A LAKE in the quake-prone mountains of central Asia is a seismic shiver away from unleashing one of the deadliest natural disasters in history, says an international team of geologists and aid workers that visited the region.
The Sarez Lake in Tajikistan formed when landslides blocked the River Murgab in the Pamir Mountains after an earthquake in 1911. The lake is now 60 kilometres long, up to 500 metres deep-and leaking. Another earthquake could breach the natural dam, say the team, who are expected to announce the results of their survey in Geneva this week.
If the dam bursts, the team predicts that a wall of water more than 100 metres high would rush down the narrow Murgab gorge into the Bartang valley, devastating hundreds of mountain villages. The wave of water would start out 170 metres high, says Scott Weber of the UN Department for Humanitarian Affairs, who coordinated the field expedition to the lake. "A thousand kilometres downstream, it would still be as high as a two-storey house."
If there was no warning of the impending disaster, the team predict that hundreds of thousands of people would be killed. The 17-cubic kilometres of water now filling the lake would also flood about 50 000 square kilometres inhabited by 5 million people in four countries-Tajikistan, Afghanistan, Uzbekistan and Turkmenistan, Weber predicts. The water would surge along the Amudar'ya River on the Tajik-Afghan border, eventually reaching the Aral Sea, which has lost half its volume over the past 30 years ("Poisoned waters", New Scientist, 21 October 1995, p 29).
Weber believes that the risk of disaster is high, as the dam leaks more each year and part of it slid into the lake five years ago. A major earthquake nearby could cause a breach, and there have been quakes registering up to magnitude 7 elsewhere in Tajikistan during this decade.
But a less severe quake could prove almost as disastrous. Hanging over the north bank of the lake is an unstable cliff with deep cracks in it. "In the event of an earthquake, it is likely that the cliff would fall into the lake," says a recent study commissioned for the UN's International Decade for Natural Disaster Reduction. "A rush of water, equalling roughly 1-2 cubic kilometres, would strike the dam and cascade over the top." Such a sudden surge is likely to inflict severe damage on the precarious barrier.
Weber believes that strengthening the dam is wishful thinking. It is 500 metres high, 4 kilometres long and located in one of the most remote and rugged areas in the world. What's more, tinkering with the structure could upset its fragile stability. Efforts to drain the lake by increasing the leakage are also likely to accelerate erosion.
That leaves the options of a pipeline down the valley or digging holes in the mountains to siphon the water into neighbouring valleys. Both options are likely to be on the table when international donors and the World Bank gather in Geneva next month to offer Tajikistan help.
For the hundreds of thousands of people living downstream, the immediate need is for safe havens and an early warning system. The former Soviet Union installed a lakeside monitoring system with a radio link to the outside world. "But it is out of action now," says Weber. "And if it worked, it would call Moscow."
Author: Fred Pearce
New Scientist 19th June 1999
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