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Landslide-created lake may overflow its natural barricade in New Zealand

A two-kilometer (1.2-mile)-long lake created by a giant landslide  in southern New Zealand last month may soon overflow its natural rock barricade. People living or visiting the area are not expected to be seriously affected by the overflow, although some tourist activities could be disrupted if the barricade bursts, officials said.

The landslide plunged hundreds of thousands of tons of rock and soil 900 meters (2,950 feet) into the valley of South Island's Young River - creating a 70-meter (230 feet)-high dam and the new lake in Mount Aspiring National Park.

The lake's water lapped within four meters (13 feet) of the top of the rocky dam face Wednesday and is expected to begin flowing over the top within days, said Otago regional council chief executive Graham Martin.

The newly formed lake near the head of Lake Wanaka, many miles from the tourist and farming town of Wanaka, rose eight meters (26 feet) over last weekend after heavy rainfall in the area. Its rise has slowed since, Martin said.

"You watch the water creeping up - there's really nothing that can be done about it - fortunately it's not in an area of habitation," he said.

The majority of houses in the sparsely populated rural area are on higher ground while residents of nearby Makarora Valley's 25 farm properties were well above the "flood line" if the dam breaches. Livestock have been moved from low-lying farm land.

"There are very big blocks of rock ... two to six meters (yards) across" in this pile, and "an enormous lot of fine, silty, sandy type of material," Martin told National Radio.

One scenario is that "the water power behind it ... will largely demolish the dam and put a very major flood wave down the Young (River) valley," he added.

The flood will carry huge amounts of silt, rock and debris including trees down the river valley.

The valley and its floor "won't be the same after this dam fails," and will leave a small lake behind, he said.

Makarora Community Board chairman Devon Millar said that "the worst-case scenario is highly unlikely to happen _ that would be a major collapse" of the natural dam.

"There's no major concerns ... it looks to be as safe as houses," he told National Radio.

But tourism activities like river jet boating and hiking could be affected by the aftermath of the dam breaking.

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