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\ 002833 etation, influences the form and condition of a stream system and valley, We can broadly describe watershed changes that occur when fluvial processes are altered by reducing natural flood flows: (1) valley floors no longer flood; (2) local water tables are no longer recharged; (3) stream bar and channel areas no longer become inundated and scoured; (4) sediment accretes on bars and channel edges and forms lower, nar- rower stream banks; (5) side channels and backwater areas become disconnected from the main channel or abandoned by the mainstream as they fill in; (6) tributary channel confluences with main stems lo- cally aggrade and push out into the main channel; and (7) the ratio of pools to riffles is significantly altered (Morisawa 1968; Platts 1979; Leopold and Emmett 1983), These factors need to be considered in any analysis of flow alteration because biotic conditions such as riparian habitat or long- term fish community structure and !ish populations may depend on them, Streams generally go through an aging process, They are seldom at equilibrium because they adjust to a wide range of fac- tors and processes within the watershed (Kellerhals and Church 1989), Once a stream approaches an equilibrium condi- tion, the controlling factors may change, Such adjustments may occur daily. season- ally, or over long periods. Nevertheless, over time a channel and associated stream- side vegetation will develop characteristics and features that balance the effects of a varied flow and sediment regime (Platts et at 1985), Channel adjustments are a natural com- ponent of the channel-forming processes in all valley bottom types (Lotspeich and Platts 1982), Hence, local channel dimen- sions and characteristics will change as a result of natural or altered flow regimes, For example, the removal of all peak flows will cause near-term channel adjustments, but over the long term reduced peak flows will impair floodplain functions, which in turn alter streamside vegetation and chan- nel conditions (Platts 1979) that provide habitat for fish, The morphology of streams, especially al- luvial ones, is controlled by the interaction of flow regime with streamside vegetation and sediment input (Hynes 1970). The magnitude and duration of the bankfull flows (and larger) are particularly impor- tant, Geology, climate, and resulting sed- iment supply (including quality and quan- tity) and size of channel bed materials, within the geomorphic setting, also pro- vide form control (Beschta and Platts 1986), In sand-bed streams, bedload transport generally occurs over a wide range of flows (Leopold et at 1964), However, in gravel- bed streams the channel materials are usu- ally stable except during relatively high flows (Beschta 1987). Therefore, if fine sed- iments that have become deposited be- tween the gravels are to be removed by flowing water, sufficiently high flows must periodically occur to cause local scour and transport of bed materials (Beschta and Jackson 1979; Beschta 1987), Natural high flow events normally provide the neces- sary level of streambed mobilization to flush fine sediments from both the bed and the gravel and rubble (Rosgen et at 1986), Regulated flows that occur downstream from water diversion and storage facilities can have both a positive and a negative effect on channel substrates. For example, a positive effect is the reduction in avail- ability of fine sediments from upstream sources that may deposit in spawning gravels, A negative effect is the reduction in gravel recruitment and loss of fines for bank-building processes, Stream bank form depends on a balance between erosive forces of flowing water and resisting forces of the bed, bank, and streamside vegetation (Platts 1979), Vege- tation buffers the stream bank from flowing water, and flowing water in turn keeps vegetation from occupying the channel (Rosgen et at 1986). The duration of over- bankfull flow is also important to channel and floodplain characteristics. Flow dura- tion determines the amount of time avail- able for deposition of sediments, recharge of subsurface moisture. and other main- tenance processes. FLOODPLAIN PROCESSES Except in under-fit or deeply entrenched streams, floodplain size is generally related tlJ 200 to the discharge of the stream and slope of valley bottom (Hack 1957). Surface erosion Rivers' Volume 2, Number 3 July 1991 I