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Contact Energy head of hydro generation Boyd Brinsdon said some water spilling late last week after an unplanned outage of a Transpower circuit meant there was insufficient transmission capacity for the dam’s generation to be exported out of the region.
On Saturday the spill continued as a result of heavy rain in the upper catchments of the Clutha and Kawarau Rivers, leading to a greater river flow than could be passed through the dam’s generators, he said.
"The volume varies but is approximately 200-500cumecs (cubic metres per second) from each station," Mr Brinsdon said.
At present the dam is discharging between 800cumecs and 2000cumecs in total downstream.
Mr Brinsdon said based on the forecast, Contact Energy expected the spill to decrease over the coming week.
However, the forecast could change, he said.
The Clyde Dam, completed in 1992, is New Zealand’s newest and third-largest hydro-electric dam.