A dam is a structure that impounds water or restricts its flow. Dams are classified into four basic types: gravity dams are massive structures made of concrete or masonry that rely on their weight to resist the force of impounded water; embankment dams are large earthworks consisting of rocks, clay, sand, soil, or gravel; buttress dams consist of a sloped, concrete face supported on the downstream side by a series of triangular buttresses; and arch dams use a curved concrete wall to transfer the weight of the water to the surrounding valley walls.