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<br />or other features that are left by
<br />infrequent high-magnitude floods
<br />,(e.g. ~ Miller 1990).
<br />Over periods of years to decades,
<br />a single river can consistently pro...
<br />vide ephemeral. seasonal, and per...
<br />sistent types of habitat that range
<br />from free-flowing, to standing; to no
<br />water. This predictable diversity of
<br />in-channel and floodplain habitat
<br />types has promoted the evolution of
<br />species that exploit the habitat ,mo~
<br />sale created and maintained by hy-
<br />drologic variability. For many river-
<br />ine species~ completion of the life
<br />cycle requires an array of different
<br />habitat types~ whose availability over
<br />time is regulated by the flow regime
<br />(e.g.. Greenberg et at 1996t Reeves
<br />et at 1996, Sparks 1995). Indeed,
<br />adaptation to this environmental dy...
<br />namism allows, aquatic and flood..
<br />plain species to persist in the face of
<br />seemingly harsh conditianst such as
<br />floods and droughts, that regularly
<br />destroy and re-create habitat elements.
<br />From an evolutionary perspective.
<br />the patte;rn ,of spatial al1d t~mporal
<br />habitat dynamics influences the rela-
<br />tive success of a species in a particu-
<br />lar environmental setting. This habi-
<br />tat template (Southwood 1977),
<br />which is dictated largely by flow
<br />re'gime, .creates both subtle and pro..
<br />found differences in the natural his..
<br />tories of species in different segmElDts
<br />of, their ranges. It also infl ue-~~es
<br />species distribution and abundancet
<br />as well, as ecosystem 'function (Poff
<br />and Allan 1995, Schlosser 1990J
<br />Sparks 1992~ Stanford et al. 1996).
<br />Human alteration. of flow .,reg~IJl.e
<br />changes the established pattern of
<br />natural hydrologic variation and dis-
<br />turbance, there by altering habitat
<br />dynamics and creating new condi-
<br />tions to which the native biota may
<br />be poorly adapted.
<br />
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<br />n 1: ~~~ .,I."::t. . I,. / " .', !./ i . .t. :
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<br />\~-..=~\ j;n f/.. ~ .. · ...... - Of .:01. I:":n~
<br />.. ~':'~:a.l.;" J.... ..+ '.... I~: '. I. ~, ,I..:: · . . I
<br />
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<br />
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<br />· B .~..~~;~.. c ~_~/ w-:~ 1.AM _f.,
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<br />(i:. .:..-I'" ; ,," b h>l 'f ~;~ ,1'.;2t.) Fl ~~.~~
<br />):1'" n:::II1 ~ , r:.... +"./ .....~d
<br />- . " './ ., .-,~;I /..,...,
<br />
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<br />
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<br />
<br />Figure 3. Stream valley cross-sections at various locations in a watershed illustrate basic
<br />principles about natural pathways of water moving downhlll and~ human influences on
<br />hydrology, Runoff, which occurs when precipitatlon exceeds losses due to evaporation
<br />and plant transpiration. can be divided into four components (a): overland flow (1) occurs
<br />when precipitation exceeds the infiltration capacitY of the s-011: shallow subsurfac'e
<br />stomiflow (2) represents water that infiltrates the soil but is'routed relatively quickly to
<br />the stream channel: saturated overland flow (3) occurs where the water table is close to
<br />the surfacet such as adjacent to the stream channel. upstream of first-order tributaries I
<br />and in &0115 saturated by prior precipitation; and groundwater" flow (4) represents
<br />relatively d~~p and stow pathways of water movement: and provid~s water to th~ stream
<br />channel even during periods of Uttle or no precipitation. Colle1=tlvely t overland and
<br />shallow subsurface flow patl1ways create the pea~ in the hydrog~aph that are a river's
<br />respo~e to storm events, whereas deeper groundwater pathways ~e responsible for
<br />base-flow. Urbanized (b) and agricultural (c) eland uses increase surface flow by incr~asing
<br />the extent of impermeable surfaces 1 reducing vegetation coverl and tnstalllng drainage
<br />systems. Relativ~ to the unaltered state, cha~nels often are scour~d to greater depth by
<br />unnaturally high flood crests and water tables are lowered. causing baseflow to drop.
<br />Side-channelst wetlands, and episodically flooded lowlands compnse the diverse flood-
<br />plain habitats of unmodified river ecosystems (d) I Levees or flooq walls (e) constructed
<br />along tlJe banks retain flood wa~ers In the main channel and lead to a loss of floodplain
<br />habitat diversity and function. Dams impede the downstream mov~ment of water and can
<br />greatly modify a river. s flow regime, depending on whether they ~e operated fOf storage
<br />(e) or ~s .. run-of-river I" such as ,fOf ~avigatlon (I) ~ . "
<br />
<br />rivers in accQrdance With both flow
<br />characteristics and the type and the
<br />availability of ~ransporta hIe materials4
<br />Within a river, different habitat
<br />features are created and maintained
<br />by a wide range_ of flows. For, ex-
<br />amplet many.channel and floodplain
<br />features. such as ri'ver bars and riffle-
<br />pool sequences.,are formed and m"ain-
<br />talned by dominant, or bankfull, dis-
<br />c charges~ These discharges are flows
<br />that can move significant quantities
<br />of bed or bank sediment and that
<br />occur ,frequently enough (e,g., every
<br />several years) to continually modify
<br />the channel (Wolman and Miller
<br />
<br />772
<br />
<br />1960). In many streams and' rivers
<br />with a small rarige of flood flows,.
<br />bankfull.. flow ca~ build and main-
<br />tain the active fJ.oodplaln through
<br />stream migration (Leopold et at
<br />1964). However~ the concept of a
<br />dominant discharge may not be ap-
<br />plicable in allflo~ regimes (Wolman
<br />and Gerson 197$). Furthermore" in
<br />some flow reginjes, the flows that
<br />build the chann~l may, differ from
<br />those that build the floodplain. For
<br />example, in river~ with a wide range
<br />of flood flows, floodplains 'may ex-
<br />hibit majer bar~ deposltst such as
<br />berms of boulder~ along the c~annel,
<br />
<br />Human alteration"of
<br />flow regimes
<br />
<br />Human modification of natural hy-
<br />drologic processes disrupts the dy-
<br />namic equilibrium between the move..
<br />ment of water and the movement of
<br />sediment that exists in free-flowing
<br />rivers (Dunne and Leopold 1978).
<br />This disruption alters both gross-
<br />and fine...scale geomorphic features
<br />that constitute habitat for aquatic
<br />and riparian species (Table 1). After
<br />
<br />BioScience Vol. 47 No. 11
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