9446 - Laserfiche WebLink

Home Browse Search

396 J, A. STANFORD ET AL. though interim flow dynamics gradually and subtly reconfigure instream structures and features (Schumm and Lichty, 1956). For example, the channel of the Snake River upstream from Hells Canyon, Idaho, USA, persists as an incised gravel bed channel containing a chain of elevated, mid-channel islands that have not been overtopped since the cataclysmic glacial flood that formed them receded over 8000 years ago (Connor, 1993), Other river channels with a greater sediment supply and frequent overbank flooding are constantly shifting, braiding or meandering on the valley bottom from year to year as the channel fills with material in one place causing the flow pathway to avulse and downcut (Best and Bristow, 1993). All rivers are fundamentally alluvial in nature as a consequence of cut and fill alluviation mediated by flooding. Most rivers have deeply bedded and expansive floodplains interspersed between constrained and often incised reaches (canyons), where the bedrock may be very near or exposed on the stream bottom. Hence, river ecosystems have three important spatial dimensions that are temporally dynamic (Figure 1), The longitudinal (upstream-downstream) dimension is described in detail in the ecological literature, includ- ing the occurrence and ecological significance (discussed below) of streamside (riparian) vegetation and asso- ciated faunal assemblages in the surficial transition zone from riverine to terrestrial environments, However, critically important lateral and vertical attributes and connections are often overlooked or ignored, Owing to the high porosity of the bed sediments in gravel bed rivers, river water penetrates the bottom and saturates the alluvial bedding of the channel and floodplain down to the less porous bedrock, thereby creating complex groundwater (hyporheic) habitats, As the valley constricts, or owing to changes in the local bedrock geome- try, the water table may intersect the surface creating floodplain (riparian) wetlands; permanent spring- brooks and ponds in up-welling areas may be observed at the downstream end of flood plains, Indeed, a pro- minent feature of alluvial rivers is sequential down- and up-welling of river water into and out of the bed sediments, which interacts with overland flooding to create complex habitat mosaics on the floodplain sur- face. The floodplain, with its hyporheic and riparian habitats, is therefore the transition zone or ecotone link- ing aquatic and terrestrial components of the river ecosystem above and below ground level. Also, groundwater flowing from uplands may mix with river water flowing within the hyporheic zone, creating yet another important lateral ecotone, These lateral and vertical transition zones alternate in juxtaposition with the channel from headwaters to mouth, forming hyporheic and riparian corridors (Naiman et ai" 1988; Stanford and Ward, 1993; Ward and Weins, in press), The mosaic of channel and floodplain structures creates a constantly changing habitat template (sensu Southwood, 1977, 1978) for a myriad of plants and animals that comprise riverine food-webs, Resources needed by particular life history stages of organisms have discrete or 'patchy' distributions within this het- erogeneous landscape, As flows change, not only does the ability of the river to move substratum change, but the way in which water moves around and/or over instream structures, such as boulders and gravel bars, also changes. Hence, biota must adapt to resources arrayed as dynamic patches that manifest from local (e,g., a single rock on a single riffle in a particular river reach: Townsend, 1989) to catchment scale, Moreover, as biota attempt to find and utilize these patches to sustain growth and reproduction over the long term, they must also adapt to the physical forces of water movement (Statzner et ai" 1988). Therefore, biota are often arrayed in precise locations within the river channel and along the river continuum (Poff and Allan, 1995), For example, a large, behaviourally dominant trout may occupy the optimal position within an eddy for capturing drifting insects; if that fish is removed, the next fish in the pecking order will move into that foraging location (Bachman, 1983), Salmonids are generally confined to the colder, rocky reaches (rhithron) of the stream continuum and are replaced by warm water species (e,g, cyprinids, ictalurids) in the slow mov- ing, sandy and often turbid reaches downstream (potamon) (lilies, 1956; Illies and Botosaneanu, 1963). The river continuum is a complex, dynamic gradient of habitat types from headwaters to oceanic conflu- ence, and flora and fauna are usually distributed rather predictably along that gradient (Figure 2) according to the requirements specified by each stage in their life cycle (Vannote et a!., 1980). Each species or unique life history type (stock or population) is most abundant where the resources they require are most abundant and/ or most efficiently obtained, They will be present (locally adapted) wherever they can maintain a positive energy balance, that is, they have enough resources to sustain growth and reproduction and thereby sustain the presence of the species or stock in the river food-web at that location (Hall et al., 1992), For some species, a positive life history energy balance can be maintained without much movement and suites of organisms a n c. S a ~ n e 1< d a I b p I; p t r' g s d f. n t e S Ii b (J a a tl o h (J P b tl fJ S' 1 n b t. c. \\ S1