The Spatial Organization of River Systems

'Although the river and hill-side waste do not resemble each other at first sight, they are only 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.' --Davis 1899, 495. Complex geomorphic, hydrologic, and biotic feedbacks operating over multiple spatial and temporal scales generate remarkable diversity in riverine ecosystems (Poole 2002). Although spatial and temporal contingencies ensure that the biophysical makeup of every system is unique (Hynes 1975; Townsend 1996), many spatial relationships and patterns are repeated from system to system (Benda et al. 2004). The ability to unravel local influences from systematic relationships provides a critical basis for informed decision-making in river management. The spatial arrangement of fluvial systems has been characterized in terms of broad-scale patterns. For example, Schumm (1977) described a general downstream transition from a zone of sediment production in the headwater region, through a zone dominated by sediment transport to an accumulation zone along the lower river reaches. More recently, the "continuum" perspective has been elaborated, incorporating notions of hierarchy and the effect of scale in all four dimensions (longitudinal, lateral, vertical, and temporal). For example, the "fluvial hydrosystem" proposed by Petts and Amoros (1996) seeks to describe and explain continua of forms and processes in both time and space, including lateral and vertical patterns alongside the longitudinal patterns described by Schumm, among others. Additionally, the biological implications of the physical gradient have been articulated, notably by Vannote et al. (1980) in their "River Continuum Concept."