‘Flushing’ to wipe out algae
It may have been barely perceptible to the eye, but releasing water from the Opuha Dam on Friday has hopefully wiped out a large quantity of didymo and other algae in the river, Opuha Water Ltd says.
Operations manager Craig Moore said the dam released a flow peaking about 65 cubic metres per second (cumecs), or 300,000 cubic metres in total during the ‘‘flushing’’ process in the Opuha River on Friday morning.
The river ‘‘pulse’’ stayed within river margins, and the wave was not really noticeable as it made its way downstream, Moore said.
Flushing had been taking place for years, but the peak flow was greater since new gates to the downstream weir were installed about 18 months ago, Moore said.
‘‘We are confident that the flow release will be very effective in significantly reducing the current high levels of didymo in the Opuha Gorge as well as assisting in removing nuisance algae from the Opuha and Opihi Rivers further down- stream,’’ he said. It was perfect timing with the high natural flows of about 100 cumecs, he said.
‘‘The conditions [on Friday morning] were ideal for us.
‘‘It’s not going to be noticed as much, because the river is already dirty and sort of in flood.
No one’s going to be in the river at 100 cumecs anyway.’’
Irrigation schemes were shut down for the flushing process.
The ‘‘pulse’’ of water took 10 to 12 hours to get right down the river, Moore said.
It was scheduled to arrive at the river mouth in time for low tide at 7.20pm.
Central South Island Fish and Game chief executive Jay Graybill said Opuha Water Ltd had been able to organise a flush on two hour’s notice, which was impressive. ‘‘It’s generally good for the health of the river, and it’s good for the users of the river.’’
During summer algae surveys were done every two weeks, and one would be done on the Opuha River next week to check how effective the flushing was, Moore said. ‘‘Opuha Water Ltd is continuing to investigate ways to improve the health of the rivers downstream of the dam and lake and flow releases like were undertaken this morning are a key element of our river operating strategy.’’
Regular algae surveys were carried out throughout the year at four key locations in the catchment.