Using a Laser-Doppler anemometer, mean velocities are measured in developing and fully developed turbulent subcritical smooth open channel flows. Experiments are conducted in a rectangular laboratory channel for 12 different test conditions with Reynolds number ranging from 28,026 to 136,842. From the experiments it is found that the boundary layer along the centerline of the channel develops up to the free surface for a flow aspect ratio b/h greater than or equal to 3. Shear velocities are calculated using the measured velocity profiles in the viscous sublayer of the boundary flow. The experiments show that shear velocity varies in an oscillatory manner across the flow section around b/h = 3. In the turbulent inner regions of developing and fully developed boundary flows, the measured velocity profiles agree well with the logarithmic ''law of the wall'' distribution when the coefficients in the expression are 2.44 and 5.5, respectively. The ''wake'' effect becomes important in the velocity profiles of the fully developed boundary layers. A reasonable agreement between the modified velocity-defect law and the experimental profile in the inner and outer regions is obtained with a profile parameter of 0.1 in the Coles's ''law of the wake.''