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406 ~!)~~n ~\,}r.'_/" : J. A, STANFORD ET AL. "~:.'~~ ."' .". -.' :J ): most of the sediment is moved on the rising limb of the hydrograph, In years of average catchment water yield, a modest peak flow can be generated, while also elevating baseflow to accomplish the purposes for which the dams were built. In dry years, peak flows can be minimal or non-existent (Figure 4), The strategy is simply to lower the baseflow a little to build peaks in relation to catchment runolf, In all years it is essential to prevent massive dewatering of the varial zone during baseflow periods; explicitly, this means that daily changes in flow (ramping rates) should not exceed the range of variation that occurred before regulation (Figure 4). Operators of hydroelectric darns may object to reregulation recommendations as depicted in Figure 6, because of the potential constraints on generation of peak power and concern often exists that the legal requirements for electrical load control cannot be met. On the contrary, load control can be performed with- out ramping flow beyond the range of variation observed in pre-regulation periods (Jourdonnais and Hauer, 1993), Loss of peaking is problematic, However, most large dams are part of/arge electrical marketing grids and alternatives to hydropower peaking exist today that were not available a decade ago, For example, mod- ern fuel turbines are very elfective peaking units, natural gas reserves are large world-wide and local utilities are finding gas-powered turbines to be preferred alternatives to the purchase of regional hydropower. The need for hydropower peaking may wane in the next decade, particularly if the cost of, and public desire for, downstream environmental mitigation increases. " ..' ~{~~i1i~ ;', .........; q' . ~ ;;\~;~i;).::~f~::~i.: :':;.;sf;:' Maximize passage efficiency to allow recovery ofmetapopu/ations Maintenance of in stream and floodplain habitats by restoration of peak flows and revitalization of shallow and slack water habitats by stabilization of baseflows will increase ecological connectivitiy along all three spatial dimensions. However. in the absence of dam and reservoir removal, optimization of dam and reser- voir passage efficiency for biota is required to reconnect the longitudinal dimension (Figure 2), Mechanisms for significantly reducing mortality of juvenile and adult fish as they pass hydroelectric dams include flowing ladders, travelling screens, surface-release a!tractors and other bypass devices (Gore, 1985). The main point is that dams with no, or'very inefficient, bypass systems maintain the discontinuum and isolate populations, thereby limiting the gene flow that may be needed to restore and maintain metapopulations, On the other hand, the presence of impassable dams in some cases has prevented immigration of non-native species into native food-webs and elfectively isolated viable native populations (Stanford and Hauer, 1992), In many large, regulated rivers, viable populations of native species remain in segments isolated by dams, Restoration of flow and temperature seasonality and reconnection of these refugia may restore critically important core areas, revitalize metapopulation structure and rapidly lead to recovery of genetically and numerically depressed populations (Sedell et al., 1990; DeVore el ai" 1995), Indeed, a primary strategy of large river restoration should be to identify, stabilize, restore and reconnect river segments to core areas con- taining native food-webs. The expectation is that native species willlecolonize restored habitat from the core area (Lichatowich el ai" 1995; Frissell and Bayles, 1996), The process can be mediated by artificial supple- mentation.(replanting) of the vestigial stock if the native gene pool is properly cultured. However, this strat- egy is fraught with risk owing to the complexity of locally adapted stocks (Lichatowich el al., 1995). Perhaps a better strategy is to reconnect the beads and allow the biota to adapt. How long this will take is a key ques- tion; biology itself can be limiting, Time frames for recovery will probably vary from years to decades depending on the degree of habitat degradation, the strength of normative conditions and the species involved. We note that biota in the rivers devastated by the eruption of Mount St. Helens, Washington, USA, in 1980 returned much sooner than expected (Anderson and Wisseman, 1987;. Lamberti et aI., 1992; Leider, 1989) and chinook salmon in New Zealand rivers developed locally adapted life histories within 50 years after initial introduction (Quinn and Unwin, 1993), . . r~ ~:;t.~~:,:"~~::; f~ '. ~.;' .'. .!' ':":~- Minimize plaming of cultured stocks Contemporary fisheries management is based on a belief system that embraces the concept that loss of bioproduction and biodiversity from stream regulation can be mitigated by construction and operation of artificial culture systems, In other words, the belief is that habitat loss caused by stream regulation can be replaced, if not enhanced, by artificial propagation, Perhaps no greater myth exists in ecology. While .... . '., ~~'~'.; ~~-'.~~'<' ..;'.;:..:.::..' ";-..::':.-. :'.~..';'.-.:.' '.:.::: .-.' ;:~:~~.'::' :.:..... ;'. ,,' .:....,' .-..... '. ~.' -;.. ~", .' .:,.... '.. ':~.-:" .: . 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