Catastrophic landslides and their effects on the Upper Indus streams, Karakoram Himalaya, northern Pakistan

The paper examines the nature and geomorphic influences of deposits from rock avalanches that obstruct rivers. Most of the 115 rockslide-rock avalanche events identified in the Karakoram Himalaya straddled one or more stream valleys. At least 73 of these landslides formerly dammed the Indus or its tributaries. Upstream of the barriers, remnants exist of once very extensive lacustrine deposits. The stability and long-term survival of dams from large rock avalanches is addressed, and the reasons for the gradual or phased, rather than catastrophic and sudden, breaching. Case histories emphasise the erosional and depositional forms associated with the barriers. They are reconstructed in terms of the emplacement and properties of the deposits from rock avalanches, and the response of the river to interruption by them. These barriers have exerted a substantial control over stream response and development for most of the Holocene at least. Today, they are associated with distinctive features of channel form and thalweg, river terrace systems, intermontane sediment sinks and sources, and local erosional forms in bedrock. More broadly, the landslide barriers have created a naturally fragmented river system. Deposition and erosion in the lower few tens or hundreds of meters of the Upper Indus stream valleys have been dominated by epicycles of aggradation, trenching, and downcutting that mainly reflect the scale, timing and history of each barrier. The landforms involved are only indirectly related to the late-glacial and paraglacial conditions that have, hitherto, been regarded as the main explanation. (C) 1998 Elsevier Science B.V. All rights reserved.