In a system, the ‘tipping point’ which brings about lasting, non-reversible changes, after which the system must develop a new equilibrium condition, adjusted to the characteristics of the altered controlling factors. If, thereafter, the system reverts to the original, balanced condition, it has not crossed a threshold. External thresholds are crossed as a result of a change in one of the external variables. Schumm (1979) TIBG 4, 4 notes that the definition has been broadened to include abrupt landform change as a result of progressive change of external controls. Ritter et al. (1999) Geomorph. 29, 3 argue that ‘if thresholds are accepted as boundaries between equilibrium and disequilibrium, they are required to be time-dependent phenomena. Thus, a steady-time cut and fill of a stream channel during the passage of a flood event is a different type of threshold crossing than the sustained, graded-time entrenchment of a channel due to distinct climate change.’

Most work on geomorphic thresholds has been on fluvial processes (see, for example, Ritter et al. (1999) Geomorph. 29, 3 and Tucker and Stokes (2004) Amer. Geophys. Union, Abstr H53E) and mass movement (see Caine (1980) Geograf. Annal. A 62, 1–2 and Friend (2000) Prof. Geogr. 52, 2). ‘If the threshold conditions can be recognized, not only will different explanations for some landforms emerge, also the ability to identify incipiently unstable landforms and to predict their change will be of value to land managers and engineers’ (Schumm, op. cit.).