The Drainage Basin as the Fundamental Geomorphic Unit

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Abstract
The need for the precise description of the geometry of landforms, particularly those of dominantly fluvial erosive origin, has been a recurring theme in geomorphology, and one of the most important aspects of this has been the search for the basic areal unit within which these data could be collected, organized, and analysed. The conceptions of the nature of these units have been very much a product of the broader methodological approaches to geography and earth science in general, and to geomorphology in particular, and can be grouped into three categories. The first important approach (Fenneman, 1914) sprang from the interest of geographers a half century ago in regional delimitation. The physiographic regions so delimited for the United States were based largely upon considerations of structural geology (e.g. the Ridge and Valley province), although certain gross morphometric attributes, notably relief and degree of dissection, were also used. The modern equivalent of this approach is provided by the terrain analogues of the U.S. Corps of Engineers, who used four terrain factors (characteristic slope, characteristic relief, occurrence of steep slopes greater than 26·5°, and the characteristic plan profile involving the ‘peakedness’, areal extent, elongation, and orientation of topographic highs) to divide up the gross landscape of a region into component landscapes in a simple taxonomic manner (fig. 2.II.1). In contrast with this basis, the second approach was concerned to identify ‘the physiographic atoms out of which the matter of regions is built’ (Wooldridge, 1932, p. 33). These ‘atoms’, however, were defined as the facets of ‘flats’ and ‘slopes’ forming the intersecting surfaces characteristic of polycyclic landscapes (Wooldridge, 1932, pp. 31-3), and, although this doctrinaire definition has been relaxed to include segments of smoothly curved surface (Savigear, 1965) and to allow the grouping of facets into landscape patterns, such as a ‘mature river valley’ (Beckett and Webster, 1962) (fig. 2.II.1), the genetic overtones and subjective character of this morphometric division limits its usefulness (Gregory and Brown, 1966). The third basis for morphometric division results from the obvious unitary features both of geometry and process exhibited by the erosional drainage basin, as recognized long ago by Playfair (Chorley, Dunn and Beckinsale, 1964, pp. 61-3) and by Davis [1899], who wrote: 78Although the river and the hill-side waste sheet do not resemble each other at first sight, they are only the extreme members of a continuous series, and when this generalization is appreciated, one may fairly extend the ‘river’ all over its basin and up to its very divides. Ordinarily treated, the river is like the veins of a leaf; broadly viewed, it is like the entire leaf. Fig. 2.II.1 Landscape units and geomorphic regions. Above: Example of a component landscape defined in terms of four terrain factors, and the relation between a component (top) and a gross landscape (From Van Lopik and Kolb, 1959). Below: The pattern of a mature river valley developed by the Upper Thames on the Oxford Clay, illustrating the facets and their relation to each other in the landscape (From Beckett and Webster, 1962). Facets of river valley and clay High gravel terrace. Spring line. Clay crest. Clay slope. Clay footslope. Unbedded glacial drift. River and banks. Local bottomland. Flood plain alluvium. Old alluvium, not flooded. Facets of scarplands bounding the river valley pattern 12 (II) Scarp slope. 13 Dipslope. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003170181/c8e302ab-b926-4a23-8899-bcb1f3c32b5f/content/fig26.tif"/>

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