A dam is a man-made barrier that is often constructed over a river to hold back water and form a lake, or reservoir, behind it. The word derives from the Latin dum which means while. Dams have been used for irrigation, power generation, flood control, and recreation. They can be divided into three main categories: natural, artificial, and hybrid. Natural dams are formed when large rocks or trees fall in the river causing them to flow around the obstacle. These dams can be found in mountainous regions where there is enough precipitation to cause flooding but not enough to need constant water management through irrigation or other means. Natural dams do not require maintenance because they will break down over time due to erosion. Artificial dams consist of structures built out of concrete or steel that hold back water. These dams can be found anywhere there is water demand that cannot be met by rainfall alone; such as cities and farms. Artificial dams are necessary for survival because without them, there would be no way to grow food or provide energy to living beings. Hybrid dams combine elements of both natural and artificial dams. Hybrid dams are commonly found in areas where there is not enough precipitation to support an entire community but also not enough precipitation to just leave things up to chance.