ABSTRACT
Channel boundary shear stress and stream power per unit boundary area are very useful concepts in assessing the role of rare, great floods in producing major geomorphic responses in fluvial systems. These variables were determined for the largest known flash floods in small drainage basins and for six historic dam-failure floods, predominantly in the United States. The largest values (shear stress exceeding 2x103 Nm−2 and power per unit area exceeding 1x104 Wm−2) occurred in narrow, deep flows in exceptionally steep bedrock streams. An optimum drainage area of approximately 10 to 50 km2 seems to be associated with the most powerful flash floods.
Even more powerful floods can be analyzed by paleohydraulic procedures. The Pleistocene Missoula floods and floods on the planet Mars approached ultimate limits of flood power defined by the onset of cavitation in fluvial flows. The geomorphic effectiveness of floods seems to be linked directly, not to their magnitude (discharge) or frequency (recurrence interval), but to the shear stress and stream power per unit boundary area relative to the resistance of the channel to erosion.
