African lakes are among the worst affected, with satellite images unveiled on Monday showing dramatic differences between the extent of some lakes and rivers today and their extent a few decades ago. Lake Chad has shrunk by almost 90 per cent, while water levels in Lake Victoria – Africa’s biggest freshwater lake – have fallen by a metre since the early 1990s. Niger has lost more than 80 per cent of its freshwater wetlands in the past 20 years.

Klaus Topfer, executive director of the UN’s Environment Programme, said: “Economically, lakes are of huge importance. I hope that [the satellite images] will ring a warning around the world that, if we are to overcome poverty and meet internationally agreed development goals by 2015, the sustainable management of Africa’s lakes must be part of the equation. Otherwise we face increasing tensions and instability as rising populations compete for life’s most precious of resources.”

Lakes and waterways are potential flashpoints for conflicts between countries, as the pressure of demand for fresh water fuels disagreements over how to share the resource. Delegates at the 11th World Lake Conference in Kenya, heard that the Volta river basin in west Africa, shared by Benin, Burkina Faso, Ivory Coast, Ghana, Mali and Togo, was a particular source of concern. Population levels around the basin are expected to double to 40m over the next two decades, causing a sharp rise in the demand for water. Rainfall and river flows in the area have declined over the past 30 years, partly due to climate change.

A report from the UN programme and the University of Oregon in the US concluded: “Current water use patterns in the Volta basin have already stretched the available resources almost to their limits and it will be increasingly difficult to satisfy additional demands.”

Legal agreements among countries sharing water sources should be strengthened in order to avoid potential conflicts in the future, the report’s authors said.

Countries can also gain from managing their inland waters for tourism and sports, as well as agriculture and drinking water. For instance, in the US, the recreational value of fresh waters for activities from rafting to fishing is estimated at about $37bn (€31bn, £21bn) a year.