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Gifts and Perils of Landslides

Catastrophic rockslides and related landscape developments are an integral part of human settlement along upper Indus streams

Kenneth Hewitt

2010-09HewittF1.jpgClick to Enlarge ImageLarge 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 4, 2010, Atabad landslide, which dammed the Hunza River about 70 kilometers north of Gilgit, Pakistan, offers a stark reminder (see sidebar, page 416). 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.

Recently, more than 340 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 geohazards they present.

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 meters elevation in climates too severe for permanent settlement. Glaciers cover 20,000 square kilometers 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 realized 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 unrecognized. Here I will focus on how the rivers are responding to the many landslides that have blocked valleys. I will also 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.

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