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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

Landslide-Interrupted Rivers

2010-09HewittF3.jpgClick to Enlarge ImageThese landslides are short-lived events, rare on human time scales or at any one location. Their detachment scars on rock walls and masses of debris below, however, can survive and influence landscape developments for millennia. 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 meters high, and debris lobes spreading far up and down valleys resist erosion and add strength to the dam.

When landslides interrupt streams two interdependent episodes follow. At first sediment is trapped above the dam, the aggradation phase. This continues until breaching occurs bringing the second, degradation phase, as impounded sediments are trenched and removed. It sounds straightforward but both phases are complex, owing partly to diverse landslide configurations and partly to the response of high energy mountain processes. Lake sediments are mainly thin layers of fine-grained material: clay, silt and sand washed into the lake and settling out slowly. Coarser material is dumped quickly around the shores. Streams build deltas or alluvial fans. The latter, an important landform first named from this region by Frederick Drew (1872), is typical where tributaries enter landslide-interrupted valleys. The larger ones, several kilometers across, usually comprise a mix of stream, torrent and debris flow materials. Smaller, steeper ones are built mainly by debris flows. Aggradation in the landslide interruption produces complex buildups with interfingering and overlapping of many different sediments.

2010-09HewittF4.jpgClick to Enlarge ImageIn the degradation phase, systems of stream terraces develop as rivers incise into the sediment pile. Terrace sequences relate to dam history, whereas their slope, width and other properties differ in up-valley sediments compared to those through the landslide and in downstream areas. Tributary channels become incised in the fans, whose outer parts are truncated by the main stream. Rock avalanche and debris fan materials, or bedrock outcrops, create “defended” terraces whose geometry reflects the locations of resistant materials. Because rivers crossing the landslide interruption need not follow the pre-landslide course, they are let down on bedrock spurs or in new valley floor sections and may cut secondary or “epigenetic” gorges, the most common type of bedrock gorge in the region. Such complications, once recognized, help distinguish landslide-generated terraces, fans and canyons from features where sediment transport and stream incision respond directly to climate and hydrological changes, or tectonics.

On average, one cross-valley rock avalanche was found for every 12 to 14 kilometers 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.

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