A dam is a structure designed to hold back water in a lake, river, stream or other water body. Dams typically include gates that can be raised or lowered, opened or closed to allow variable amounts of water to pass downstream or leave the lake. The path the water takes to leave the lake through the gates is called a spillway.

There are several styles of dams used for different purposes. 

• An arch dam is made of masonry or concrete that is convex towards the upstream. Its stability depends on the arch, which transfers the water pressure to abutments on each end of the dam.
• A buttress dam has a watertight face upstream supported by buttresses along its downstream side.
• A coffer dam is a temporary watertight enclosure that is pumped dry to expose the bottom of a water body so construction can be done.
• A gravity dam is made of concrete or masonry and depends on its weight for stability. A variation of this is a crib dam, which has chambers made from wood or other material and those chambers are filled with stone or heavy materials. 
• An embankment dam is built of fill material such as earth or rock. It has sloping sides, with a length greater than its height.
• A weir is a dam in a river or stream that raises the water level for a mill, creating a fish pond or other smaller-scale uses.