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I I I I I I I I I I I I I I I I I I I 29 and D is the particle diameter. According to eqn. 1, a particle begins to move when the critical value of 'r*c is exceeded (or equivalently when the available shear stress, 1', exceeds the critical shear stress, 'rc). In rivers with poorly sorted sediment, Le. sizes ranging from sand to gravel, the value of 1'* c has been shown to vary from < 0.01 to> 0.2 depending on whether particles are larger or smaller than the median grain size, Dso (Komar, 1987; Andrews, 1983). As it turns out, however, larger particles tend to be more exposed to the flow while smaller particles tend to be hidden in pockets, and thus most particles will begin to move at nearly the same shear stress (Wilcock and Southard, 1988; Andrews, 1983; Parker et a1., 1982). Under this assumption, a single value of Shields parameter corresponding to a particular grain size (e.g. Dso) can be used to determine the threshold for motion. For Dso, a value of 'r*c = 0.03 is commonly used as the criterion for initial motion. At this level of shear stress, sediment transport is very weak, involving the sporadic movement of just a few particles. As the flow and shear stress increase, more and more particles become entrained, until at a value of about 'r*c = 0.06, there is significant motion and almost all particles on the bed will be moving. To evaluate when the critical shear stress is reached, we must determine the range in shear stress for different flows. The average boundary shear stress can be defined by the equation 't = P g h sf ( 2 ) where h is the flow depth, and sf is the friction slope, or streamwise energy gradient. Over very long reaches (> 1km), sf can be approximated by the bed or water-surface slope, but over shorter reaches (e.g. one pool-riffle-run sequence), sf . must be calculated from a step-wise solution to the I-dimensional momentum equation