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Because of competing forces and variations through time the relative importance of geomorphic processes responsible for the long term topographic evolution of a mountain range is not always obvious. Here we perform a space for time substitution with the western escarpment of the Andes between 10 and 20°S to identify the mechanisms of plateau destruction over geological timescales. We use this setting to propose that variations in the precipitation rate play a primary role in setting hillslope relief in uplifted mountainous landscapes. We find that in dry climates local topographic relief grows with increasing precipitation independent of the underlying lithology and given an overall uniform rock uplift history. We proceed by differentiating Andean landscapes with generally low precipitation rates (80–500 mm a- 1 Peruvian Andes 10–20°S) where local relief correlates positively with precipitation from those with higher precipitation rates (400–1400 mm a- 1 Chilean Andes 35–40°S) where increases in precipitation lead to topographic decay. We suggest that these trends result from dominant bottom up control (channel incision is faster than hillslope response) giving way to an increasing top down control (hillslope lowering is faster than channel incision). With low precipitation relief growth is controlled by stream incision and knickzone retreat into a largely undissected plateau. With higher precipitation rates relief is set by the steepness of graded streams and the rates of sediment production and transport on hillslopes. Trends of topography can also be interpreted in temporal terms in which the higher precipitation results in shorter response times such that the Peruvian Andes between 10 and 20°S are still responding to Miocene uplift while in the Chilean Andes between 35 and 40°S these knickzones have already propagated through the entire fluvial network. We anticipate that such changes also operate during the formation and destruction of other mountainous plateau landscapes.

This paper explores the mechanisms by which inner gorges in the Alps were formed. It focuses on the ca. 1.5 km long 80 m deep and a few hundred meter wide Lammschlucht located in the northern foothills of the central Alps. We reconstructed the glacial cover using lateral moraines and hanging talus cones that record the elevation of the ice surface at the deglaciation stage of the LGM (Last Glacial Maximum). We used the reconstructed ice thickness patterns to calculate the erosional potential of the subglacial meltwater. The applied model is based on the principle of energy conservation and yields the pattern of downstream changes of the dynamic pressure which is considered a measure for erosion potential. The model results suggest a maximum of the dynamic pressure at the end of the inner gorge. We interpret therefore that the subglacial meltwater scoured the reach toward the end of the Lammschlucht because of the enhanced dynamic pressure which was ultimately controlled by the ice overburden. Post glacial fluvial erosion then resulted in a readjustment through a regressive shift of the erosional front along the inner gorge farther upstream. The current location of this front lies almost in the middle of the Lammschlucht inner gorge where a step pool channel changes into a straight plane bed channel flowing on a deeply scoured bedrock.