Abstract

The widening and deepening of river valleys occur at different rates. This co-evolution is largely undocumented over thousands of years. Uncertainties on incision and widening laws limit the ability of models to reproduce erosion rates and responses to tectonics and climate over geologic timescales. We studied two 1 km-deep canyons in northern Chile, both characterized by valley widening after the upward migration of a major knickzone but with a diachronous incision initiation. We use Be-10 concentrations measured in colluvial deposits to quantify the erosion rates E of along-stream valley flanks. We observe that E increases in the knickzone, with well-defined relationships (R2 = 0.7): E increases quasilinearly with the valley-bed slope and decreases with the valley width (exponent similar to-0.4). Our results suggest that E decreases with time but that valley flank weathering may sustain a significant lateral erosion rate. Our results provide data for testing erosion laws governing valley widening.