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The Waitaki, Clutha and Waiau Rivers are all controlled to optimise electricity production from hydro power stations, which have fed the smelter over the past 50 years.
Those with an interest in the rivers’ ecology have just started contemplating what reduced hydro generation might mean.
The 80km lower Waiau River was the only outlet of Lake Manapouri before Meridian Energy’s Manapouri power station — the main supplier of electricity to the smelter — created a second outlet to the sea at Doubtful Sound.
Jan Riddell has chaired the Waiau River Working Party for Environment Southland since 1990. She said she did not want to diminish the economic hurt the smelter closure would have on Southland but could see the possibility of the river making some gains.
"Undoubtedly, given the transmission limitations in terms of sending power north, there will be more water [flowing] down the lower Waiau River."
That, she believed, would be a good thing.
"There’s a great deal of passion and feeling for the state of that river in Western Southland in terms of the inadequacy of the flow regime."
While there was a voluntary "flushing flow regime" designed to clear the river of algae during summer, it had mixed success and was "just not adequate".
She was "not too sure" how the change brought by the closure of Tiwai might "pan out".
As for the Clutha River, Contact Energy expects more water to be spilled through the Clyde and Roxburgh dams as a result of Tiwai closing, although a company spokeswoman said on Friday there were "still too many moving parts" to be able to say what effect that might have.
Former Otago Fish & Game Council environmental officer Peter Wilson said he did not expect a noticeable difference for Clutha River users but the exception might be Lake Hawea, which rose and fell with electricity demand.
"It might be they just choose not to store [water] in a lake like Hawea."
One short-term effect — until Transpower completes its upgrade of transmission lines to the North Island — might be a delay to the multibillion-dollar idea of turning Lake Onslow into a storage lake, with water pumped from Roxburgh.
The idea was revived by the Government last year as part of a push to have 100% renewable electricity in New Zealand by 2035.
"The critical decisions around storage might be put off for a decade or so because you’ve now got Manapouri integrated with the rest of the electricity market."
He believed there was unlikely to be much change to the Waitaki River because electricity from hydro stations there was more easily shipped north.
But there might be an environmental benefit for Doubtful Sound, "which wasn’t exactly designed to have New Zealand’s largest river artificially flowing into it".