Massive landslip changes colour of Lake Wakatipu

Lake Wakatipu has changed colour following the massive slip at Sandy Bluff, above Chinamans Bluff. Photo / Tracey Roxburgh

The massive landslip in the Dart Valley has changed the colour of Lake Wakatipu - temporarily, at least.

Queenstown Lakes District Council harbourmaster Marty Black said the change in water colour was "quite noticeable" following Saturday's slip, changing to a "more turquoise" colour, similar to Lakes Tekapo and Pukaki, near Mount Cook.

While heavy rain in the headwaters had caused water to look like "pea soup", Mr Black said there was "no question" the lake had already changed colour.

"Across the lake from Cecil Peak towards Kelvin Heights [the other day] you could see a distinct line from the real blue to the turquoise."

The phenomenon had been caused by the glacial rock falling within the Te Koroka/Slip Stream area at Sandy Bluff, about 10km above Chinamans Bluff.

While Mr Black said he had asked if the lake would return to its previous colour, he had been told it would but "not in our lifetime".

However, GNS principal scientist Simon Cox doubted the colour change would be anything more than temporary.

An "awful lot" of fine glacial "flour" had come down the face of the slip and into the lake, where it was being suspended in the water, refracting the light.

In lakes like Tekapo and Pukaki, the glacial flour was so fine it was unable to flocculate together and sink, remaining suspended in the water.

Mr Cox said GNS had noticed the colour change and some people had become interested in whether the lake would "become like Lake Pukaki".

While the slip material had not been tested, he suspected the glacial flour in Lake Wakatipu would not be fine enough to remain suspended.

"You might have something like that temporarily, but I would be very surprised if it was permanent, so perhaps enjoy it while you can.

"The lake is really big, you've got a huge amount to get scattered ... I suspect the [landslip] is not big enough to sustain that amount of material in the water over a long period of time."

Mr Cox said GNS was still working to establish the volume of rock which came crashing down about noon on Saturday, forming a new lake upstream.

He believed more material came down early yesterday, increasing the slip's ability to dam the river for a while. However, the river flow had increased again by noon.