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<br />
<br />RESTORATION OF REGULATED RIVERS
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<br />appear to occur in zones along the river continuum; others must move long distances in search of resources
<br />needed for each life stage, sometimes involving migrations into the lakes (e,g, adfluvial bull charr, Salvelinus
<br />confiuentus) or the ocean (e,g, anadromous salmon and trout: Onchorynchus spp,; Sa/mo sa/or and S, Irutta;
<br />Sa/velinus spp,),
<br />Widely dispersed species often exist as metapopulations because local populations are linked by dispersal
<br />and gene flow into larger regional populations that may encompass the entire catchment (Hanski, 1991;
<br />Hanski and Gilpin, 1991), For example, metapopulation structure is thought to be particularly evident in
<br />many salmonid populations (Reisenbichler el 0/., 1992; .Rieman and Mcintyre, 1993) and most likely influ-
<br />ences the probability of persistence for a species (Stacy and Taper, 1992). Metapopulation linkages allow for
<br />local extinction of populations, which can be re-established via colonization from adjacent populations (Lei-
<br />der, 1989; Milner and Bailey, 1989), The spatial arrangement of large- and small-scale habitat features within
<br />a catchment may serve as a template for metapopulation organization of fishes (Schlosser and Angermeier,
<br />1995), The mosaic of floodplain reaches and constrained segments (Figure 2) within the mains tern and tri-
<br />butaries influences size, spatial distribution and proximity of local spawning populations. Proximity of
<br />populations and favourability of connecting habitats can affect exchange of individuals between local popu-
<br />lations (Reiman and McIntyre, 1993; Li ec 0/" 1995; Schlosser and Angermeier, 1995) and thus influence
<br />potential for recolonization of habitats where local extinction has occurred,
<br />Since most river fauna are ectotherms, growth and reproduction is also vitally influenced by river tempera-
<br />ture. Most organisms adapted to the cold climes of the headwater reaches simply cannot survive in warmer
<br />reaches downstream, and vice versa. Indeed, species found in a particular thermal environment in one river
<br />generally will be found in very similar environments in other rivers within the geographical range of that
<br />species, if all other resource needs are also met. Because growth of ectotherms is strictly temperature depen-
<br />dent, temperature is a critical habitat attribute (Ward, 1985; Hall eC 0/" 1992), Stream insects and fish witl be
<br />found in areas of the stream where their thermal needs are met and substratum, food and other resources are
<br />marginal, but rarely the inverse, at least for individuals that ultimately reproduce successfully, This is
<br />because of the basic thermal energetics of growth and the fact that many life history stages, such as insect
<br />emergence (ecdysis) and fish spawning are initiated by precise temperature cues (Brett, 1971; Vannote and
<br />Sweeney, 1980; Ward and Stanford, 1982), In addition, because few riverine organisms have highly specia-
<br />lized food requirements, food limitation may be less prevalent than thermal limitation most of the time.
<br />For plants of the river food-web, availability of light and nutrients is crucial. In headwater streams shaded
<br />by riparian plants, decomposition of allochthonous (terrestrially derived) coarse particulate organic matter
<br />(leaves, grasses) usually drives instream bioproduction (Cummins el 0/,,1984,1989). Plant growth nutrients
<br />are released into transport by the decomposition of particulate organic matter entrained on the bottom, and
<br />are utilized by aquatic plants in better light environments downstream where the stream channel is wider and
<br />the riparian canopy opens, Of course, nutrients and other dissolved solids are also derived from dissolution
<br />of the bedrock and other geochemical reactions, Indeed, streams with high alkalinity from limestone disso-
<br />lution generally are more productive than streams draining more inert bedrocks, such as granite massifs
<br />(Kruger el 0/" 1983; Waters ec a/.. 1990). Dissolved solids that are required for growth by algae and macro-
<br />phytes spiral downstream, alternatively retained and released into transport by the river food-webs (New-
<br />bold el 0/" 1981, 1982), Conditions may shift back to heterotrophy in turbid, slow moving reaches near
<br />the river mouth as a consequence of planktonic microbial decomposition of organic matter transported
<br />from upstream reaches, reduced light reaching the bottom owing to deep and often turbid water and shifting
<br />substratum (Vannote and Sweeney, 1980; Minshall el 0/" 1983; Naiman et aI., 1987).
<br />All of this underscores the complex linkages between the spatial dimensions of river ecosystems (Figure I),
<br />These interactive components and attributes are repeated throughout the river course, from headwaters to
<br />mouth. Floods maintain channel and floodplain habitals and pulse nutrient-enriched waters laterally into
<br />backwaters and on to floodplains, as well as downstream into the estuary. Because it is a continual habi-
<br />tat-forming process, river biota are adapted to frequency and duration of flood pulses (Copp, 1989; Junk
<br />el 01" 1989), Rivers that flood frequently (annually or more often) maintain different species and food-
<br />webs than systems that are more ecologically benign by rarely or never experiencing scouring floods (e,g,
<br />spring-brooks and lake outlet streams). Food-webs are complex and change predictably along the stream
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