Water was being spilled from the Benmore, Aviemore and Waitaki dams.
Benmore proved the most spectacular, with two of its four spillway gates open and the water hitting the concrete footing and shooting high into the air.
The river was carrying about three times its mean flow at the Waitaki dam.
Meridian has gone from worryingly low lake levels in August-September in the controlled storage lakes of Pukaki and Tekapo to coping with over-full lakes because of heavy westerly rainfall.
With a further wet front expected on Thursday, it had to release water to lower the lakes to provide a buffer to cope with any further major inflows.
Since early January, the Waitaki River has been running high, but the decision to spill even more water was made last week after consultation with Environment Canterbury (ECan), Central South Island Fish and Game Council and the Department of Conservation - it was decided to release more water over a 24-hour period as a "flushing flow".
Yesterday morning, Meridian started boosting the river, reaching a flow of about 970cumecs by midday.
That high flow is due to remain in place until about noon today.
It is believed the flushing will benefit the river - for example clearing didymo before the peak of the salmon angling season and reopening the river's mouth in its normal place.
ECan senior engineering officer Bruce Scarlett said yesterday a digger opened a new mouth for the river on Friday with the aim of using the high flow to shift it back south.
Over the past few years, the mouth had shifted steadily north until it was about 3km above the Glenavy huts on the north side of the river.
Mr Scarlett said the new channel was "working well", with the new mouth opening up and the old one to the north slowly closing.
Central South Island Fish and Game officer Graeme Hughes said the high flows and discoloured river would not initially help anglers.
However, the new mouth would be easier to access and didymo "should take a fair pounding", which would improve fishing when lower flows resumed.
Meridian's external relations manager, Claire Shaw, said yesterday that after the "flushing flow" eased, the river could still remain high as "controlled spills" continued to provide a buffer in Lakes Pukaki and Tekapo.
The flushing flow would bring benefits for the river.
"It made sense to deliver benefits from water we could not use to generate electricity," she said.
For Meridian, it was a balancing act between having the storage lakes full and providing for any major in-flows so resource consent conditions were not breached.
How long higher flows in the river would continue depended on inflows into the storage lakes, she said.
About 4pm yesterday, Lake Pukaki, the largest of the storage lakes, was at 532.43 metres above sea level.
Its maximum operating range is 532.0m. In early September it was 521m.
Lake Tekapo was 709.78m yesterday. In early September it was about 704m.
In December 1995, when the Waitaki hydro system was operated by ECNZ, it was caught with its storage lakes full when there was heavy rain and it was forced to release up to 3000cumecs of water, causing major damage to the twin bridges on SH82 at Kurow, forcing their closure while piles were repaired.
