Constant-discharge reservoir releases on the Trinity River, California, provide an unusual opportunity to unambiguously relate flow and gravel entrainment on a large gravel-bed river. Bed shear stress &tgr;0 was estimated using local observations of depth-averaged velocity. Gravel entrainment was measured using large tracer gravel installations. Lateral variability of &tgr;0 is large, even for straight channels with simple, trough-like geometry. No simple relation exists between local and cross-section mean values of &tgr;0. Fine grains (less than 8 mm; 20--30% of the bed material) are transported at lower discharges than coarse grains. Scour to the base of the bed surface layer occurs at a dimensionless shear stress &tgr;g*≈0.035, for &tgr;g* formed using local &tgr;0 and the median grain size of the gravel portion of the bed. The dimensionless reference transport rate W*=0.002, often used as a surrogate for the threshold of grain motion, occurs at nearly the same &tgr;g*. At smaller &tgr;g*, entrainment and transport rate decrease rapidly, becoming vanishingly small at &tgr;g*≈0.031. Even at very small gravel transport rates, all sizes are transported, although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained. Both entrainment and cumulative transport observations suggest that maximum scour depth for plane-bed transport is slightly less than twice the surface layer thickness. ¿ American Geophysical Union 1996