Data obtained in an annular shear cell are interpreted to evaluate a dynamic Coulomb yield criterion at the boundary separating flowing and stationary grains in a steady granular fluid shear flow. The ratio of the shear stress to the normal stress is nearly constant at the depth of no motion, suggesting a dynamic balance between the mobilizing effect of the applied shear stress and the stabilizing effect of the normal stress which results from the immersed weight of the moving grains. We suggest that the yield criterion can be applied to predict the immersed weight of grains undergoing bed load transport.