A dam is thicker at the bottom than at the top because the forces exerted are strongest close to the ground. Water pressure increases proportionally with depth, so the lower portion of the dam has greater water pressure exerted upon it. The increasing thickness of the lower dam also helps it support its own weight.
Pressure is directly proportional to depth and the deeper the length the high the pressure. Thus at the bottom, water pressure is very high and the walls must be made thicker to accommodate that pressure.
