The use of relative high velocities reduces the cost of canal per unit length. The limit of velocities permissible is somewhat less than velocities that will erode a canal bed. The determination of permissible velocities is not possible from data on transporting velocities or mere non-silting. velocities. The presence or absence of colloidal matter is of prime. importance. The opinions of irrigation engineers as determined from their experience are given, as well as deductions from available data and the final recommendations of the Special Committee on Irrigation Hydraulics.