Large landslides and their effect on sediment flux in South Westland, New Zealand

Landslides and runoff are dominant erosional agents in the tectonically active alpine South Westland area of New Zealand, characterized by high uplift rates and extreme orographic precipitation. Despite a high density of shallow debris slides and flows, the geomorphic imprints of deep-seated bedrock failures are dominant and persistent. Over 50 large (> 1 km(2)) landslides comprising rock slide/avalanches, complex rotational and rock-block slides, wedge failures, and deep-seated gravitational slope deformation were detected on air photos and shaded-relief images. Major long-term impacts on alpine rivers include (1) forced alluviation upstream of landslide dams, (2) occlusion of gorges and triggering of secondary riparian landslides, and (3) diversion of channels around deposits to form incised meandering gorges. Remnants of large prehistoric (i.e. pre-1840) landslide deposits possibly represent the low-frequency (in terms of total area affected yet dominant) end of the spectrum of mass wasting in the western Southern Alps. This is at odds with high erosion rates in an active erosional landscape. Large landslides appear to have dual roles of supplying and retaining sediment. The implications of these roles are that (1) previous models of (shallow) landslide-derived sediment flux need to be recalibrated, and (2) geomorphic effects of earthquake-induced landsliding may persist for at least 10(2) years. Copyright (c) 2005 John Wiley & Sons, Ltd.