Cosmogenic nuclide-derived rates of diffusive and episodic erosion in the glacially sculpted upper Rhone Valley, Swiss Alps

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 (Be-10) 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 Be-10 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 (C) 2010 John Wiley & Sons, Ltd.