ARCHEOLOGIA NUOVA

The people living beside the Colonia River in the Aysén region of Chilean Patagonia are under constant threat of a sudden catastrophic flood sweeping down from the mountains above them.

The region has experienced an unprecedented seven events called glacial-lake outburst floods since April 2008. Each time, Lake Cachet 2, which lies on the Colonia glacier, has drained its 200 million cubic metres of water in a matter of hours into the Colonia Lake and River, sending the water on to the river's confluence with Baker River, Chile's largest in terms of volume, and generating a wave as far as 25 kilometres upstream and 100 kilometres downstream to the Pacific Ocean.

"You can review the scientific literature and you will notice that these phenomena are known worldwide in the Himalayas, the Alps, but the difference here, and what is striking, is its recurrence," says Fabián Espinoza, regional director of the country's Bureau of Water Management.

The most recent flood was on 4 March this year. The water level in Colonia Lake rose 3.5 metres in just 28 hours and divided the region in two after cutting across the Austral Highway, the main north-south route in Aysén. The Baker River doubled in volume.

Glaciologist Gino Casassa of the Center for Scientific Studies (CECS) in Valdivia has studied the floods in collaboration with the Chilean Meteorological Office. After three years, he has a clear culprit in mind: the repeated opening and closing of a tunnel 8 kilometres beneath the Colonia glacier, connecting Lake Cachet 2 above the glacier and Colonia Lake below it¹.

But why has this phenomenon occurred repeatedly over the past few years in Cachet 2? Casassa's answer is climate change: "The glaciers of Patagonia in general have receded and thinned greatly during the past decades," he says. "The lakes grow at the expense of the ice and generate situations in which there is a danger of draining. In the case of the Colonia glacier, this thinning has weakened the natural dam the glacier constitutes."

César Portocarrero, head of the Department of Glaciology and Water Resources at the National Water Authority of Peru, has studied this phenomenon elsewhere in the Andes. He agrees with Casassa. "We can affirm that there is a direct relation between climate change and the phenomena that have been occurring," he says. "The water generated by the melting of the glaciers circulates between the glacier and rock and serves as a lubricant for the shifting of the ice mass."

So, as the glacier melts, the melting water forms a tunnel between the two lakes, allowing Cachet 2 to drain quickly, after which the tunnel collapses and the lake begins to refill. But the last flood seems to have been different. Espinoza says that it looks as if the tunnel formed in a different area, not at the glacier's base, but nearer the top.

In collaboration with the Dresden University of Technology in Germany, the CECS has installed two cameras that take photos every two hours[and allow precise calculations of lake levels as it is draining. Together with the Federal Institute for Geosciences and Natural Resources in Hanover, Germany, they have measured the thickness of the ice in the Colonia glacier and, in conjunction with NASA, have studied the depth of the lake and the height of the glacier. The researchers conclude that the discharges will continue until the ice has receded or thinned sufficiently to generate a permanent natural drainage channel.

In response to the threat, public agencies, together with the local population, have developed protocols for averting a repeat of the catastrophe caused by the first discharge, when farmland was flooded, roads were cut off, houses were carried away by the water and people were isolated for days — although no lives were lost. Water-level monitoring stations have been installed on Cachet 2, Colonia Lake and Baker River that provide text message warnings to the authorities who pass them on to the public.

According to Portocarrero, the risks associated with glacial-lake outburst floods should be taken into account in plans to locate cities or infrastructure in potential flow zones. "If we want to cultivate or build a culture of prevention, we must analyse these pretty mountain decorations known as glaciers, which can occasionally cause lethal phenomena," he says.