The “inevitable” return of the Kawarau Arm of Lake Dunstan to a braided river is happening – like it or not.

That calls for a collaborative approach between stakeholders to manage the transition, Contact Energy and the Lake Dunstan Charitable Trust say.

Sediment build-up throughout the Kawarau Arm, the establishment of the lake weed lagarosiphon in popular swimming areas and driftwood covering the shoreline have all raised the ire of Cromwell residents, many placing the blame squarely on the Clyde Dam and consequently its owner, Contact Energy.

The Clyde Dam was built between 1977 and 1992, and flooded the Cromwell Basin with water from the Clutha and Kawarau Rivers, forming Lake Dunstan. Cromwell sits at the junction of the two rivers.

The third-largest hydro-electric dam in the country, Clyde Dam combined with the downriver Roxburgh Dam produces 9% of the New Zealand’s electricity.

Contact Energy formed in 1995, taking ownership of the dam and the management of the new lake.

At the time of the planning and consenting of the dam, it was foreseen sediment build-up would eventually return the lake to a river – something three decades later many Cromwell residents remained unaware of.

Contact Energy head of hydro Boyd Brinsdon said sediment calculations made by Niwa at the time of the dam’s construction aligned with what was now happening in the Kawarau Arm, and had been since the lake was formed in 1993.

The difference was, now it was more apparent to residents.

It was calculated one million cubic metres of sediment –  the equivalent volume of the concrete used in the construction of the Clyde Dam – flushed through the Kawarau Arm annually from 1994 to 2018.

Mr Brinsdon said it was known as the tipping face –  where river velocity slowed, causing sediment suspended in the river to “tip out” on to the riverbed. Sediment build-up would eventually result in a braided river.

Where two rivers meet . . . The confluence where the Kawarau and Clutha Rivers meet. PHOTO DENIS MCENTYRE

“[It was] expected that in 30 years’ time it would reach the confluence [where the Clutha and Kawarau Rivers meet] and we’re pretty much there – that was based on expectation of about one million cubic metres per year.”

The visual effects of the transition have been a source of ire for Cromwell residents and lake users for years –  a short-lived protest at the opening of the Lake Dunstan Cycling and Walking Trail in May highlighted the frustration many feel about a perceived lack of lake care by Contact Energy.

That was far from the truth, Mr Brinsdon said, and the company had created a landscape and visual amenity plan for the bed of the Kawarau Arm as part of its resource consent requirements for operating a hydro dam in the area.

“We are proud members of the communities we live and operate in and focus on ensuring that we comply with our resource consents and being the neighbour you would want.”

An example of this was the driftwood removal Contact had recently undertaken from the Cromwell Heritage Precinct and above the Butchers Dr boat ramp as part of the landscape and visual amenity plan.

Cleaning Up . . . Fulton Hogan worked with Contact Energy on the recent clean up of driftwood on Lake Dunstan. PHOTO: DENIS MCENTYRE

The plan has been signed off by both the Central Otago District Council (CODC) and Otago Regional Council (ORC).

Created in 2009 and reassessed in 2014, the plan is under its second review at present.

A draft will be released to the community via the CODC and ORC for consultation, and Mr Brinsdon said he expected to hear community views as part of the process.

“We are finding with the reassessment this time [there] is stronger engagement from community groups such as Lake Dunstan Trust since we last reviewed the plan.”

Contact recognised the needs of the community had changed over the past decade and welcomed that input, Mr Brinsdon said.

Part of Contact’s responsibility was being clear about the future of the Lake Dunstan area.

“The reality is the lake bed will continue to change and that is not unexpected.”

Contact also worked with Land Information New Zealand (LINZ) on the management of the invasive lake weed lagarosiphon, contributing financially to help keep it at bay.

LINZ deputy chief executive crown property Jerome Sheppard said the organisation had a number of Crown land management roles and responsibilities in relation to Lake Dunstan, including lakeweed control, administering responsible camping areas, issuing permits for activities on the lake and surrounding lands, and general maintenance on land it managed.

LINZ worked collaboratively with local agencies in the area including ORC and CODC, he said.

He said LINZ’s investment in Lake Dunstan and the Kawarau Arm continued to reflect its responsibilities in the region.

“That is to help protect and maintain the recreational and economic values of these waterways for local communities now and in the future,” he said.

Lake Dunstan Charitable Trust community engagement co-ordinator Megan Phillips agreed transparency in communicating the transition from lake to river was an important step.

It was about people realising the return to a braided river was “inevitable”, Miss Phillips said.

Formerly Guardians of Lake Dunstan, the trust was formed to bring together the “needs of community and the activity of stakeholders to achieve a healthy and happy environment for people and wildlife”.

Important step . Lake Dunstan Charitable Trust community engagement co-ordinator Megan Phillips says transparency in communicating the transition from lake to river is an important step.

The group worked alongside LINZ, the CODC and ORC as well as Contact to ensure the needs of the community were being understood and met as the lake went through the transition.

“There’s a lot of things that upset the Cromwell community with the lake – what the Kawarau Arm impact is- so we’ve chosen to be the voice of the community,” Miss Phillips said.

Earlier this year the trust took had a barrister raise with ORC it believed Contact was not meeting its obligations to actively manage the visual amenities of the lake and ORC agreed.

The result meant the company had now stepped up its management of the lake, increasing the driftwood removal from around the shore and reassessing the visual amenity plan for the Kawarau arm to manage the transition, Miss Phillips said.

“From the trust’s perspective, it’s really good they’re taking their civil responsibility seriously and actually stepping up,” she said.

Contact was now planning on how best to deal with silt through potential dredging or speeding it up by depositing more on the shoreline and creating wetlands, she said.

The return to a braided river has already begun in the upper reaches of the Kawarau Arm with the formation of silt islands. PHOTO: SHANNON THOMSON

The blame did not lie solely with Contact – both the CODC and ORC had been remiss in the way they enforced the management of the lake’s visual amenities, Miss Phillips said.

It had taken the trust to highlight the poor management on the part of all parties involved, she said.

The return of the lake to a river could be a positive thing if it was managed well and once a clear river was defined, it could be used safely for fishing, swimming and boating –  the key was collaborating throughout the transition so locals weren’t left with a mess for years while it took place, she said.

“If done right, it would create a beautiful area for the community to enjoy.

“At the moment the Kawarau Arm is all about what has been lost, but we want to change it to what we gain.”

The CODC and ORC were contacted for comment but did not respond by deadline.