Hydro-lakes spill flushes didymo out

A controlled flood has flushed much of the didymo in the Waitaki River out to sea but the reprieve may be short.

The invasive algae in the river's main channels has been washed away after a recent hydro-lakes spill.

However, Fish and Game New Zealand warns didymo may return within months.

The flood, initiated by power company Meridian Energy two weeks ago, came after a request from Fish and Game that water not needed for power generation be used to clean the braided river.

Excess water from Lake Tekapo, Lake Pukaki and Lake Benmore was spilt down the river valley, with the river's mean flow of about 250 cubic metres a second (cumecs) almost quadrupling to 950 cumecs.

Environment Canterbury also took the opportunity to bulldoze a new path for the Waitaki at its mouth, where a shingle spit had developed, pushing the river north for 3.5 kilometres.

Fish and Game Central South Island chief executive Jay Graybill said the result was "fantastic".

More than a dozen mostly salmon anglers had reported that didymo had disappeared in the main channels and the river was much cleaner, he said.

"Much, if not all, of the didymo has been cleared off the gravels. Some of what has been washed away could still be hung up in quiet backwaters, but in more active channels it's largely cleared out.

"Some of the anglers are making comments it's `like the old days', they can cast 10 or 12 times without cleaning off their hook.

"Not every last scrap of it (didymo) will have gone though. The cells are still there. We are still watching to see how and when it comes back. We're not sure how long it will take could be a matter of weeks or months."

Environment Canterbury southern region duty flood controller Tony Henderson said the flushing flow had been a success.

"The biggest thing for us was the opening of the (river) mouth. It certainly has cleaned out the river.

"The reports we have had are that not a lot of vegetation was cleared out, except vegetation on the edge of the flow gorse and rubbish like that. The flow was constrained mainly to the central part of the river."

Meridian spokeswoman Claire Shaw said the company had stopped spilling from the Waitaki hydro scheme late last week. "We're back to normal. There's a very, very dry forecast for this week."

Contact Energy spokesman Jonathan Hill said Contact was no longer spilling water from the Clutha River scheme at Lake Roxburgh and Lake Dunstan.

- © Fairfax NZ News