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<br />396
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<br />J. A. STANFORD ET AL.
<br />
<br />though interim flow dynamics gradually and subtly reconfigure instream structures and features (Schumm
<br />and Lichty, 1956). For example, the channel of the Snake River upstream from Hells Canyon, Idaho,
<br />USA, persists as an incised gravelbed channel containing a chain of elevated, mid-channel islands that
<br />have not been overtopped since the cataclysmic glacial flood that formed them receded over 8000 years
<br />ago (Connor, 1993), Other river channels with a greater sediment supply and frequent overbank flooding
<br />are constantly shifting, braiding or meandering on the valley bottom from year to year as the channel fills
<br />with material in one place causing the flow pathway to avulse and downcut (Best and Bristow, 1993),
<br />All rivers are fundamentally alluvial in nature as a consequence of cut and fill alluviation mediated by
<br />flooding, Most rivers have deeply bedded and expansive floodplains interspersed between constrained and
<br />often incised reaches (canyons), where the bedrock may be very near or exposed on the stream bottom,
<br />Hence, river ecosystems have three important spatial dimensions that are temporally dynamic (Figure I),
<br />The longitudinal (upstream-downstream) dimension is described in detail in the ecological literature, includ-
<br />ing the occurrence and ecological significance (discussed below) of streamside (riparian) vegetation and asso-
<br />ciated faunal assemblages in the surficial transition zone from riverine to terrestrial environments. However,
<br />critically important lateral and vertical attributes and connections are often overlooked or ignored, Owing to
<br />the high porosity of the bed sediments in gravel bed rivers, river water penetrates the bottom and saturates
<br />the alluvial bedding of the channel and floodplain down to the less porous bedrock, thereby creating complex
<br />groundwater (hyporheic) habitats. As the valley constricts, or owing to changes in the local bedrock geome-
<br />try, the water table may intersect the surface creating floodplain (riparian) wetlands; permanent spring-
<br />brooks and ponds in up-welling areas may be observed at the downstream end of flood plains, Indeed, a pro-
<br />minent feature of alluvial rivers is sequential down- and up-welling of river water into and out of the bed
<br />sediments, which interacts with overland flooding to create complex habitat mosaics on the floodplain sur-
<br />face, The floodplain, with its hyporheic and riparian habitats, is therefore the transition zone or ecotone link-
<br />ing aquatic and terrestrial components of the river ecosystem above and below ground level. Also,
<br />groundwater flowing from uplands may mix with river water flowing within the hyporheic zone, creating
<br />yet another important lateral ecotone. These lateral and vertical transition zones alternate in juxtaposition
<br />with the channel from headwaters to mouth, forming hyporheic and riparian corridors (Naiman el al., 1988;
<br />, Stanford and Ward, 1993; Ward and Weins, in press),
<br />The mosaic of channel and floodplain structures creates a constantly changing habitat template (sensu
<br />Southwood, 1977, 1978) for a myriad of plants and animals that comprise riverine food-webs, Resources
<br />needed by particular life history stages of organisms have discrete or 'patchy' distributions within this het-
<br />erogeneous landscape, As flows change, not only does the ability of the river to move substratum change, but
<br />the way in which water moves around and/or over in stream structures, such as boulders and gravel bars, also
<br />changes, Hence, biota must adapt to resources arrayed as dynamic patches that manifest from local (e,g" a
<br />single rock on a single riffle in a particular river reach: Townsend, 1989) to catchment scale, Moreover, as
<br />biota attempt to find and utilize these patches to sustain growth and reproduction over the long term,
<br />they must also adapt to the physical forces of water movement (Statzner el ai" 1988). Therefore, biota
<br />are often arrayed in precise locations within the river channel and along the river continuum (Poff and Allan,
<br />1995), For example, a large, behaviourally dominant trout may occupy the optimal position within an eddy
<br />for capturing drifting insects; if that fish is removed, the next fish in the pecking order will move into that
<br />foraging location (Bachman, 1983), Salmonids are generally confined to the colder, rocky reaches (rhithron)
<br />of the stream continuum and are replaced by warm water species (e,g, cyprinids, ictalurids) in the slow mov-
<br />ing, sandy and often turbid reaches downstream (potamon) (IlIies, 1956; IlIies and Botosaneanu, 1963).
<br />The river continuum is a complex, dynamic gradient of habitat types from headwaters to oceanic conflu-
<br />ence, and flora and fauna are usually distributed rather predictably along that gradient (Figure 2) according
<br />to the requirements specified by each stage in their life cycle (Vannote el ai" 1980). Each species or unique life
<br />history type (stock or population) is most abundant where the resources they require are most abundant and/
<br />or most efficiently obtained, They will be present (locally adapted) wherever they can maintain a positive
<br />energy balance, that is, they have enough resources to sustain growth and reproduction and thereby sustain
<br />the presence of the species or stock in the river food-web at that location (Hall el ai" 1992), For some species,
<br />a positive life history energy balance can be maintained without much movement and suites of organisms
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