They are the strongest and most massive dams built today. A gravity dam is built on a solid rock foundation. The dam transfers the force of the water downward to the foundation below. Gravity dams can hold back enormous amounts of water. However, they are costly to build because they require so much concrete.
Arch dams curve outward toward the flow of water.
They are usually built in narrow canyons. As the water pushes against the dam, the arch transfers the water's force outward to the canyon wall. An arch dam requires much less concrete than a gravity dam of the same length. Buttress dams depend for support on a series of vertical supports called buttresses. The downstream face of a buttress dam usually slopes outward at about a 45-degree angle.