The Pukaki and Ohau canals on the upper Waitaki power scheme are to be drained this week, for an outlet gate to be inspected, only the third time since they were built about 30 years ago.
The large, empty canals will be a spectacular sight for people used to driving over them on State Highway 8 when they are full of water.
Draining the canals is scheduled to start on Wednesday.
Meridian Energy has co-ordinated the project with a Transpower shutdown of the HVDC link between Benmore and Haywards, which shifts power to the North Island.
The shutdown will affect generation at least six of the eight power stations on the Waitaki River.
The only two still capable of getting water would be Tekapo A and B. Meridian spokesman Alan Seay said when contacted yesterday, the two canals would be "de-watered" so "Gate 18", the outlet gate from Lake Pukaki into the canal, could be inspected.
It would take five days to drain the canals, carry out the inspection before refilling them, the process requiring the shutdown of the Ohau A, B, and C power stations, which have a combined generation of 688Mw.
The level of Lake Benmore is also likely to fluctuate more than usual, although Meridian can pass water down the Pukaki spillway into the lake.
Flows in the lower Waitaki River would be maintained within the limits set by Meridian's resource consent.
Mr Seay said draining the Pukaki and Ohau canals would enable Meridian to take a close look at the concrete structure at gate 18, where it was suspected there could be some erosion.
Divers could not do the job because of poor visibility.
The inspection would give Meridian a clearer idea what, if any, work would be needed at the gates.
It may be able to carry out temporary repairs while the canals are drained or have to schedule more major work later.
The two canals were built between the mid-1970s and 1980s as part of the upper Waitaki power scheme.
The last time they were drained was in the 1980s.
The Pukaki canal runs from Lake Pukaki to Lake Ruataniwha, where the Ohau A power station is located.
The outflow from Lake Ruataniwha flows into the Ohau canal through the Ohau B and C power stations before going into the Haldon arm of Lake Benmore.
Mr Seay said stakeholders with an interest in the canals and Lake Benmore would be kept informed of what was happening.
The Pukaki and Ohau canals are also popular fishing water, with trophy-sized trout being taken.
Central South Island Fish and Game Council would have to monitor the operation to rescue any stranded fish.
In November last year, Meridian made major repairs to the Tekapo canal between Lakes Tekapo and Pukaki, but did not have to drain it.
