In some cases the Keulegan equation is utilized to estimate the average velocity for in-bank flows for gravel-bed rivers. To apply the equation, the effective boundary roughness, k5, is usually estimated in terms of some characteristic bed material size.Data from 67 gravel-bed river reaches in Alberta are used to determine an optimum method of defining the effective boundary roughness in the Keulegan equation. The analysis indicates that k5 is best estimated as 6.8d50, 5.2d65, 3.5d84, or 3. Id90. These findings are compared with the work of other investigators who used gravel-bed river data from Britain and the United States.Errors in the estimation of average velocity by the Keulegan equation range from about 20% for D/d90 = 200 to about 100% for D/d50 = 5 if the characteristic bed material size, d0, is used to estimate the effective boundary roughness, k5, rather than the optimum definition of the effective boundary roughness, Cd0, as determined in this note.