|
<br />:.....,'1....'
<br />;:
<br />
<br />404
<br />
<br />"r\~ l.""
<br />... ....-.. -.......
<br />
<br />/....
<br />(....
<br />
<br />J A. STANFORD ET AL.
<br />
<br />strong interactors capable of structuring food-webs: Power et aI., 1995) may clarify spatial and temporal
<br />scales in large river restorations. .
<br />In general, the entire catchment, from headwaters to the ocean, is relevant. In the case of rivers that sup-
<br />port anadromous fisheries, estuarine and ocean habitats connect functionally to the riverine components,
<br />Mathematical models can be used very effectively to formalize understanding of the effects of regulation
<br />and interactions with pollution and food-web manipulation within the river continuum, but models should
<br />not be used exclusively to define a restoration strategy (Hall, 1988b), The process must be inferential from the
<br />entire body of quantitative and qualitative information about how river regulation and interactive effects
<br />have altered ecosystem capacity,
<br />
<br />ReslOre em'ironmental (habitat) heterogeneity but let the river do the work
<br />The main goal is to reduce the range of human disturbances so that interconnected riverine habitats (Fig-
<br />ures I and 2) can support diverse and productive food-webs, including species of special social and economic
<br />interest. Management should strive to restore environmental heterogeneity and reduce or eliminate sources
<br />of mortality from pollution and overharvest. Keep in mind that riverine biodiversity and bioproduction are
<br />largely controlled by abiotic drivers (Figure 3) and that density-dependent relationships, such as the stock-
<br />recruitment relations often used to determine fisheries harvest prescriptions, rarely manifest predictably
<br />owing to natural abiotic variation (Hall, 1986; Hall, 1988a; Pimm, 1991; Huston, 1994),
<br />Owing to the importance of flow to habitat maintenance, and temperature to food-web energetics, highly
<br />significant restoration is possible simply by reregulation to allow more natural seasonality of flow and tem-
<br />perature, We call this restoration of normative habitat conditions, where the norm or standard is established
<br />from what is possible in a natural-cultural context as opposed to striving for pristine conditions which are
<br />difficult, if not impossible, to define or achieve, at least for entire catchments. Removal of dams certainly
<br />should be considered and, where possible, done; but, restoration of normative habitat conditions is possible
<br />in many ifnot most regulated rivers without taking dams out. However, channel revetmenls are problematic
<br />because the objective is to reconnect channels and floodplains,
<br />Peak flows are needed to scour and rearrange substratum and reconnect floodplain habitats with the chan-
<br />nel; spatial and temporal temperature variability promotes re-establishment of native biodiversity (Figures 2
<br />and 5). Peak flows needed to re-establish cut and fill alluviation (called effective flows by geomorphologists)
<br />mayor may not be equal to bank-full, and gravel supply may be limiting owing to storage of bedload in the
<br />reservoirs (Ligon el aI., 1995), Effective flows can rapidly degrade (downcut) entire segments when sediment
<br />mass balance relations change as a consequence of regulation (Andrews and Nelson, 1989). Adding sedi-
<br />ments to regulated rivers (e,g. using slurry pipelines from reservoir deltas) should not be out of the realm
<br />of consideration in situations where instream sediment supply is limited by years of regulation. However,
<br />overbank flows in many cases will initiate cut and fill alluviation in an ecologically effective manner, supply-
<br />ing sufficient sediment from lateral erosion.
<br />We emphasiZe'lhat reregulation of flows requires careful evaluation of channel morphometry, bed-sedi-
<br />. menl size distribution and shear stress in relation to the range of possible flows, A great deal of geomorpho-
<br />logical study and modelling has been devoted to this problem in recent years (Andrews, 1980; Andrews and
<br />Nelson, 1989; Kellerhals and Church, 1989; Deitric!J et al., 1993; Church, 1995) and more work is needed. In
<br />general, flows that mobilize substrata of median particle diameter will build bars, cut overflow channels and
<br />dig pools, Determination of peak flows is complicated by dense, often senescent, thickets of riparian
<br />vegetation on the floodplains of regulated rivers. Repeated scouring flows will most likely restore riparian
<br />successional vitality, Annual temperature patterns similar to pre-regulation conditions, which will directly
<br />mediate restoration of biota, can often be attained by depth selective withdrawal structures on the dams
<br />(Gore, 1985),
<br />Of course, restoration of overbank flows may be problematic in many rivers where humans have colonized
<br />the floodplains. In these cases, revetments have often been extensively built to restrain flood flows. Reregu-
<br />lation to produce overbank flows may not be practical. However, floods of record will most likely result in
<br />overbank flow even in intensely regulated rivers, because natural storage on floodplains throughout the con-
<br />tinuum has been drastically diminished, Revetments tend to act as dams during very large floods on
<br />
<br />.....,..
<br />"
<br />...:..:'.',;. ..'
<br />.'. ...:.....
<br />
<br />". .......'..
<br />............,'..
<br />'-." '.:' .,-;~::..:::.
<br />
<br />.......'..
<br />
<br />--
<br />'..<...-".
<br />
<br />'., ;. '.~, .' ':'
<br />
<br />:,~' :.:-:".:;:'-:
<br />
<br />"~.'.'.' ....":::.
<br />. .'. .~"..~
<br />
<br />--
<br />.:. .);;
<br />..>:>:.-....!;::: .
<br />.' ....
<br />........
<br />
<br />.'.'
<br />.......-....
<br />
<br />........{.....-..;..
<br />
<br />.........
<br />....,... '>/'
<br />....'. ...:'
<br />;>.:....
<br />.: .;~ .....",
<br />~'. :.~.:. .~. '. .'
<br />. ',' ....-
<br />
<br />~~::. r~:~.:\ :~..:\::I
<br />
<br />~~:(f~:'t~
<br />
<br />. '.' '-,.'~
<br />..'...:..'-..-;.....
<br />~:'\:~(;~i~<3~i
<br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />..::.(.~_:\:-: :>:.
<br />
<br />
<br />ri~~
<br />
<br />~,~
<br />
<br />..::..~.:~-..~..:.::: /::
<br />
<br />''';..:J<C'.
<br />
<br />.~ '-'1
<br />
<br />"
<br />'...
<br />
<br />.".,
<br />
<br />
<br />....-
<br />c''.
<br />--
<br />,'.'
<br />>:-<.~. <~
<br />
<br />:; .... ,~,..;.. ,.' '..'
<br />.J'. .:......
<br />.? ,.>.
<br />
<br />.;,
<br />
<br />
<br />
<br />'. '.'
<br />
<br />
<br />'" ..........c.
<br />..' ....
<br />..,....
<br />
<br />
<br /><,'
<br />
<br />, '
<br />. ."
<br />
|