An experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0.3 m wide tilting flume. The concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport. The experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter. A revised Shields diagram relating critical stress, grain Reynolds number, and depth to grain size ratio has been derived. This has implications for the calculation of sediment transport rates, design of stable alluvial channels, and physical hydraulic modeling. ¿ 2000 American Geophysical Union