Thresholds and lags in geomorphologic changes

Summary The concept of thresholds in geomorphology is developed with reference to several different systems. Thresholds are classified as transitive or intransitive, according to whether a change of geomorphic state or process is caused by a persistent change of external boundary conditions or by a relatively short‐lived perturbation of boundary conditions. Lags enter the system when a geomorphic change persists for some time after disruption of boundary conditions. The concepts are first developed with reference to a simple physical system where water changes state from water to ice, or vice versa. Geomorphic examples discussed range from small‐scale rapid changes on sandy surf beaches, through larger‐scale fluvial examples, to the case of semi‐arid dune systems. Use of the concept focuses attention on climatic questions such as geomorphic change: climatic change or climatic perturbation?