Laserfiche WebLink
<br /> <br />406 <br /> <br />J, A. STANFORD ET AL. <br /> <br />most of the sediment is moved on the rising limb of the hydrograph. In years of average catchment water <br />yield, a modest peak flow can be generated, while also elevating baseflow to accomplish the purposes for <br />which the dams were built. In dry years, peak flows can be minimal or non-existent (Figure 4). The strategy <br />is simply to lower the baseflow a little to build peaks in relation to catchment runoff. In all years it is essential <br />to prevent massive dewatering of the varial zone during baseflow periods; explicitly, this means that daily <br />changes in flow (ramping rates) should not exceed the range of variation that occurred before regulation <br />(Figure 4). <br />Operators of hydroelectric dams may object to reregulation recommendations as depicted in Figure 6, <br />because of the potential constraints on generation of peak power and concern often exists that the legal <br />requirements for electrical load control cannot be met. On the contrary, load control can be performed with- <br />out ramping flow beyond the range of variation observed in pre-regulation periods (Jourdonnais and Hauer, <br />1993) Loss of peaking is problematic. However, most large dams are part of large electrical marketing grids <br />and alternatives to hydropower peaking exist today that were not available a decade ago. For example, mod- <br />ern fuel turbines are very effective peaking units, natural gas reserves are large world-wide and local utilities <br />are finding gas-powered turbines to be preferred alternatives to the purchase of regional hydropower. The <br />need for hydropower peaking may wane in the next decade, particularly if the cost of, and public desire <br />for, downstream environmental mitigation increases. <br /> <br />Maximize passage efficiency to allow recovery of metapopulations <br />Maintenance of instream and floodplain habitats by restoration of peak flows and revitalization of shallow <br />and slack water habitats by stabilization of baseflows will increase ecological connectivitiy along all three <br />spatial dimensions, However, in the absence of dam and reservoir removal, optimization of dam and reser- <br />voir passage efficiency for biota is required to reconnect the longitudinal dimension (Figure 2), Mechanisms <br />for significantly reducing mortality of juvenile and adult fish as they pass hydroelectric dams include flowing <br />ladders, travelling screens, surface-release attractors and other bypass devices (Gore, 1985), The main point <br />is that dams with no, or very inefficient, bypass systems maintain the discontinuum and isolate populations, <br />thereby limiting the gene flow that may be needed to restore and maintain meta populations, On the other <br />hand, the presence of impassable dams in some cases has prevented immigration of non~native species <br />into native food-webs and effectively isolated viable native populations (Stanford and Hauer, 1992) <br />In many large, regulated rivers, viable populations of native species remain in segments isolated by dams. <br />Restoration of flow and temperature seasonality and reconnection of these refugia may restore critically <br />important core areas, revitalize metapopulation structure and rapidly lead to recovery of genetically and <br />numerically depressed populations (Sedell et al., 1990; DeVore et al., 1995), Indeed, a primary strategy of <br />large river restoration should be to identify, stabilize, restore and reconnect river segments to core areas con- <br />taining native food-webs, The expectation is that native species will recolonize restored habitat from the core <br />area (Lichatowich et al., 1995; Frissell and Bayles, 1996), The process can be mediated by artificial supple- <br />mentation (replanting) of the vestigial stock if the native gene pool is properly cultured, However, this strat- <br />egy is fraught with risk owing to the complexity of locally adapted stocks (Lichatowich et aI" 1995), Perhaps <br />a better strategy is to reconnect the beads and allow the biota to adapt. How long this will take is a key ques- <br />tion; biology itself can be limiting. Time frames for recovery will probably vary from years to decades <br />depending on the degree of habitat degradation, the strength of normative conditions and the species <br />involved. We note that biota in the rivers devastated by the eruption of Mount St. Helens, Washington, <br />USA, in 1980 returned much sooner than expected (Anderson and Wisseman, 1987; Lamberti et al." <br />1992; Leider, 1989) and chinook salmon in New Zealand rivers developed locally adapted life histories <br />within 50 years after initial introduction (Quinn and Unwin, 1993). <br /> <br />Minimize planting of cultured stocks <br /> <br />Contemporary fisheries management is based on a belief system that embraces the concept that loss of <br />bioproduction and biodiversity from stream regulation can be mitigated by construction and operation of <br />artificial culture systems, In other words, the belief is that habitat loss caused by stream regulation can be <br />replaced, if not enhanced, by artificial propagation. Perhaps no greater myth exists in ecology, While <br />