The maximum velocity in any channel cross section might be as important as the mean velocity. It is easier to measure the maximum velocity than the mean velocity, and many flow rate sensors measure maximum velocity and convert it to mean velocity for the evaluation of the discharge. The experimental results obtained from two actual sites and the comparison with their estimated values, are presented in this study. The plots of isovel lines of the primary velocity from each site are presented. Concerning narrow channel properties, it was observed that the maximum velocity occurred below the free surface. Several series of measurements from these sites were collected to explore the relationship between the cross-sectional mean (U(mean)) and maximum velocity (U(max) under different hydraulic conditions. Additional velocity data and measurements in flumes and rivers were also collected from work of other researchers in order to compare this relationship in different cases. It was found that the ratio of the U(mean) on U(max) in narrow channels was higher than that in rivers with a large aspect ratio (width/water height).