An arch-gravity dam or arched dam is a dam with the characteristics of both an arch dam and a gravity dam. It is a dam that curves upstream in a narrowing curve that directs most of the water against the canyon rock walls, providing the force to compress the dam. It combines the strengths of two common dam forms and is considered a compromise between the two. They are made of conventional concrete, Roller Compacted Concrete (RCC), or masonry. Arch-gravity dams are not reinforced except at the spillway. A typical example of the conventional concrete dam is the Hoover Dam. Changuinola Dam is an example of the RCC arch-gravity dam. A gravity dam requires a large volume of internal fill. An arch-gravity dam can be thinner than the pure gravity dam and requires less internal fill.