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<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
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