By Peter Greste
BBC News, Ethiopia
Most people in Ethiopia's lower Omo River Valley continue to exist much as they have done for hundreds of years with virtually no concession to the 21st Century, with one disturbing exception: automatic weapons.
Almost every male carries a Kalashnikov or an M-16 assault rifle, and what might in the past have been a fairly innocuous dispute over grazing or water-rights between different groups, now frequently escalates into bloody warfare.
Some fear the potential for dispute could be about to increase, because a huge dam - the second biggest in sub-Saharan Africa - is being built upstream.
The government denies that the river's flow will be affected and indeed says the Gilgel Gibe III Dam will reduce flooding.
"It increases the amount of water in the river system. It completely regulates flooding in the Omo, which has been a major problem," said Prime Minister Meles Zenawi.
But local people - and some academics - simply don't believe it.
The Mursi people are one of about two dozen groups who depend, either directly or indirectly, on the river and its annual cycle of flood and recession for their survival.
They are famous for the coaster-sized clay disks that the women insert into their ear lobes and lower lips.
In the shade of a fig tree, a group of Mursi elders gathered to discuss "rumours" of the dam.
One of the senior community priests, Bargaeri, said although they were aware of the dam, they had heard nothing official.
"We will suffer because there will be no more floods," he said. "I don't think the government likes the Omo tribes. They are going to destroy us."
The floods lie at the very heart of the dispute over the dam.
The government plainly believes they will continue pretty much as they always have, except that the dam will allow the authorities to manage the timing and the height of the flood in a way that nature never did.
Richard Leakey - the renowned ecologist and most vocal critic of the dam - was blunt in his assessment of its consequences.
"My problem is that the dam is going to affect a huge number of people who have no voice, a huge number of people who will fight over the decreasing resources.
"Innocent people will be killed in conflict over those resources, and I don't believe it is necessary."
Mr Leakey's criticisms echo those of a collection of European, American and East African academics who have banded together as the "African Resources Working Group".
The group has released a highly detailed commentary on the electricity company's environmental impact assessment (EIA) that criticises almost every element of both the dam and the study.
In a section dealing with the impact on indigenous communities, the commentary asserts:
"Additional dispossession and disruption of the ethnic groups of the lowermost Omo basin, from the planned irrigation agricultural schemes and industrial projects described in the downstream EIA and planned by the Ethiopian government
will precipitate waves of new conflicts among groups already competing with one another over the shrinking natural resource base available to all of them."
The Nyangatom is amongst the most heavily armed of the communities in the Omo Valley.
Half of the group lives over the border inside South Sudan, where most young men fought with the rebel Sudan People's Liberation Movement during its long civil war with Khartoum. They brought back training, experience and weapons, raising the stakes even further.
Mursi women are famous for their lip-plates
In the village of Kangaten, the Nyangatom's elder spokesman called Kai shook with rage as he condemned the authorities.
"Let them first bring helicopters to kill us all; then the government can build its dam," he said.
Another elder bluntly declared: "If the river goes down, there will be war."
According to anthropologist Marco Bassi, of Oxford University, the tribes have developed sophisticated agricultural techniques that have allowed them to live comfortably and sustainably for centuries.
Each wet season, the riverside communities retreat to higher ground, waiting for the flood that inevitably comes.
Once the waters retreat, the communities move back to plant their crops on the damp and newly replenished soils.
Their cattle feed on the fresh grasses. The higher the flood, the more land is inundated, and the more becomes available to farm.
Even the highest of floods are necessary to replenish the outlying bush lands that the communities use to feed their livestock during the equally inevitable droughts.
"It looks very primitive from the outside," Mr Bassi said. "But when you investigate it, you discover that they have a very intimate knowledge of the land and its fertility.
"Each family has maybe seven or eight different varieties of sorghum that responds to different conditions. And combined, the community has 20 or 30.
"They know how to plant in a way that guarantees enough food whatever happens through the year."
But the tribal lands have become increasingly squeezed between newly gazetted national parks and large commercial landholders, and growing populations on the other.
The government has promised irrigation schemes as a way of mitigating any negative effects of the dam, but that too is dismissed by the community elders like Mursi priest Biyatongiya.
"It's not true," he said. "I haven't seen anything like irrigation before. They're just lying to us. Maybe they come and tell us these things but it's not true."
And anyway, Marco Bassi doubts the communities will be able to adapt to irrigated farming which would mean a wholesale transformation of their centuries-old nomadic cultures.
"The issue is how to empower these communities to face this change in a way that they can manage. How do you empower, enable these people to deal with this change?
"Under the current circumstances, they will not be able to do that
Simply, they will die."