The role of extreme floods in shaping bedrock channels

Alluvial river channels adjust readily to formative discharges of moderate magnitude and frequency. Bedrock river channels present various thresholds to effective channel adjustment, such that only relatively rare, highmagnitude flood discharges contribute to shaping their morphologies. Very high values of power per unit area of bed, exceeding 10² Wm^-2, result in high-energy erosional processes, including cavitation and macroturbulent plucking. Although these processes are best exemplified in the Channeled Scabland and other late Pleistocene cataclysmic flood channels, they also can be achieved in modem bedrock gorges where very resistant rocks are acted upon by unusually large floods. Examples include the Katherine Gorge, northern Australia, and various gorges of the Narmada and Tapi Rivers in central India. Distinctive scabland-like morphologies occur, including wide, shallow bedrock surfaces with inner channels and narrow, deep gorges. As in alluvial rivers, there may be an adjustment to energy expenditure, but at a much higher level of energy associated with rare floods.