Detention dams are constructed to either stop or slow the amount of water in a river. This is mostly to prevent flooding. The most important load that a dam must be able to support is the water behind it. How much the water pushes on the dam is called water pressure. A dam's shape can help it to withstand the pressure.
Water pressure increases with the depth of the water.
In deep water, there is more water backed up, which causes the pressure to be greater at the bottom than at the surface. A dam's design must enable it to withstand the pressure at the bottom. As a result, many dams are built in a triangular shape. The wide bottom withstands the great load of the water deep below the surface, while the top of the dam can be thinner so as not to use unnecessary costly materials.