Barrier built across a stream, river, or estuary to conserve water for such uses as human consumption, irrigation, flood control, and electric-power generation. The earliest recorded dam is believed to be a masonry structure 49 ft (15 m) high built across the Nile River in Egypt c. 2900 BC. Modern dams are generally built of earth fill, rock fill, masonry, or monolithic concrete. Earth-fill (or embankment) dams, such as Egypt's Aswan High Dam, are usually used across broad rivers to retain water. The profile of an earth-fill dam is a broad-based triangle. Concrete dams may take various forms. The gravity dam uses its own dead weight to resist the horizontal force of the water. Concrete-buttress dams reduce material in the wall itself by using support buttresses around the outside base. An arch dam, such as Hoover Dam, is built in a convex arch facing the reservoir, and owes its strength essentially to its shape, which is particularly efficient in transferring hydraulic forces to supports.