Laboratory flume experiments were undertaken to measure the vertical profiles of mean flow velocities for two different flow discharges and four different densities of Hydrilla beds. These data were also used to calculate three hydraulic parameters, i.e., Manning’s roughness coefficient, the Reynolds number, and the Froude number. Results show that (i) the vertical distribution of measured flow velocities exhibits three-zone profiles; (ii) Manning’s roughness coefficient decreases with increasing depth-averaged flow velocity; (iii) the relationship between Manning’s roughness coefficient and the depth-averaged flow velocity is within the same smooth inverse curve; (iv) Manning’s roughness coefficient significantly changes with increasing density of Hydrilla beds; (v) the Froude number is independent of the density of Hydrilla beds; and (vi) both the Reynolds number and the Froude number increase with increasing depth-averaged flow velocity.