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<br />46 <br /> <br />scour, but in time, the deposits can become increasingly stable, vegetated, <br />and able to attract more sediment. Greater sediment 'lccumulation is <br />encouraged when water velOCities decrease by natural or man caused <br />manipulations i.e., irrigation diversions, regulated !flows below dams, etc. <br /> <br />Pools, Riffles and Glides <br /> <br />Variations in water depth and water vE.locity are inherent in all <br />stream channels regardless of their straight, braided or meandering <br />configurations. Even straight reaches of II channel hllve a more or less <br />uneven streambed that consist of depressions, humps or deeps and shallows <br />commonly called pools and riffles. Pools, .riffles, and glides ar.! defined <br />by their basic morphology based on su_r low flow habitats for IClwer order <br />streams that combines channel morphology, J:low characteristics, substrate, <br />and cover (Bisson et al. 1981). Pools, riff les and pe>int bars ar.! bes t <br />developed in gravel-bed alluvial stream channels with a small sand fraction <br />in the streambed. Stream chsnnels with significant ~lounts of sand in the <br />channel produce migrating ripples and duneE: at low flow which may be <br />superimposed on and partly mask the more st,able bed forms (Keller 1980). <br /> <br />Pools <br /> <br />A pool is a topographically low area produced by scour which generally <br />.contains relatively fine bed ..teria! and b usually l:ssoc1ated wHh a <br />point bar. ' At low flow pools are recogni:u.d by regulllrly repeatil1g deeps, <br />with slow moving water resulting from a lo..-surface gradient. As discharge <br />increases, the difference in water-surface gradient ~'tween pools and <br />riffles gradually disappears until about blink-full stElge when the <br />water-surface gradient is constant and the pool-rifflE' sequence ill said to <br />be "drowned out" (Leopold et al. 1964). Hc,wever, the basic undulllting <br />bottom topography of the pool riffle sequence does not disappear .It high <br />flow (Keller 1980). <br /> <br />A number of distinct habitat types can be recognIzed as a rellult of <br />the interaction between flow and a variety of physical channel <br />characteristics. For example, pools may often be assc,ciated with the <br />presence of large boulders, bedrock outcroppings, large woody steals, <br />rootwads etc. Based on habitat type at leut six pool types have been <br />recognized (Bisson et al. 1981). Trench pools are essentially slots in a <br />stable channel bottom (generally bedrock) and"may be very large O'igure <br />16). Plunge pools occur where the main flow passes over a complete channel <br />obstruction and drops vertically into the streambed below. Lateral scour <br />pools differ from plunge pools only in the direction of scour, wlU,ch is <br />deflected to one side of the stream by a partial channel obstrucUon. The <br />scour may leave a depression in the streambed or it may undercut the bank; <br />either way, lateral scour pools are usually found at bends in the channel. <br />Dammed pools consist of deep water, impoundlld upstream. fro. a full or <br />partial channel blockage, such as those caused by debris jams or <br />land.lid~s. Backwater pools are found along channel marlin. and are caused <br />by eddies behind larle obstructions such a. roots, boulders and rootwads. <br />, " .~, 4 <br />