Variation in the reference shields stress for bed load transport in gravel-bed streams and rivers

[1] The present study examines variations in the reference shear stress for bed load transport (tau(r)) 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 m(3)/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 tau*. The reference dimensionless shear stress tau(r)* was then estimated by selecting the value of t* corresponding to a reference transport rate of W* = 0.002. The results indicate that the discharge corresponding to tau(r)* averages 67% of the bank-full discharge, with the variation independent of reach-scale morphologic and sediment properties. However, values of tau(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 tau(bf)*, formulated with data from 159 sites in North America and England, mirrors the relation between tau(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, tau(bf)* - tau(r)*, across a wide range of slopes.