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<br />33 <br /> <br />, <br />",..-I <br /> <br />amount and duration of the sediment being earr1ed. '~erefore thll discharge <br />and load over time 1& responsible for creating channEll patterns. <br /> <br />A stream channel develops its own geomorphic pattern that includes <br />width, depth, gradient, and meandering, based on lon~:-term pattel~ns of flow <br />(Blench 1972). Most channels undergo changes during high flow arid lesser <br />modifications at low flow, however, period.1c catastrc'phic floods are not as <br />important as the bankfull states that occur at 1 to 2: year intervals <br />(Wolman and Millar 1968). Maximum bank erosion in'st:reams generlLlly <br />follows periods of bankfull flow. During I:he bankfull stage watE,r helps <br />support the banks, but as the flood diminishes unstable banks become <br />increasingly susceptible to erosive forces (Keller 1977). The erLd result <br />is bank slumping and a washing away of the sediment c1luring the ne!xt <br />bankfull or flood flow condition. L <br /> <br />Longitudinal Gradient <br /> <br />Natural streams are in a continual state of dyns.lIl1c equilibrium with <br />discharge, sediment load, and gradient through the processes of erosion and <br />deposition (Maddock 1976). The extent to "hich,a stream can erocle its <br />channel depends on the physical features that serve a,s baae levels such as <br />bedrock or ultimately the oceans. A streaOl is considered graded when there <br />is a balance between erosion and deposition and any change in the, balance <br />leads to signs of disequilibria. Headwater streams (thought to be <br />youthful) with an irregular longitudinal profile are considerell. u,ngraded <br />whereas mature downstream reaches with smooth longitudinal profiles and <br />defined meanders, are considered graded (Leopold et al. 1964, Morisawa <br />1968) (Figure 2). Grade implies physical stability and natural gradual <br />change with continual adjustments of width, depth and slope maintaining a <br />balance between erosion and deposition. <br /> <br />Substrate <br /> <br />When streams are viewed as longi tudim,lly linked systems, from <br />headwaters to mouth, the mean substrate particle size decreases ill a <br />downstream direction. Beginning elevation" slope, dhcharge, velclc1ty and <br />geomorphology of drainage basins form the physical conditions within the <br />channel that change progressively and predl.ctably with increasing stream <br />size. Boulder, rubble, and gravel substrates tend to predominate in steep <br />gradient beadwater mountain reaches tbat at'e annually subjected tCl high <br />water velOCities or in downstream reaches "ith cbaract:eristics sbdlar to, <br />tbe beadwaters. The size of streambed components is elbviously a function <br />of local drainage basin geology (Table 1). Sand, silt:, and clay materials <br />may be found in these systems, but are usu~Llly restrie:ted to wider, deeper, <br />backwater and shallow areas of low gradient, or low VE!locity arelUl of less <br />tban 1 cfs. Even those materials will be moved downst:ream during high <br />spring runoff and flood flows. The finer materials, Eluch as silt, sands, <br />clay, and suspended organics, tend to remai,Q suspended at slightl), elevated <br />flows until tbe stream approaches base level. The ultimate effect: of the <br />numerous factors that act upon the movement of substrBLte materialEL' results <br />