The lake, which provides the livelihood for some 30 million people in the shoreline countries of Kenya, Tanzania and Uganda, has suffered a dramatic fall in water levels since 2003.

Levels have plummeted 1.2 metres (3.9 feet), bringing the lake to its shallowest since 1951, exposing muddy banks that have stranded ferry boats and fishing vessels and causing water shortages for shoreline towns and farmers.

A total of 75 cubic kilometres (18 cu. miles) of water, equivalent to about three percent of the lake's normal volume, has been lost in just three years.

The report says that more than half of the lake's drop in 2004 and 2005 is attributable to the Owen Falls dams in Uganda, across the only outlet for the mountain-fringed lake.

In breach of an international agreement, the dam's operators have been releasing more water than the lake can sustain, it charges.

The reason, says the report, is to maintain sufficient flows of water over the dam's turbines and keep the lights on in Uganda.

"The resultant over-release of water... is contributing to the severe drop in water level in Lake Victoria," says the report, written by Daniel Kull, a hydrological engineer based in Nairobi, for International Rivers Network, a US green group.

The Owens Fall hydroelectric project dates back to 1954.

Until then, the lake spilled out over a natural rock weir, to form the Victoria Nile, which eventually becomes the White Nile.

Britain, Uganda's colonial power, blasted out the weir and replaced it with the first dam, now called the Nalubaale dam, thus effectively transforming Lake Victoria into a vast hydroelectric reservoir.

At the time, engineers agreed a common-sense formula that the flow going through the dam should mimic that of the flow that went over the weir, a rate that ranged from 350 to 1,700 cubic metres (12,250-59,500 cu. feet) per second depending on the water depth in the lake.

The formula, called the "Agreed Curve", remains in force today under a treaty with Egypt, the downstream user of most of the water from the Nile.

In 2000, Uganda built an extension to the Nalubaale dam, called the Kiira extension, to generate more power. The scheme was backed with money by the World Bank, a favourite target for environmentalists for its traditional favouring of big hydro projects.

The turbines are located below those of Nalubaale, using the same "head", or water drop, which is channelled via a 1.3-kilometre (one mile) long canal.

But the demands of the Kiira extension, magnified by a reduction of inflow to Lake Victoria caused by a 10 percent fall in regional annual rainfall and tributary flow, have made it "difficult, if not impossible" to adhere to the Agreed Curve, says Kull.

According to his calculations, 55 percent of the severe drop in Lake Victoria's levels in 2004 and 2005 was due to letting out too much water to feed the Nalubaale and Kiira turbines. Only 45 percent of the fall could be blamed on the drought.

The report raises serious doubts about Uganda's overwhelming dependence on the big dams, given that global warming may lead to drier conditions and lower the lake levels further still.

"It is unknown if Lake Victoria will recharge to the high levels and outfloow experienced during 1961-2000, and if such a recharge could occur, whether it would be in the next years or only in 100 years," it says.

Frank Muramuzi, a member of a Ugandan NGO, the National Association of Professional Environmentalists (NAPE), said the dam complex "is now pulling the plug on Lake Victoria, with implications for millions."

Asked to respond to the report, Ugandan Information Minister James Nsaba Buturo said on Wednesday: "We have no evidence at all that the second dam is the cause of the low water levels. What we have from experts is the prolonged drought as the reason for the low water level."

He added: "Uganda is open to any advice if what we think is the cause is not what it is."

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