HIERARCHIES AND SPATIAL SCALE IN PROCESS GEOMORPHOLOGY - A REVIEW

The form and functioning of a geomorphic system is the end product of a set of interacting processes which operate at a large variety of spatial and temporal scales. An appreciation of the nested, hierarchical structure of geomorphic systems offers insight into the effect of scale in process geomorphology. A literature review led to the formulation of a series of propositions concerning the spatial scale problem in process geomorphology. These propositions are presented in a theoretical framework indicating their association. Central to the spatial scale problem in process geomorphology is that a geomorphic system must be viewed in its complex, hierarchical context: every geomorphic system consits of an array of ever smaller, lower-level systems, and is at the same time part of a sequence of ever larger, higher-level system. In a geomorphic system there exists a similarity between the scales of process and form. Furthermore, within a geomorphic system there is a similarity between spatial and temporal scales so that the timespan relevant to present system behaviour and morphology increases with system size. In a geomorphic system, differences occur in the sensitivity of morphology to changing process conditions. Depending on the time elapsed since a change in process conditions, morphology in a geomorphic system may be either polygenetic, when only partial adjustment of the morphology (i.e. only at smaller scales) has occurred, or characteristic of the process domain, when all aspects of morphology (i.e. at all scales) are adjusted to present process conditions. In the latter case, dominance of landscape evolution by high-frequency, low-magnitude processes results in a smooth landscape having small-scale irregularities, whereas dominance by low-frequency, high-magnitude processes produces an irregular landscape having large-scale irregularities. When spatial scale is held constant, the model of evolution of a geomorphic system varies with the timescale of interest. Conversely, when temporal scale is held constant, the model of evolution varies with the spatial scale of interest. In both cases, the canon (fixed rules) and options of a geomorphic system vary with the scale, temporal or spatial, of interest. The linkages between spatial and temporal scale and between the scales of form and process of geomorphic systems lead to four additional points. First, whether or not an event is catastrophic depends on the hierarchical level of the geomorphic system. Second, because of increased travel time and a larger variety of possible pathways, differences between the temporal patterns and material properties of input and output increase with the hierarchical level and scale of a geomorphic system. Third, frequent, small-scale processes may cause infrequent, large-scale events when viewed at the same hierarchical level. Fourth and last, differences between geomorphic systems at the same level are controlled by variables varying over distances equal to or smaller than the distance between the system, but equal to or larger than the spatial dimensions of the systems.