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<br />44 <br /> <br /> . <br /> ,. <br /> 30 <br /> 25 <br />; 20 <br />~ <br />~ 15 <br />. <br />,g 10 <br /> 5 <br /> 0 <br /> <br />I 3 !5 7 9 II 16 <br />Pool-riffle lpox'"9lmchorntl ""dltl) <br /> <br />Figure' J, Fre:qumc:y of pool-rlffle spacing <br />for four .1lwt.l stree.. (10Z <br />pool.rlfne Seftljencts). <br /> <br />fro~ E.A. Keller. 1980. The fluvial system: selected observntlons. <br />pp. 39-46. In A. Sands (ed). Rlpnian forests in California, their <br />ecology and conservation: A symposium. Pub, No. 4101. A~riculture <br />Sciences PubUcatlons, U. of California, Ber.-Illey. CA 94720. <br /> <br />8/5 for a meander factor of 1.6. In other words, a perfectly str'aight <br />stream would have a meander factor of 1.0 "hereas a stre.. with I1l8ny bends <br />will have a meander factor greater than 1.0. <br /> <br />Pattern Migration <br /> <br />Stream channel pattern IIligration is dl!fined as the natural increase in <br />meander amplitude in the lateral direction and the gr,adual down valley <br />movement of the overall stream pattern (Skinner and Stone 1983). The <br />lateral and down valley movement of meander bends is ia characteristic <br />feature of alluvial streams and one of the most conspicuous changes <br />affecting fluvial landscapes (Gregory 1977)1. Laterally confined meanders <br />are restricted to down valley movement only (Neill 19:73). Alluvial streams <br />continually change their position as s consequence of hydraulic forces <br />acting upon the streambed and banks. Changes will oCI:ur because banks <br />erode, sediments are deposited, and floodplains, islands, chutes, and side <br />channels undergo modification with time. Meanders shUt position by <br />crossing the valley and the entire pattern may sweep downstream. The <br />result is a complete reworking of the floodplain allu',ium. The rate at <br />which a single meandering bend IIligrates and grows dep.mds on its shape and <br />bank stability (Figure 14). Meander growth is at firllt essentially <br />lateral. As the amplitude of the bend increases, dO~lstream shift of the <br />meander becomes increasingly important. Natural, but dramatic changes of <br />stream position by cutoffs and erosion are common and can be greatly <br />accelerated by man's acti vi ties. The shif t:Lng of a channel across a flood <br />_plain is considered natural and streams undergoing thl!se changes should not <br />be considered unstable. However, s stream reach that is changing from high <br />sinuosity to low sinuosity should certainly' be consid'lred unstable even if <br />the change is only local (Schumm 1977). <br />