The fishermen and herders eking out an existence on the shores of the majestic Lake Turkana risk having their way of life destroyed by a giant dam under construction in Ethiopia, their neighbour to the north.

Glittering jade under the scorching sun, Lake Turkana is a fragile jewel in an arid environment already hit by global warming. At 250 kilometres (150 miles) long by 60 kilometres wide at its largest point, it is the world's biggest desert lake.

"This is a precious lake, an amazingly beautiful one and maybe in 60 years from now you will not see the people here, nor the fish. and you will have a dead lake," Joseph Lekuton, a local legislator, warns.

Flowing down from the north, the river Omo supplies Lake Turkana with 80 percent of its water. Since 2006, Ethiopia has been building a dam several hundreds of kilometres upstream that will on completion be Africa's highest.

The 243-metre-high Gibe III dam will create a reservoir covering 210 square kilometres (80 square miles).

In 2006 Kenya, which struggles to cover its energy needs, signed an agreement with Ethiopia to import up to 500 MW produced by the dam.

For the people living around Lake Turkana that was seen as an act of betrayal.

UNESCO -- which classes part of the lake as a World Heritage site -- condemned the Ethiopian dam project.

China stepped in to finance the project and around 50 percent of the dam has already been built.

Crusading environmentalist Ikal Angelei, who founded the Friends of Lake Turkana pressure group in 2008, estimates that water levels in the lake will go down by two to five metres as the dam's reservoir fills up and will never return to normal.

"We are really definitely duplicating the Aral sea (devastated since the 1960s when water was pumped out to grow cotton) -- building a dam and now putting sugarcane and cotton plantations downstream in the Omo basin, all things that will reduce the amount of water flowing into the lake," Angelei said.

The surface area of the lake has already shrunk by dozens of metres over the past few years as rising temperatures have led to increased evaporation. That is in a region where temperatures already climb to over 40 degrees Celsius (104 degrees Fahrenheit) for most of the year.

-- 'Really scary to think what could happen'--

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Fighting between communities for control of watering holes for livestock and grazing land has become more common as water has become scarcer and a year ago Turkana was the area of Kenya hit hardest by the drought and famine that struck East Africa.

"We have adapted to the changes over the years and we have built a sense of resilience but now we have reached a tipping point," said Angelei, who earlier this year won the prestigious Goldman prize -- considered a sort of Nobel prize for environmentalists.

"Should we have an abrupt change, it is really scary to think what could happen," she went on, raising the spectre of local people becoming dependent on food aid or being herded into displaced camps.

Lake Turkana is "a very fragile ecosystem", and data on the dam's potential impact has been limited, according to Achim Steiner, executive director for the Nairobi-based United Nations Environment Programme.

"There is a reason to be concerned [because] the environmental assessment, the hydrological data, the models have not been as public as perhaps some would have wished them to be," Steiner said.

"If at the end the result of the dam being constructed and operated is that the ecosystem can no longer function the way it had over hundreds or thousands of years, then clearly you have a major disruption, and neither the Kenyan nor the Ethiopian authorities would like it to happen," Steiner said.

"But these things need to be studied, discussed and assessed in advance, not after the fact."

Meanwhile, some activists are already resigned to the fact that the dam will be finished and are already looking ahead to what can be done next.

"To be very honest it is only a matter of time before the Chinese release the money to complete the dam... so our next plan of action is to develop something akin to the Nile water basin whereby we would have a stake in what happens upstream," said Gideon Lepalo, director of the Save Lake Turkana Campaign project.

"I have very good memories of the lake as a child," he said, adding that it pained him that his children would not have similar memories to hold on to.