Changes, perturbations, and responses in geomorphic systems

In recent decades views of change, disturbance, response, and recovery in geomorphology have expanded considerably. Conceptual frameworks emphasizing single-path, single-outcome trajectories of change have been supplemented - not replaced - by multi-path, multi-outcome perspectives. Geomorphology has also seen a transition from the idea of normative standards such as characteristic, (steady-state) equilibrium, zonal, and mature forms to the recognition that some systems may have multiple potential characteristic or equilibrium forms - and that some may have no particular normative state at all. These trends are not presented as a replacement of outmoded ideas, but rather as a broadening of approaches. The single-path single-outcome frameworks can generally be viewed as special cases of the broader pluralistic analytical structures. In this context, two perspectives - an adaptation of White's hazards matrix, and the landscape sensitivity concept-are suggested which lend themselves to studies of recent and contemporary changes in earth surface systems. These perspectives can be synthesized into a framework for the assessment of geomorphic changes and responses based on the 'four Rs': response (reaction and relaxation times), resistance (relative to the drivers of change), resilience (recovery ability, based on dynamical stability), and recursion (positive and/or negative feedbacks).