Low flow no cause for woe
UNUSUALLY low flow levels in the Clutha River should not be a cause for concern, observers say.
The waterway — known commonly as the ‘‘Big River ’’— was a less than impressive sight in Balclutha yesterday as rarely seen shingle banks and disused bridge pilings were exposed.
Otago Regional Council data showed it reached a sixmonthlow flow rate of just 297cumecs on Monday, and was running only slightly higher at about 340cumecs yesterday lunchtime.
The river, New Zealand’s second longest, and its largest by volume, has an average flow of 613cumecs, but in flood can exceed 4000cumecs.
ORC environmental monitoring team leader Pete Stevenson said although 300cumecs was ‘‘low’’, it was not exceptional.
The level was a result of low outflows from feeder lakes Wakatipu, Wanaka and Hawea — the latter at present only 0.5m above its minimum operating level.
‘‘In the last 10 years, the Clutha flow rate has dropped below 300cumecs in nine. Low lake levels this time of year are an annual occurrence, as precipitation in the headwaters is locked up as snow.’’
He expected warmer spring and summer weather to restore flows to closer to normal levels.
Seasoned Clutha Mouth whitebaiter Christine Melvin, of Balclutha, said although low flows did not seem to affect whitebait numbers in the river, it could have an effect on catches.
‘‘If there’s no current, like now, the bait goes out wide and misses the nets. And depending on your stand, low flow can mean you have to wait a bit longer for the tide to bring the bait up to your net anyway. But it’s not been too bad so far this season.’’
A perception that hydroelectric dams upstream could inhibit flows was incorrect, a Contact Energy spokesman said.
Its dams at Clyde and Roxburgh were ‘‘run of river’’, meaning they operated on natural flow only.
Contact was required to retain a minimum flow of 250cumecs from Roxburgh, unless natural flows dropped below that level.
Data from Niwa’s National Climate Centre suggests recent low inland rainfall could instead be responsible.
Rainfall levels in the upper Clutha River catchment ran at 40% to 60% of their 30year seasonal average for the period from July 1.
Lower Clutha tributaries have been running at 80% to 100%.
However, Niwa’s spring climate outlook predicts rainfall in inland Otago will be near normal.
Rivers are also expected to reach near normal levels between now and November.