ABSTRACT
A geomorphic threshold is a point or period of time that separates different modes of operation within part of a landscape system. The threshold conceptual framework includes within it the concept of equilibrium, because equilibrium occurs during time periods of no net change in landforms. A threshold may be regarded as a balance between opposing tendencies. Ratios are a convenient way of describing thresholds if numerator and denominator describe the opposing tendencies. A threshold that describes changes in dominant geomorphic processes is hillslope runoff threshold. The threshold of critical power in streams occupies a key position in complex interactions between the hillslope and stream subsystems, and it is affected by feedback mechanisms and complex responses operating in either subsystem. In conclusion, geomorphic thresholds may be defined in terms of ratios, the numerators and denominators of which describe opposing tendencies, and which may be simple or complex depending on the needs of the investigator or the complexity of the real world.
