Impacts of Lithologically Controlled Mechanisms on Downstream Bedrock Valley Widening

Abstract Wide, fluvially carved bedrock valleys are found around the world, but little is known about the processes and mechanisms controlling their formation. We know that rates of valley widening are controlled by stream discharge and bedrock lithology, with larger streams and softer rocks resulting in wider valleys. But lithology also controls widening mechanisms that cause valleys with contrasting bedrock strength to widen in fundamentally different ways. The effect of distinct widening mechanisms on bedrock valley width remains unquantified. We analyze a new topographic data set from various lithologies that demonstrates that bedrock valleys in soft lithologies widen downstream twice as fast as valleys in intermediate lithologies. These downstream valley‐widening rates also emerge from landscape evolution model simulations using two end‐member valley‐widening mechanisms. The topographic data show that lithology and mechanism of valley widening are linked and that different widening mechanisms can control bedrock valley development along river courses.