A critical condition or tipping point that, when exceeded, triggers a significant change in landform processes or morphology. Geomorphic thresholds explain why landscapes can undergo abrupt changes in response to gradual forcing.
A geomorphic threshold is a critical limit of a controlling variable (such as slope, shear stress, sediment supply, or discharge) beyond which a significant and often irreversible change in landform processes or morphology occurs. The concept, formalized by Stanley Schumm in the 1970s, distinguishes between extrinsic thresholds triggered by external changes (climate, tectonics, land use) and intrinsic thresholds crossed due to internal system evolution without external forcing. For example, a meandering river may abruptly cut off a meander bend when sinuosity reaches a critical value, or a hillslope may fail catastrophically when pore water pressure exceeds a threshold during a storm. Channel pattern thresholds describe the conditions under which a river transitions from meandering to braided or vice versa, governed by the relationship between slope, discharge, and grain size. Bank stability thresholds determine when a streambank transitions from stable to actively eroding, often related to critical bank height, soil cohesion, and vegetation root density. Understanding thresholds is essential for predicting responses to environmental change, as gradual changes in climate or land use may produce little visible landscape response until a threshold is crossed, at which point change can be dramatic and rapid.