Hydraulic geometry, river sediment and the definition of bedrock channels

A growing literature deals with bedrock channels, erosion processes within them, and their role in landscape evolution. Formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin. This is equated with a physical property of the flow. Using up-to-date erosion laws and descriptions of channel dynamics, we show that existing definitions are difficult to apply and may lead to conflicting classifications. We propose the definition: A bedrock channel cannot substantially widen, lower or shift its bed without eroding bedrock. Applied to channels in Taiwan, this leads to a different classification than previous definitions. By analysing at-a-station hydraulic geometry of these channels, we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock.