THE RANGE OF LANDSCAPE DISTURBING PROCESSES

Disturbances to landscape forms and processes range in extent and intensity from intermittent alterations of the surface condition with intervening periods of ‘recovery’ to radical reshaping of landforms that alter the path of landscape evolution and regional sediment distribution. The former may be thought of as composing the ‘normal’ frequency distribution of geomorphic events, even though geomorphologists have only recently begun to formalize and quantify the role that such discrete events play in landscapes. Examples include the intensification of runoff and erosion after wildfire, during extreme storms, or during fluctuations in the storm climate. Forensic, and even experimental, studies of such events are slowly yielding information on their mechanics, increasing the capacity to model their behavior, and to predict occurrence and cumulative role in landscape functioning over the long term. More dramatic and extraordinary changes are triggered by collapse processes, such as cone failure on volcanoes or massive rockslide-avalanches in active orogens, or by drainage diversions associated with knickpoint recession or ice-sheet drainage. These often result in replacement of one type of landscape with another, rather than ‘recovery’. Or the stabilization requires periods of time usually associated with landscape evolution rather than reversion to some normal form and chronic tempo. The difficulty of observing or even interpreting such processes and of parameterizing such important physical controls as shear strength or resistance to bedrock incision severely limits our ability to assess the risk of such catastrophes occurring, their magnitude and extent. Even in these cases, however, some progress is being made in interpreting processes quantiatively, and measurements are beginning to accumulate. An important task for geomorphologists is to organize a coherent program of observations, material property characterization, and theory development to improve our capacity for prediction of the full spectrum of landscape disturbing processes.