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<br />hill cranes (Gr.us canadensis) . which
<br />made the Platte River .famousj have
<br />· abandoned river segments that have
<br />"narrowed the most (Krapu ~t aL 1984)'1
<br />Chang'es in the duration of flow
<br />conditions also have significant bio-
<br />logical consequences~ Riparian plant
<br />species respond dramatically to chan-
<br />nel dewatering, which occurs fre-
<br />quently in arid regions due to surface
<br />water diversion and 'groundwater
<br />pumping. These biological and eco..
<br />logical responses range from altered
<br />leaf morphology to totallo~s of ri-
<br />parian vegetation cover (Table 2) r
<br />Changes in duration of inundationi
<br />independent of changes in annual
<br />volume of flowt can alter the abun-
<br />dance of plant cover types (Auble et
<br />al. 1994). For example. Increased
<br />duration of inundation has contrib-
<br />uted to the conversion of grassland
<br />to forest along a regulated Austra-
<br />lian river (Bran 1992). For aquatic
<br />" species, prolonged flows of particu-
<br />lar levels can also be damaging~ In
<br />the regulated p~cos River of New
<br />Mexic 0 J _artificially prolonged high
<br />, summer flows for irrigatiqn displace.
<br />the flca ting eggs of the threatened
<br />Pecos bluntnose shiner (Notropls sinlus
<br />pecosensis) into unfavorable habitat,
<br />where none "survIve" (Robertson in
<br />press) ,
<br />Modification of natural flow tirn-
<br />ing~ or predict~bility. can affect,
<br />aquatic organisms both directly and
<br />indirectly. For example, some native
<br />fishes in Norway use seasonal flow
<br />peaks as a cue for egg hatching, and
<br />river regulation that eliminates these
<br />pe~ks can directly reduce local popu...
<br />lation sizes of these species (Nresje et
<br />. at 1995)~ Furthermore, entire food
<br />webs, not just single species, may be
<br />modified by altered flow timing. In
<br />regulated rivers of northern Califor-
<br />nial the" seasonal shifting of sCQuring
<br />flows from winter. to summer Indi.
<br />reetly reduces th'e growth rate of Juve-
<br />nile steelhead trout (Oncorhyncus
<br />myktss) by Increasing the relative
<br />abundance of predator..resistant in-
<br />vertebrates that divert energy away
<br />from the food chain leading to trout
<br />(Wootton .et at 1996). In unregu-
<br />lated rivers~ high winter flows re-
<br />duce these predator..resistant insects
<br />and favor species that are more pal..
<br />atable to fish.
<br />Riparian plant 'sp~cies are ~lso
<br />strongly affected by altered flow tim-
<br />
<br />778
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<br />.~igure 5. A brief hl~tory . of fl ow alteration, in th:e United States.
<br />
<br />. T - ~ -
<br />
<br />ing (Table 2). A ~hift in timing of
<br />peak flows from spring to summer I
<br />as often occurs when reservoirs are
<br />managep to supplr irrigation water.
<br />has prevented rees,tabl1shmerit of the
<br />Fremont cotto~wood (Populus
<br />fremontli) I the d~minant plant spe..
<br />c:ies in .ArizoQa, b~cause flow peaks
<br />now occur after, rather than before,
<br />its germination p~riod (Fenner et at
<br />1985) ~ Non-nativ$plant species with
<br />less specific germination require-
<br />ments may benef~t from changes in
<br />flood timing. F~r example'~ sal t
<br />cedarl,s (Tamarlx~p~) long seed dis-
<br />persal period allqws it to establish
<br />after floods occurIing any time during
<br />the growing seasoq, contributing to its
<br />abundance on floottplains of the west-
<br />ern Un_ited States {Harton 1977)~ -
<br />Altering the r~~e of change in flow
<br />can negatively affect both aquatic
<br />and riparian species. As mentioned
<br />above~ loss of natural flashiness
<br />
<br />threatens most of the native fish fauna
<br />of the American Southwest (Minckley
<br />and Deacon 1991), and artificially
<br />incteased rates of, change caused by
<br />peaking power hydroelectric dams
<br />on historically less flashy rivers ere-
<br />ates numerous ecological problems
<br />(Table 2: Petts 1984) ~ A modified
<br />rate of change can devastate riparian
<br />species, such as cottonwoods, whose
<br />successful seedling growth depends
<br />on the rate of groundwater recession
<br />followIng floodplain Inundation. In
<br />the ,St~ Mary River in Alberta,
<br />Canadat for example; rapid draw-
<br />downs of ,river stage during spring
<br />have prev'ented the recruitment of
<br />young trees (Rood and Mahoney
<br />1990). Such effects can be reversedt
<br />however. Restoration of the spring
<br />flood and its natural. .slow recession
<br />in the Truckee River in California
<br />has allowed the successful establish-
<br />ment of a new generation of cotton-
<br />
<br />BioScience Vol. 47 NOf 11
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