Since the forces of water on a dam are hydrostatic forces, they vary linearly with depth.  The total force on the dam is then the integral of the pressure multiplied by the width of the dam as a function of the depth.  The center of pressure is located at the centroid of the triangular shaped pressure field  from the top of the water line. The hydrostatic force and tipping moment on the dam about some point can be computed from the total force and center of pressure location relative to the point of interest.