Rock-slope failure and the river long profile

Abstract

This study examines the geomorphic effects of large (>10⁶ m³) rock-slope failures on long profiles of rivers in the Swiss Alps and the New Zealand Southern Alps. Regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock-slope failures. For a fixed concavity index, the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river-damming rockslide debris as old as 10 k.y. Assuming that the knickpoints do not predate slope failure, data show that high profile steepness and inferred specific stream power are not always the cause, but often a result, of large river-blocking rock-slope failure in mountain basins. Omission of rockslide data from slope-area plots lowers the steepness index and increases the concavity index on average, yet only in few cases more than one standard deviation.