"It's better to kill us first," says Olikoro, a Mursi tribesman, naked apart from the piece of cloth slung over his shoulder. An AK-47 rests by
his side. He is talking about the Gibe III dam, the latest in a series being built along the Omo River in south-western Ethiopia.

In Addis Ababa, the capital, the dam is considered essential for progress. But in the Omo Valley, far downstream of the dam's planned location, people depend on the river that begins in Ethiopia's emerald highlands, dropping through steep gorges before twisting towards Lake Turkana on the border with Kenya. Fifteen tribal groups depend on the seasonal floods to nourish their crops of maize and sorghum, and to provide grazing for their cattle. Gibe III will affect half a million lives. "If the dam is built, we will die," is how Olikoro puts it.

Yet along the Omo River, many of the people I meet don't even know that a dam is being built. "The government has no interest in these people," says Terri Hathaway, of the environmental organisation International Rivers. "The fact that many wander around wearing few clothes is an embarrassment to officials." When the government began building the dam, environmental impact assessment papers were prepared. However, little mention was made of the people living downstream.

Ethiopia needs electrical power if it is to develop quickly. At a cost of $1.7bn, Gibe III will be the country's biggest-ever infrastructure investment and one of the world's largest dams. Gibe I and II have already been built; IV and V are planned. They will allow expansion of the national grid and should stop the power shortages that have hampered manufacturing output. Ethiopia has few exploitable natural resources, but its river basins and high central mountains have huge potential for hydro­power. Energy can be exported to neighbouring Kenya and Sudan.

“Anyone opposed to the dams should suggest alternative solutions to creating vast amounts of energy to feed the fastest-growing non-oil economy in Africa," says Gail Warden, an official at the Ethiopian embassy in Nairobi.

But, in the short term, the extra power will mostly benefit those in the cities. The communities living along the Omo will still have no electricity. "We know the power is not for us," Olibisini, a Mursi elder, tells me. "We would prefer the river." Yet the government maintains that local communities stand to gain over time. "Electricity is essential for rural transformation, providing the basis for businesses in small towns and mechanised agriculture," says the energy minister, Alemayehu Tegenu. "Children need light for studying. We have identified 6,000 rural towns and villages in an ambitious rural electrification plan, penetrating half the country within five years."

These days, it seems as though everyone wants a piece of the Omo. Missionaries pour in, as do tourists in 4x4s. Recently formed national parks along the river limit the space for crops and grazing, and the area is being explored for oil. The tribes already fight over increasingly scarce water and land - but the dam could plunge them into more serious conflict. Weapons, which continuously flood over the border from Sudan, are worn like handbags.

Gibe III is more than just a problem in Ethiopia: its aftermath will stretch to Kenya. Approximately 300,000 Kenyans rely on Lake Turkana for their livelihood, catching tilapia, Nile perch and catfish. Reduced water flow will cause the lake to shrink and become saltier, destroying its ecosystem.

Wash away

The Ethiopian government has promised an annual ten-day artificial flood to help the farmers. But experts doubt this will fix the problem. "The natural flood builds slowly, rising and falling over several months, depositing nutritious silt all the time and letting the moisture sink in deep," explains David Turton, an anthropologist specialising in Mursi culture. "It's difficult to believe that ten days will be enough. It will act like a flash flood, washing away the silt and causing erosion."

The government insists that big, natural floods are damaging. But Ashote, who belongs to the Dassenech tribe, disagrees. "Big floods are celebrated," he tells me. "We just move to higher land when the floods come."

Last year's flood was not big, and even though my visit is during harvest time, cultivation sites along the river lie empty. Many people are hungry, selling their cows to buy grain or living on blood and milk. "What you see is the result of a low flood, a foretaste of what is to come if the dam is constructed," warns Will Tate, an Addis Ababa-based expert in displacement. "In the future, there will be starvation, economic loss and death."

Irrigation schemes have been proposed, but the idea makes Shoro, of the tiny Kara tribe, laugh. "We have so much experience of the government promising us things but never seeing them," she says. Food aid has also been pledged. "We don't want relief, we want food made with our own hands."

The dam's critics are urging financial institutions not to fund the project. The European Investment and African Development Banks are carrying out studies on its impact. Some still hope that the project can be stopped, but Turton says it is too late. Now, he says: "The crucial thing is for donors to make a condition of real compensation for the people downstream. They should be the main beneficiaries."