ensure the ecology of the Lee River is maintained.

Alasdair Mawdsley is the dam’s environment and sustainability manager. He moved down from Auckland with his family to take up the role.

“This was an opportunity to come down and work in a beautiful river setting with trees and animals,” he says.

“There’s quite a big ecological restoration programme that goes with the dam, so that was really attractive.”

His job is to coordinate designers, engineers, and contractors to ensure the dam remains in compliance with its resource consents.

“It’s a lot of procurement, organisation, generally just managing this process through. I’ve probably planted two or three trees in my time here, but we’ve put more like 45,000 in the ground,” he says.

About 39-hectares of native forest is being planted by dam manager Waimea Water Limited (WWL) to offset the trees lost from the dam’s reservoir.

Two of the most advanced planting sites are at Rough Island and along the Waimea River. Mānuka, kānuka, and cabbage trees are being planted and over the next four to five years will grow until they “close out the canopy and shade out the weeds.”

From there, taller tree species that would struggle to grow in weedy environments will be planted.

“While the dam has a lifespan of 100 years… the forest we’re putting in place is something you could go and look at a hundred, two hundred, three hundred years from now,” Alasdair says.

“So that’s a satisfying aspect to this job.”

Back on the dam site, WWL has a plan for allowing climbing fish to be able to continue travelling up- and downstream.

Fish trying to travel up river will be trapped and then transported up to the reservoir manually before they’re released and can continue their journey.

The original design for the dam included a fish ladder so they could continue to travel up river.

“But no one was ever really sure that the design was going to work,” Alasdair says. “NIWA [the National Institute of Water and Atmospheric Research] has guidelines for up to four metres tall, and we’ve got a 53-metre-high dam.”

Given the uncertainty around the ladder’s effectiveness, the transportation method was opted for.

“Which gets over the elevation problem, but it also gives us a bit more opportunity to look at what’s there, count the fish that are moving, and be a bit selective about getting rid of any species that shouldn’t be passing.”

The bottom of the dam’s spillway, where the excess reservoir water goes to ensure the river keeps flowing, is now planned to have a plunge pool rather than a concrete apron, which will provide a “softer landing” for anything moving downstream, and the dam’s intake structures are also blocked by a fine screen with a large surface area to prevent fish from being getting sucked against it and becoming stuck.

The number of macroinvertebrates in the river water, an indicator of river health, is also measured in annual water quality reports which continue to show “excellent” scores.

“That’s been quite a satisfying thing to have on a project where we are moving lots of dirt around and we’re doing concrete works,” Alasdair says.

“Things that are considered risky for that kind of environment.”

Rare samples of shovel mint, rock coprosma, and scented broom have also been taken from the reservoir site and given to Brightwater’s Titoki Nursery to propagate.

“We’re working out how to re-establish those.”

While there have been successes with the shovel mint and the broom, the coprosma is proving more difficult to re-establish, but the work is giving valuable learning opportunities to WWL’s ecologists.

Once the dam is complete, Alasdair will stay on and run it as the operations manager.

“I’m really quite excited,” he says.

“I like the idea of solving a water issue and ensuring that that resource is there for everybody to use going forward.”