A $5-$7 million plan to significantly realign Lake Manapouri's Waiau Arm outlet above the Mararoa Weir, in an attempt to provide more reliable flows down the Lower Waiau River, is being proposed by the operators of the Manapouri underground hydro electric power station - Meridian Energy (Meridian).

Meridian Energy's proposal to redirect water through a new channel above the Mararoa Weir. Photo: Meridian Energy

Meridian Statutory Advocacy Strategy Manager Andrew Feierabend said effective flows to the Lower Waiau River had been a concern to stakeholders for a number of years and the Meridian proposal was in response to that.

"The depth of the channel and its current alignment isn't really conducive to providing the types of flows that are needed to, amongst other things, manage nuisance periphyton."

Feierabend said nuisance periphytons (material growing on submerged surfaces in freshwater such as toxic algal bloom and Didymo) had only been considered since about 2000 with Didymo first becoming apparent in 2004.

"The need to provide flows to manage [periphytons] wasn't even in frame... but over time, the way to manage that organism is through providing regular flushing flows, which we haven't been able to do."

Feierabend said Meridian would be lodging a resource consent application, which included a protocol to provide four or five flushing flows a year, with Environment Southland.

If successful, a new 1km channel would be constructed parallel to the current river that would be better shaped and aligned to allow flushing flows, even at low lake levels, through the Mararoa Weir.

Projected to start in January 2024 or 2025, work would involve up to 225,000m3 of gravel being excavated and relocated onsite, in an around-the-clock operation that is expected to take four to five months.

Once operational the channel would carry 70% of Lake Manapouri's Lower Waiau Arm outflow and provide significant improvement and reliability to flows, Meridian said.

Fish and Game Southland manager Zane Moss said he supported the intent to provide better and more consistent flushing flows down the lower Waiau River.

"At the moment the ecosystem is challenged by excessive periphyton growth when we have long periods of stable flows," Moss said.

"The ability to provide those flushing flows is better able to mimic a more natural flow regime, which would flush off and displace that Didymo and algal growth in a more natural environment."

"Creating the alternate channel will provide more certainty and consistency, and being able to provide those flushing flows should improve the ecosystem health on the lower river."

Moss said the ecosystem was inherently dynamic and variable and should respond quite rapidly if the prevalence of Didymo in particular was reduced.

"We'll see a response in macroinvertebrate communities, which drive the small fish communities, which drive the big fish communities," Moss said.

However, Waiau Rivercare Group (WRG) Co-chair Paul Marshall said Meridian's proposal was deserving of both bouquets and brick bats.

"The bouquet is that Meridian has acknowledged what the local community has known for a long time - that the flushing flow regime that's in place for the lower Waiau, and that Meridian was directed to implement, has fallen well short of the mark."

"In fact, we understand that over the last nine years only 30% of the agreed flushing flows have actually been delivered," Marshall said.

"What that means is that the river more often than not, through the summer, is choked with extensive Didymo blooms."

"And what's worse, from a recreational perspective, with instances of toxic cyanobacteria blooms from immediately below the Mararoa Weir at Excelsior Creek, right down as far as Tuatapere."

However, Marshall was critical of the consultation that Meridian had embarked upon, saying it had been highly selective.

"In spite of assurances in the media that Meridian was prepared to talk with the Waiau Rivercare Group, Meridian has chosen not to and from our point of view that's really disappointing."

"It means that we can't tell whether Meridian's proposal will actually improve the reliability of the flushing flows into the Lower Waiau or not," Marshall said.

Marshall said the group was still open to meeting with Meridian.

In response, Feierabend said Meridian had spoken with the Waiau Working Party, which had captured all the key stakeholders.

"Members of the Rivercare Group were at that session," he said.