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