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<br />reach average slope of 0.0014, which is the average slope of the line connecting surveyed <br /> <br />elevations of the high-water marks, and of the flood plain and terrace surfaces. <br /> <br />We used the Shields relation, as discussed by Andrews (1983), to calculate the <br /> <br />critical shear stress necessary to entrain the D50 or median particle size. Andrews (1983) <br /> <br />showed that in a naturally sorted gravel-bedded stream, a range of particle sizes is <br /> <br /> <br />mobilized at nearly the same discharge. Assuming "equal mobility" of a range of particle <br /> <br /> <br />sizes allows the use of a "reference" particle in the Shields relation and provides a means <br /> <br /> <br />to calculate the critical shear stress necessary to entrain a range of bed material sizes. <br /> <br />The Shields relation is calculated as: <br /> <br />tcr = t* 50 (gs-gw)D50 <br /> <br />where tcr is the critical shear stress necessary to entrain a particle, in N/m2, t* 50 is the <br /> <br />critical dimensionless shear stress, which for most gravel-bedded streams is between <br /> <br /> <br />0.033 and 0.086 (Andrews 1983; Buffington and Montgomery 1997), gs is the specific <br /> <br /> <br />weight of the particle, which is assumed to be 2650 kg/m2s2, and gw is the specific weight <br /> <br /> <br />of the fluid. Shields (1936) assumed t* 50 values of 0.06, and this determination has been <br /> <br /> <br />widely used (Andrews 1983). However, recent findings by Buffington and Montgomery <br /> <br />(1997) concluded that a universal t*50 for gravel-bedded rivers does not exist. In this <br /> <br />study we used 0.034 and 0.06 as the values for t* 50 for consistency with similar analyses <br /> <br />(Smelser 1997; Andrews 1983; and Wilcock etal. 1996). When to is greater than or <br /> <br />equal to tcr , the discharge is sufficient to entrain the particle. <br /> <br />RESULTS <br /> <br />17 <br />