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Contact generation manager Graham Quinn said the lake would be out of bounds while flushing was being carried out and the company had locked the access gate to the Roxburgh dam boat ramp and put up warning signs at the Alexandra boat ramp.
It was expected the flushing would be finished in three to five days, subject to the flow of the river, Mr Quinn said yesterday.
The flushing moved sediment which had deposited in the upper reaches of Lake Roxburgh further downstream, reducing the potential effects of flooding in Alexandra, he said.
During the flushing, the level of Lake Roxburgh at the Roxburgh dam site was lowered from its usual level of 132m above sea level to about 126m above sea level.
Sediment flushing was carried out during periods of high river flow, with opportunities coming about about once a year, he said.
Flushing was permitted under the company's resource consents and Otago Regional Council was being kept informed.