The present study examines variations in the reference shear stress for bed load transport (τr) using coupled measurements of flow and bed load transport in 45 gravel-bed streams and rivers. The study streams encompass a wide range in bank-full discharge (1--2600 m3/s), average channel gradient (0.0003--0.05), and median surface grain size (0.027--0.21 m). A bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate W* versus the reach-average dimensionless shear stress τ*. The reference dimensionless shear stress τ*r was then estimated by selecting the value of τ* corresponding to a reference transport rate of W* = 0.002. The results indicate that the discharge corresponding to τ*r averages 67% of the bank-full discharge, with the variation independent of reach-scale morphologic and sediment properties. However, values of τ*r increase systematically with average channel gradient, ranging from 0.025--0.035 at sites with slopes of 0.001--0.006 to values greater than 0.10 at sites with slopes greater than 0.02. A corresponding relation for the bank-full dimensionless shear stress τ*bf, formulated with data from 159 sites in North America and England, mirrors the relation between τ*r and channel gradient, suggesting that the bank-full channel geometry of gravel- and cobble-bedded streams is adjusted to a relatively constant excess shear stress, τ*bf - τ*r, across a wide range of slopes.