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Keywords:

  • central Swiss Alps;
  • cosmogenic 10Be;
  • denudation rates

Abstract

Denudation rates of small tributary valleys in the upper Rhone valley of the Swiss Central Alps vary by more than an order of magnitude within a very small distance (tens of kilometers). Morphometric data indicate two distinct erosion processes operate in these steep mountain valleys. We determined the rates of these processes using cosmogenic beryllium-10 (10Be) in pooled soil and stream sediment samples. Denudation in deep, glacially scoured valleys is characterized by rapid, non-uniform processes, such as debris flows and rock falls. In these steep valleys denudation rates are 760–2100 mm kyr−1. In those basins which show minimal previous glacial modification denudation rates are low with 60–560 mm kyr−1. The denudation rate in each basin represents a binary mixture between the rapid, non-uniform processes, and soil creep. The soil production rate measured with cosmogenic 10Be in soil samples averages at 60 mm kyr−1. Mixing calculations suggest that the debris flows and rock falls are occurring at rates up to 3000–7000 mm kyr−1. These very high rates occur in the absence of baselevel lowering, since the tributaries drain into the Rhone trunk stream up-stream of a knickzone. The flux-weighted spatial average of denudation rates for the upper Rhone valley is 1400 mm kyr−1, which is similar to rock uplift rates determined in this area from leveling. The pace and location of erosion processes are determined by the oscillation between a glacial and a non-glacial state, preventing the landscape from reaching equilibrium. Copyright © 2010 John Wiley & Sons, Ltd.