A watershed, or drainage basin, is an area of land which serves as a funnel collecting water from many smaller feeder funnels into a larger tributary which ultimately delivers the water to the ocean via a river or river system. Water is the only substance on earth that can be found naturally in three forms – solid, liquid and vapour. Water falls in the form precipitation, such as rain or snow, and then collects in different water stores, including rivers, lakes, wetlands and aquifers. A particular watershed consists of every creek, stream, river and lake that conveys water within that individual drainage basin, including the surface lands which capture, utilize and shed water. One watershed is separated from an adjacent watershed by a drainage divide, usually a geographical formation such as a mountain, ridge or hill, but on flat ground the demarcation between drainage basins is often more subtle, such as the line created by a river coursing through a landscape. Along its journey, water may be stored in man-made detention basins, such as a small reservoir, or in a retention basin - a reservoir in which water is permanently held and obstructed from flowing downstream.