Disturbance regime landscapes: mountain drainage systems interrupted by large rockslides

Abstract

The paper examines the role of rockslide-rock avalanches in mountain landscapes, and the landforms associated with them. While the landslides are extremely short-lived events, rock wall detachment scars and rock avalanche deposits can persist for long periods as influences on landscape development. Especially significant are rock avalanches with complex runout and emplacement related to interactions with rugged terrain or deformable substrates. Their characteristics greatly increase the scope of landscape disturbance. Hundreds of rock avalanches are now known, worldwide, that have formed crossvalley barriers interrupting mountain drainage systems. Many have done so for millennia or tens of millennia. They give rise to distinctive sediment assemblages, constructional and erosion landforms generated by other processes responding to the landslides and constrained by them. A landslide interruption epicycle of five phases is described, and related sediment assemblages. These provide the basis for defining a landslide interrupted valley landsystem. Its full significance is seen in mountain drainage basins affected by multiple landslide interruptions. These create naturally fragmented fluvial systems, in which a disturbance regime geomorphology is identified. Stream profiles, sediment delivery, and related landforms are kept in a chronic state of disequilibrium with respect to climatic and geotectonic controls, and drainage organization. The transHimalayan Upper Indus Basin provides an example, a large high mountain drainage system fragmented by more than 170 late Quaternary rock avalanches. In this case, as elsewhere, misidentification of rock avalanches led to neglect of their role in Quaternary histories. The nature and limitations of disturbance regime geomorphology are discussed, and broader implications for mountain landscapes.