Return of Kawarau Arm to braided river ‘inevitable’

• Published date: 22 July 2021
• Date: 22 July 2021

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.

‘‘[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 that 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...