Lakes contain about 90% of all the surface water found on Earth (not including oceans). Lakes form when water finds its way into a basin. In order to continue existing, lakes must have a continual source of new water, otherwise they will eventually dry up.
Most lakes contain fresh water. However, in some cases, the water found in a lake can become salty, just like the ocean. This happens when a lake does not have a stream, either above ground or underground, draining water away from it. As water enters a lake, it carries minerals with it. As this water dissolves, it leaves the minerals behind.
Most lakes only last a few thousand years and then disappear. This is because as streams and rivers carry water into the lake, they also carry sediment. This sediment slowly fills the lakes, causing them to become shallower. At the same time, outbound streams cut deeper and deeper channels, causing the lakes to drain more quickly.
Many lakes are man made. These are referred to as reservoirs. Reservoirs allow cities and nations to store water for later use. Most of these reservoirs are small in size, but some are very large, spanning several hundred miles, or kilometers.