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<br />v: '.~ ......,. <br />:.~.~.~.P:~)f <br /> <br />396 <br /> <br />!JJ~3'J3 <br /> <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 <br /> <br />~::. ~'. ...' .'..-.: <br />~'~f;}/1~~~~:~ <br /> <br />?? ~~:::'-',~,,;:':', <br /> <br />-. <~.: <br /> <br />...... '.'. <br />..... . . <br /> <br />.~....~ .' <br /> <br /><, . <br />\t~.~;;:(:..:: <br /> <br />~-.:.::'.''';/'..i.:-'' <br /> <br />..... '. 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