9446 - Laserfiche WebLink

Home Browse Search

402 J, A. STANFORD ET At. As a consequence of reservoir storage of peak flows for flood control, navigation, irrigation and hydro- power production, baseflows increase substantially and often fluctuate so erratically that aquatic biota can- not survive in shallow, near-shore habitats. The varial zone, shown in Figure I, constricts owing to loss of peak flows and is depopulated by cyclic dewatering and rewatering that occurs on weekly, daily or even hourly schedules (Cushman, 1985; 10urdonnais and Hauer, 1993). In stark contrast, a natural river pulses water on to often expansive floodplains within a range of variation that allows a diversity of aquatic and riparian biota to exist in multiple successional stages in a complex array of habitats. Persistent shallow or slack water habitats are especially important for the survival of early life history stages of fishes that cannot survive in the strong currents of the channel thalweg. Storage of bedload in the reservoir and constant clear water flushing downstream artificially depletes gravel and finer sediments in the tailwaters, causing armour- ing of the bed with large cobble and boulder substratum (Simons, 1979), Large rocks eroded from the can- yon walls and coarse bedload from tributaries jam the channel and increase the size of rapids over time, because peak flows are insufficient to scour and transport the largest materials downstream (Dolan et aI., 1978). Channel constrictions and habitat simplification occur as the channel downcuts and riparian vegeta- tion invades to the top of the varial zone in aggraded reaches, owing to loss of upstream sediment supply and loss of scouring flood flows (Johnson, 1994; Church, 1995). The general conclusion is that regulation creates a discontinuum of environmental conditions and severs the connectivity of channel, groundwater, floodplain and upland components of the catchment ecosystem; habi- tats for riverine biota become spatially homogenous, limited to the permanently wetted portion of the channel thalweg that is dominated by conditions dictated by operations of upstream storage reservoirs (Figure 5), Indeed. serial construction of low-head dams has converted virtually all the mainstems of the largest rivers in USA, Europe, Sweden and Finland into shallow reservoir habitat that is neither truly lacustrine nor riverine, 2. Native biodiversity decreases and non-native species proliferate Native biodiversity almost always decreases after regulation (Minckley and Deacon, 1991; Ward and Stanford, 1991; Moyle and Leidy, 1992: Stevens et ai" in press), as conceptualized in Figure 5 compared with Figure 2, Vital core populations may be extirpated and satellite populations may become increasingly isolated by regulation schemes. Moreover, for anadromous species of fish mortality resulting from passage through dams and reservoirs on the mainstem may be selective for certain of the geographically diverse populations that use the mainstem as a common migratory pathway, thereby reducing biodiversity and increasing the probability of metapopulation extinction (Harrison and Quinn, 1989; Reiman and McIntyre, 1993), Altered temperature patterns and continual export of very fine organic matter and dissolved nutrients, coupled with simplification of the channel, stabilization of bottom substratum and loss of floodplain inun- dation. promotes environmental conditions to which native species are poorly adapted, opening opportu- nities for non-native plants and animals to establish robust populations (Stanford and Ward, 1986; Li et at.. 1987; Pflieger and Grace, 1987; Bain et ai" 1988; Shannon et al., 1994). In some cases one or a few native species are more abundant than they were before regulation (e.g. Poe et aI" 1991), But, the most pervasive result of habitat change produced by regulation is the proliferation of non-native species, Non-native inver- tebrates and fishes are consistently more abundant in regulated compared with unregulated river reaches (Li et aI" 1987; Bain et ai" 1988). Native riparian plants cannot exist on dewatered floodplains, which opens niches for exotic, dryland plants, Moreover, owing to loss of scouring flows, exotic and some native riparian plants choke the periodically saturated area of the shoreline above the narrowed varial zone and exotic hydrophytes usually invade and quickly dominate shallow water habitats (Decamps and Tabacchi, 1994; Johnson, 1994). Explicit reasons for non-native proliferation in regulated rivers vary, but, in general, non-natives are simply better competitors in the homogenous habitats of regulated rivers, plus the fact that a wide array of non-natives have been purposely introduced into regulated rivers, ], Biophysical conditions reset pr.edictably in relation to influences of tributaries and as distance downstream from the dam increases, The serial discontinuity concept (SDC) (Ward and Stanford, 1983, 1985b) explicitly acknowledges the inherent connectivity of the river continuum and predicts that the conditions described above will ameliorate downstream as a natural consequence of the biophysical energetics of rivers, The spatial rate at which reset occurs and its manifestation relative to position within the river continuum (Figure 5) is related to the limnological attributes (depth, volume, water retention time, trophic state) of