In a geomorphological system, the magnitude of an input—object or process—required to cross a threshold. Thus, critical shear stress in an overland flow is that point where the shear stress is great enough for incision to occur; or within a channel, for mobilizing sediment (entrainment). This point occurs at the critical distance from the drainage divide, where the flow reaches the critical depth. When the velocity of a flow is less than critical, it is considered a subcritical flow. Subcritical flows are characterized by a water surface that is smooth and on which a wave produced at the water surface is quickly damped out; subcritical flows are also termed ‘tranquil’ (K. J. Gregory and A. S. Goudie 2011). Supercritical flows are highly erosive, moving rapidly and efficiently through the channel, often over-shooting bends. Turbulent mixing is less intense (M. Kay 1998).

As quantified by Bagnold (1980, Proc. R. Soc. Lond. A 372, 453), critical stream power is the amount of stream power needed to transport the average sediment load. Ferguson (2005) Geomorph. 70, 33 amends Bagnold’s equation, and Parker et al. (2011) Geomorph. 126, 51–65 present an interesting review on critical stream power. This paper is critiqued and supplemented by Camenen (2012) Geomorph. 139–40, 34. Gob et al. (2010) ESPL 35 investigate the relationships between sediment size and critical stream power.