This chapter presents the hydrodynamics pressures caused by the flow on the spillway. The spillway will operate at different heads of the design discharge: for low heads, the pressure will increase, and for high heads, the pressure will decrease. For optimum performance of the spillway, the flow must be well accommodated, without detachment and disturbance of the liquid vein, for the various operating conditions of the structure. The instantaneous pressures fluctuations, caused by the high velocity flow on the rough surface of the concrete in the turbulent boundary layer, are measured in model and prototype. The forces acting on the chute are of a random oscillatory nature due to the turbulent characteristics of the high velocity flow, which are divided into average values and floating values. The mean values of the vertical and tangential components are due to the static pressure due to the water rod and the shearing stress due to the friction.