Rock avalanches cause destruction, but also provide benefits to people living in Asia’s mountains. Opening a three-part article, Kenneth Hewittexplores the impacts in the Indus valley.
Large rockslides and the rock avalanches they can generate will destroy any living thing or built structure in their path. In mountain valleys, they can form dams impounding lakes that may burst suddenly with devastating consequences. The January 2010, Attabad landslide – which dammed the Hunza River in northern Pakistan, destroyed 26 homes and displaced a further 20,000 people – offers a stark reminder. Not surprisingly, the hazards of landslides have tended to drive scientific study. Less often noted is how they also create resources and opportunities for mountain people.
In the past couple of decades, great numbers of large landslides have been found in the world’s high mountains, including most of the Inner Asian ranges. More than 375 large landslides have been identified along the Indus streams in the Karakoram, Hindu Kush and northwest Himalayan ranges.
Most predate historical records but continue to influence landscape developments. Land use is closely adapted to landforms controlled by the landslides. Many villages and some small towns sit amid landslide rubble, as do ancient cultural sites, modern roads, airfields and tourist facilities. The environmental knowledge and stories of local people reveal an understanding of these events, and the geo-hazards they present.
My particular interest is in those involving “catastrophic rock slope failures” or “rock avalanches”; collapses on steep rock walls, and a descent of hundreds of metres. They are catastrophic not only because no living thing or built structures in their path will survive, but also in the geophysical sense: they are very large, at least one million cubic metres and reaching tens of cubic kilometres in some cases. They occur suddenly, often with no warning, in part because they originate in rugged terrain and partly because they are usually triggered by earthquakes or extreme weather. They travel at very high speeds, with velocities between 100 and 250 kilometres an hour, comparable to an express train.
The crushing forces involved are so great that, in less than a minute, huge volumes of solid bedrock are reduced to rubble, sand and dust. This transforms the collapsing mass into a rock avalanche – a high-speed run out of broken and crushed rock that appears to flow, plunge and surge forward, creating immense dust clouds like the more familiar snow avalanches that bury large areas in crushed and broken rock.
And all of this occurs within two or three minutes – a geological instant. However, they have huge impacts on landscapes and landscape development, notably by damming rivers, with consequences lasting thousands, possibly tens of thousands of years.
On the one hand, these landslides are among the more extreme mountain hazards; though they are comparatively rare events in human timescales or in any given region. On the other hand, because their consequences remain in the landscape for millennia, they have an enduring effect on human habitats and activities in the mountains.
This three-part article focuses on these longer term consequences of rockslides, in particular on the remarkable story of how mountain people have adapted to life among the landslides. It emphasises how, thanks to human adaptability, the landsides not only bring “perils” but also “gifts” or benefits to human settlements. This is notably by creating landforms suitable for human habitation in otherwise very extreme and rugged environments.
The landslide-related benefits are notable because so much of the region is inhospitable. Two-thirds of the upper Indus basin lies above 3,500 metres elevation, in climates too severe for permanent settlement. Glaciers cover 20,000 square kilometres and permafrost an even larger area. Rain-shadowed valley floors tend to be arid or semi-arid whereas, high above, snowfall is heavy. Precipitous rock walls are the dominant landform. The landslides themselves reflect this rugged terrain, the mountain-building forces at work and valleys deeply excavated in recent geological time. Indeed, where rivers are cutting down vigorously into bedrock to match high rates of tectonic uplift, life has very few and precarious footholds.
Surprisingly, however, most sections of the upper Indus streams flow not in bedrock, but over thick and extensive valley-fill sediments. Scientific observers have long been impressed and puzzled by so much deposition within rugged mountains. They realised that the deposits must record events that overwhelm or interrupt stream incision, causing sedimentation along the valleys. Until recently, however, very few of the landslides were identified as such, and their role went unrecognised.
This article looks at the trans-Himalayan upper Indus basin, where more than 375 such landslides have been identified in recent years. However, the phenomena are not confined to that region. Such landslides have also been identified in the Hindu Kush, Greater Himalaya, Pamirs, Tian Shan and other Inner Asian ranges. In many cases it is apparent that human communities have adapted to the consequences.
I will describe for the first time the positive effect on the availability of habitable land. Although the article outlines the science behind the discovery and nature of the landslides to introduce their place in settlement geography, we will remain mindful of the major disaster risks that accompany them.
Landslides and rivers
Rock avalanches that travel across river valleys will usually dam them. Unstable, short-lived dams and floods from sudden breaching are of singular concern, but along the Indus streams remnants of lake-bed deposits record dozens of former landslide impoundments that lasted decades or centuries.
More than 150 landslide barriers remain incompletely cut through today, continuing to act as local base levels for valley development. Their composition, and the shock of the sudden halting of the rock avalanche, make for a highly compacted mass, able to form strong, impermeable landslide dams. Where stalled against opposing slopes, barriers can be hundreds of metres high, and debris lobes spreading far up and down valleys resist erosion and add strength to the dam.
On average, one cross-valley rock avalanche was found for every 12 to 14 kilometres of upper Indus streams surveyed. The landslide interruptions are encountered in every possible stage of development, depending on age, size and local conditions. Each case is affected by others, some older and some younger. Intact dams upstream will starve downstream impoundments of sediment. Stream flows are moderated by passing through a lake or over flatter, aggraded areas above dams. Conversely, degrading interruptions can send huge pulses of sediment down the valley. Since there are many overlapping sequences of aggradation and degradation from landslide interruptions in different stages of development, they modify and complicate sediment delivery throughout the river system.
As yet, the full story of these landslides has not been investigated; when they occurred, and how likely or when others may occur in the future. The extent to which some thousands of years of human habitation has been interfered with, and assisted by the landslides also remains largely unexplored. If the upper Indus basin examples described in the article are anything to go by, a substantial story waits to be uncovered other parts of the greater Himalaya.
Kenneth Hewitt is professor emeritus in geography and environmental studies and research associate at the Cold Regions Research Centre at Wilfrid Laurier University in Ontario, Canada. This is an updated version of an earlier article, published in the American Scientist in 2010.
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