Lithological Thresholds Changing Geomorphic Effectiveness in Andean basins

Abstract:

Mountain rivers connect the rugged mountainous terrain of the Andes with the Amazon basin. River incision into bedrock and deep-seated landsliding are the principal mechanisms by which the Andean escarpment is adjusting to regional tectonic forcing. Steep channel gradients in the escarpment zone facilitate upstream propagation of the fluvial knickzone into the Andean Amazon headwaters. Past studies on landscape evolution have often considered knickzone migration to be a steady process, responding to changes in external tectonic and climatic boundary conditions.

This study on the Eastern Andean Cordillera in Ecuador uses empirical data on fluvial erosion rates measured on subannual, decadal and millennial time scales to document spatio-temporal variability of river incision and sediment transfer. Our data on the Coca River indicate that the non-steady character of the erosion rates can be attributed to autogenic variability strongly mediated by river bed lithological strength. Beyond illustrating the effect of bed lithology on fluvial erosion rates, our work also shows the limitations of forecasting future erosion rates based on short time series because of non-steady erosion processes in Andean Amazon basins.