Landscape under attack: Changing shape of Africa's lakes

"Business is very bad; there is no fish here any more," said John, one fisherman repairing his nets on the lake. "Everything in this area is getting worse."

His gloomy observation is backed by research that reveals an unfolding catastrophe.

Satellite photographs taken by Unep, the UN Environment Programme, show that more than 600 lakes in Africa have shrunk dramatically over past decades, drained by deforestation, pollution and farming.

Water levels at Lake Victoria, source of the White Nile, have dropped by a metre in 10 years. The Kenyan and Ugandan lake supports over 30 million people - one of the densest and poorest populations in sub-Saharan Africa - and has suffered badly from an invasion of water hyacinth, which clogged shipping lanes and destroyed the region's fishing industry.

Most of the water hyacinth has now been cleared from the lake, but local populations, most of whom are subsistence farmers, now face problems of soil degradation, and agronomists estimate that 150,000sq km of land has been affected. People like John, who grow their own food to eat at times when the fishing business is slow, struggle to feed their families.

The Unep photographs and research, released to coincide with the start of the 11th annual World Lakes Conference in Nairobi, warns that some of the continent's greatest freshwater lakes could turn into swamps if action is not taken soon.

Klaus Topfer, the executive director of Unep and a former German environment minister, said: "I hope these images of Africa's lakes will galvanise greater action to conserve and restore these crucial water bodies. Economically, lakes are of huge importance. I also hope these images will ring a warning round 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 precious resources."

One of the starkest changes can be seen at Lake Djoudj, 60km from St Louis in Senegal. The waters were once a series of thin lakes surrounded by streams, ponds and backwaters that provided a haven for three million birds, including the great white pelicans and the Arabian bustard. But since the Diama Dam was built in 1986, barely 23 kilometres from the mouth of the Senegal river, the lake has changed shape dramatically, and local farmers have moved from seasonal, flood-based farming to year-round irrigation-based agriculture that wreaks further havoc with water levels.

The report also reveals how salt-mining has destroyed Lake Songor in Ghana. The lake, home to rare turtles such as the olive ridley, was once clearly visible from space as a solid blue mass of 74sq km of water in 1990. Ten years later, the surface area has shrunk.

Man-made structures such as the Cabora Basa dam in Mozambique have also altered the flow of the Zambezi river, source of the Victoria Falls, southern Africa's main tourist attractions. Mr Topfer also warned that water shortages could lead to political tensions between countries that have to share diminishing water resources. All African lakes except Lake Tana in Ethiopia are shared across international borders, and national governments will find it increasingly difficult to agree how to share the waters.

The Volta river basin in west Africa, shared between Benin, Burkina Faso, the Ivory Coast, Ghana, Mali and Togo, is one potential flashpoint. The population around the Volta is projected to double to 40 million within 20 years, and rainfall and river flows in the region have declined steadily over the past 30 years.

A Unep report on hydropolitical vulnerability and resilience along international waters in Africa warns: "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. With the sustainability of the Volta basin under threat, there is urgent need for basin states to co-operate more closely to jointly manage the basin's water resources."

The disappearance of freshwater lakes will also have a dramatic impact on the region's economy. Africa's lakes hold 30,000 cubic kilometres of water, and yield 1.4 million tonnes of freshwater fish each year. But dams, sewage and industrial pollution have cut the amount of fish caught, especially in the Nile delta and Lake Chad.

The report also warns that wetlands crucial for wildlife have been destroyed and countries such as Niger have lost more than 80 per cent of their freshwater wetlands over the past 20 years.

Sub-Saharan Africa suffers from erratic rainfall, and large numbers of people depend on groundwater for drinking and farming. If the lakes, which supply most of the ground water, dry up, swaths of the continent would become uninhabitable. The UN estimates that already two thirds of the rural population and a quarter of the urban population in Africa are without safe drinking water.

The water shortage facing Africa is compounded because the continent has the highest rate of population growth in the world, and is also one of the regions most vulnerable to climate change. At least 25 African countries are expected to experience severe water shortages over the next 20 to 30 years. The continent still has a hugely unequal distribution of water, and large numbers of people have to walk long distances each day to fetch fresh water.

The companies and farms that do have access to water are given no incentive to use water carefully or to recycle it. The report estimates that as much as 90 per cent of Africa's water is used in farming, of which 40 per cent to 60 per cent is lost to seepage and evaporation.

A group of scientists and environmental experts launched a campaign, which coincides with the World Lakes Conference, to save the region's five most endangered lakes.

The African Living Lakes Network warned that not only were Lake Victoria, Lake Tanganyika, Lake Malawi, Lake Chad and Lake Tana losing water at an alarming rate, but also that the quality of the water that remained was deteriorating.