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Grams and Schmidt 15 dam flow regime. Because this stage did not occur during our study, we used the U.S. Geological Survey (1924) profile to estimate reach average slope at each cross section. Boundary shear stress was compared with estimates of critical shear stress for median particle size of each gravel bar. A first-order approximation of flows necessary to mobilize the gravel bars was made by estimating critical shear stress using the Shields relation (Shields, 1936), 'Cc50 = 'C*c50(Ys-Yf)D50 where, 'Cc50 is the critical shear stress in N m-2, 'C* c50 is the critical dimensionless shear stress for the median partical diameter of the bed surface, Ys is the specific weight of the solid in Nm-3, and Yf is the specific weight of water in Nm-3, and D50 is the median particle diameter of each sampled bar, in m. Values for 'C*c50 have been found to range over an order of magnitude and are affected by the bed-material size distribution (Andrews, 1983). For this approximation we used 0.033,. the value found by Andrews (1983) to be the most common for coarse-bedded streams. Figure 8 shows the downstream variation in critical shear stress necessary to entrain the median particle size of each gravel bar and the estimated reach average boundary shear stress at each cross section. The average boundary shear stress and the critical shear stress are larger in canyon reaches than in meandering reaches. The .:<:alculated shear stresses for the pre-dam lO-yr flood approximate and sometimes exceede the estimates for critical shear stress in both canyon and meandering reaches. 6.2.3 Pine-Grained Alluvium Fine-grained alluvium is abundant in the canyons of the Green River in spite of the river's high gradient, high average boundary shear stresses, and narrow alluvial valley because of the high frequency of fan-eddy depositional environments. The wide alluvial bottoms of meandering reaches, however, contain an order of magnitude more alluvium than the canyons. In the canyons between 66 and 94 percent of all alluvium exposed at low discharge is at least partially composed of [me-grained material (Table 3). Although