SEDIMENTARY PROCESSES/Catastrophic Floods 635 Figure Progressive increase in stream power for various erosional channel morphologies in the Columbia River gorge After Benito (1997) Energy expenditure and geomorphic work of the cataclysmic Missoula flooding in the Columbia River gorge, USA Earth Surface Processes and Landforms 22: 457 472 Figure 10 (A) Cataract erosion in the Jokulsa´ a´ Fjollum system, northern Iceland (B) Erosional bedforms sculpted by fluvially driven abrasion in high resistant bedrock, Kangerlus suaq, West Greenland wash load within the flow A joă kulhlaup in Greenland with a discharge of 1100 m3 s and powers of 3000 W m transported sediment of up to 10 cm in diameter in suspension, whilst the November 1996 joă kulhlaup in Iceland with a peak discharge of 45–53 000 m3 s and a power of 40 000 W m was capable of moving 2-m-diameter boulders in suspension These modern joă kulhlaups only maintained such high stream powers locally and for a short duration Giant Quaternary outbursts in the Altai Mountains of Siberia and from glacial Lake Missoula have generated flood powers of 105–106 W m over larger areas As erosional and sediment transport capacities are so high during floods, large amounts of sediment can alter flow conditions Typical alterations include reduction of resistance to flow as sediment in transport blankets roughness elements, reductions of fall velocities of sediment in transport, and transition to upper flow regime bed conditions Sediment concentrations often exceed the values normally associated with fluidal flow conditions and become hyperconcentrated Sediment bulking during floods may transform floods into debris flows, which have their own strength Debris flow deposits directly reflect the entire flow thickness in contrast to water flood deposits, which only represent what was deposited at the bed of the flow Sediment concentrations within catastrophic floods may also increase towards the base of the flow, leading to hyperconcentrated near-bed conditions, often referred to as a traction carpet This may account for the deposition of units more typical of hyperconcentrated flow conditions by seemingly turbulent high-energy fluvial flows High sediment concentrations within floods can only be achieved if there is sufficient sediment readily available for entrainment Sediment availability to catastrophic floods varies, depending upon flood generation mechanism, lithology, and weathering environment Sediment bulking is usually most efficient within catchment headwaters, where precipitation inputs are high and stream and hillslope systems are