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ecosystem dimension (i.e., catchment size) is logically determined by the question being examined or the resource being managed. The time frame encompassing the research question or management problem is of course also important. In geologic time, as a result of orogeny and erosion, watersheds were bisected and catchments reorganized, which clearly had enormous zoogeographic consequences (Stanford and Ward 1986). In a much shorter time frame, engineering projects artificially connected catchments via trans-watershed diversions of rivers in many areas (Stanford and Ward 1979; Davies and Walker 1986), allowing differently adapted organisms to comingle (Guiver 1976) or greatly accelerating immigration of nonnative biota introduced by other means (Stanford and Ward 1986, Mooney and Drake 1986). Given that catchments may be referred to as ecosystems and that the ecosystem is dynamic in time and space as well as in its relation to environmental problem solving, it is fundamentally important to recognize the major structural features and dimensions of river ecosystems (Figure 1). Ecologists have appreciated for many years the importance of microhabitats encompassed by the run-riffle-pool sequence as influential on the distribution and abundance of biota within the river channel. Zonation of biota within the longitudinal continuum has long been recognized as a fundamental feature of the lotic environment (Hynes 1970), although mechanisms explaning specific distribution patterns often remain contentious (Alstad 1982, 1986, Thorp et al. 1986). Within the last decade, the connection between riparian zones, including the surficial floodplain dynamics, and ecological structure and function has been clearly demonsrated (see reviews in Dodge 1989, Gregory et al. in press). Microbial transformation and transport of solutes in groundwaters has been shown to be an important source of plant growth nutrients for channel biotopes in streams (Stanford and Ward 1988, Ford and Naiman 1989, Dahm et al. 1990, Stream Solute Workshop 1990, Grimm et al. in press, Valett et al. in press); and, penetration of groundwaters (i.e., the hyporheic zone, Figure 1) by amphibiotic stream biota has been documented (Schwoerbel 1967, Stanford and Gaufin 1974, Williams and Hynes 1974, Bretschko 1981, Danielopol 1984, Pugsley and Hynes 1986, Stanford and Ward 1988). But, the presence of large-scale hyporheic zones and the critical importance of groundwater - surface water interchange as a major landscape feature of catchments has only recently been demonstrated (Stanford and Ward 1988, Danielopol 1989, Gibert et al. 1990). River floodplains are often, if not always, penetrated by interstitial, subterranean flow (Figure 2). Water penetrates (downwells) at the upstream end of the floodplain, 5