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398 ], A, STANFORD ET AL. Catchment Geology, Climate, Land-use Natural.Cultural Setting t Discharge Channel-Floodplain Geometry Temperature Substratum Nutrients Pollutants Harvest Primary Controlling Variables t t Feedbacks (t,g., ""tritlll spiraling) Riverine Foodweb Biotic Inreractions Figure 3. Primary controlling variables and biophysical interactions of river ecosystems continuum in direct response to variations in the strength of interconnections between channel, ground- water, floodplain and upland elements of the catchment (Ward and Stanford, 1995a). In our view the primary variables driving the distribution and abundance of animals and plants in flood prone rivers are usually abiotic and primarily determined by the geological and climatic setting of the catch- ment basin (Figure 3), Biotic interactions (e,g. competition, predation, parasitism), while they obviously con- tinually occur within food-webs in all habitats, may become progressively more important and apparent as the time between abiotic disturbances increases, and hence are most pronounced in spring-brooks and lake outlet streams where abiotic drivers are comparatively non-variable (Ward and Stanford, 1983b; McAuliffe, 1983, 1984; Reice, 1994). All big rivers that are not influenced by large on-channel lakes are naturally flood prone, and ultimately biophysical structure is controlled by the inexorable, but highly dynamic, scouring process of cut and fill alluviation. Environmental heterogeneity (complexity) maximizes in the alluvial (aggraded) reaches of the river con- tinuum, Owing to the energetics of materials transport through large catchment basins from high elevation to sea level, alluvial reaches are arrayed along the stream continuum between canyon segments like beads on a string (Figure 2). The hyporheic and riparian corridor is expansive on alluvial reaches and seasonal tem- perature patterns vary within the wide array of aquatic habitats that exist laterally from the channel across the floodplain (Ward, 1984). Large floodplains appear to function as centres of biophysical organization within the river continuum (sensu Regier et ai., 1989). They are likely to be 'hot spots' of biodiversity and bioproduction that are structurally and functionally linked by the river corridor (Copp, 1989; Gregory et ai" 1991; Zwick, 1992; Stanford and Ward, 1993; Ward and Stanford, 1995a,b). Indeed, intermountain and piedmont valley floodplains world-wide are characterized by nutrient-rich floodplain soils and diverse and productive backwater and mainstem fisheries (Welcomme, 1979; Davies and Walker, 1986; Lowe- McConnell, 1987; Sparks et ai" 1990; Junk and Piedade, 1994; Welcomme, 1995), These reaches are also