Abstract: Experiments with clay suspensions flowing over smooth, gravel and stone beds prove that resistance of the flow may be considerably reduced by the suspended sediment if the flow boundary is rough The measured mean velocity of clay suspensions is much higher than the flow of water The friction factor of flows of high clay concentrations over the gravel bed was less than half of the clear water flow on the same hydraulic conditions The high viscosity and yield stress of clay suspensions suppressed the de...