Today, nearly all masonry dams are built of large blocks of concrete. Gravity dams depend entirely on their own weight to resist the tremendous force of the oncoming water. 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.