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analysis of bed load transport thresholds by Mueller et al. (2005) indicates that the movement of <br />framework grains on the bed surface (partial transport) occurs at flows equal to about 67% of the <br />bankfull discharge. The third bed load transport phase, involving motion of most all particles on <br />the bed is termed fully mobilized transport[ Wilcock and McArdell, 19931. This transport phase <br />has been equated with the bankfull discharge [Pitlick et al., 1999; Pitlick and Cress, 2000; Pitlick <br />and Wilcock, 2001], the rationale being that these flows shape the channel and thereby mobilize <br />most all of the sediment on the channel bed. <br />The flow levels or discharges required to reach the transport phases discussed above are <br />determined by selecting a threshold value of r* and solving (1) for the corresponding shear <br />stress, r. In previous studies of the Colorado River and the Gunnison River, Pitlick et al. [1999] <br />set the threshold for initial motion at r* = 0.03. Results from field studies elsewhere served as <br />the basis for selecting that value; however, the value of 0.03 is not a hard number, and recent <br />work suggests that there may be substantial variation in the critical r*. Indeed, this study was <br />motivated in part by uncertainties associated with the choice of the critical T*. For the purposes <br />of the present study, the threshold for initial motion was determined using an empirical relation <br />developed by Mueller et al. [2005]. This relation is based on an an alysis of flow and bed load <br />transport measurements taken in 45 gravel-bed streams and rivers in the western USA and <br />Canada. The analysis focused on variations in the threshold for bed load transport which arise <br />from changes in flow structure as channel gradient and bed roughness increase. For each data <br />set, Mueller et al. [2005] plotted the relation between bed load transport rate and dimensionless <br />shear stress, and, following the procedure of Parker et al. [1982], estimated the reference <br />dimensionless shear stress, r*„ associated with a small, non-zero transport rate The resulting <br />19