Such information is very valuable in remote regions and poor countries. It shows how water levels are affected by farming and weather. It also gives warning of floods to people living nearby.
Satellites also provide important information about the health of lakes, including water temperature, colour and pollution.
One of the most dramatic examples is the Aral Sea in Kazakhstan. Once the fourth largest lake in the world, it has shrunk to less than half its original size over the last 45 years.
Radar instruments in space can also measure the changing heights of lakes and the rivers that feed them. In most of the Aral Sea, the water level has dropped 14 metres since the 1960s. However, efforts to raise the water level and reduce the saltiness in the northern part of the lake have had some success in recent years.
Last update: 15 November 2010