A lake is a body of standing water, defined by the geography of an area. Flow may be reduced by low relief or by a narrowing of the channel, in either case allowing water to accumulate (Pidwirny 2006).
A reservoir is a body or water that collects behind a dam wall or weir. These man-made barriers to flow are usually erected to gather water for water supply for domestic, industrial or agricultural purposes. The controlled release of water can also be used to generate electricity. Dams can vary in size from small farm dams, designed to retain small bodies of water for irrigation and livestock watering, to enormous dam walls constructed by national water authorities.
The amount of time water stays in a reservoir or lake is known as the residence time; the time it takes to change all the water in a lake or reservoir is known as the replacement rate. Residence times and replacement rates can range from years in large natural lakes, to weeks in large reservoirs, and days in run-of-the-river dams.
The reservoir at Massingir Dam, at sunset.
Source: Hatfield 2009
( click to enlarge )
