First there is the endless mooing, then the parade of hundreds and hundreds of cattle swimming across the Tana River, their heads bobbing above the brown water. The bulls and cows keep coming in wave after wave for about an hour.

Draped in red cloth and carrying long sticks, herdsmen lead the cattle up the riverbank and into this drowsy farming village, where women are shucking corn and men are surveying their crops. The herders try to be mindful of the crops and houses as they lead their livestock both around the village and along a path that winds among the farmers' homes.

The farmers just shake their heads.

"Why are they here again?" asks Emma Gwinyo, 27, as she watches the cattle stomp over the muddy earth. "Their animals are going to eat all of our crops. They never go away. We don't want to share with them."

Here along the Tana River, and in countless other places across Africa, farmers and herders are in conflict. Their ways of life, as old and familiar as any on Earth, differ in almost every way imaginable, yet they have one thing in common: They depend on water and land for their livelihood.

Often these natural rivals live side by side, competing for the same resources. Sometimes the rivalry turns violent, as it did here on March 7, 2001, when a fight over one patch of land near the Tana River ended with 100 people being slaughtered.

"This is our wealth and nothing else," Kolde Abashora, 58, said while herding his cattle out of the river. "We know there are tensions. But we can't graze in the air."

All over the developing world, the struggle for scarce resources causes disputes that sometimes lead to well-documented wars and sometimes to skirmishes that never make headlines.

Last month, for example, fighting broke out in northeastern Congo between the traditionally pastoral Hemas and the Lendus, who are mostly farmers. Violence between the two groups has cost hundreds of lives over the past four years. The same week as the Congo violence, eight people were killed in Nigeria when a group of gunmen identified by locals as Muslim Fulani herdsmen attacked the Christian farming village of Maza.

Not quite a decade ago, one of modern Africa's bloodiest episodes erupted from similar causes. Hatred between Rwanda's Tutsi and Hutu tribes, stemming from colonial rulers' preferential treatment of the pastoralist Tutsis over the agriculturalist Hutus and from their competition for land and water in the tiny country, erupted into genocide in 1994. A state-sponsored campaign of killing by Hutu extremists led to the slaughter of more than a half-million Tutsis in 100 days.

Here in Ngao, the people are thankful that violence has been absent for more than a year. But "the tensions are here and will be here," said Abraham Daniel Mabombe, the chief of the area, who toured the village one recent day, supervising the reconstruction of houses burned in the clash last year. "The farmers want land and water to grow crops," Mabombe said. "The herders want land and water to raise their cattle. Does that make conflict hard to solve? Well, yes. Very. But we try anyway. We are aware of the problem."

Some of the tensions stem from the fact that the herders and the farmers have completely different economies. Many of the farmers grow food only to feed their families. They usually settle on plots of land for generations. They have strong communal responsibility, rarely selling what they grow to the outside world.

In contrast, cattle herders are nomadic. They are driven to different regions by where they can best feed and water their animals. They operate as capitalists, selling their cattle for around $300 each when they need cash. They are unattached to the land, ready to move around the country to earn a living.

Culturally and physically the groups are also stunningly different. Most cattle herders are from Nilotic tribes, descended from the tall, slender, narrow-featured peoples of the Nile Valley. Farmers typically are shorter and stockier because their roots extend from the Bantu tribes that migrated from western Africa to almost every part of the continent below the Sahara.

In a village called Sare B, herders of the Wardei tribe are regally draped in colorful gold and red locally made cloth. They are Muslims, and the women cover their hair with shimmering scarves that flow over their long bodies.

The children wear bright silver hoops in their ears, and the women wear beaded bracelets. The men wear used wingtip shoes with no laces and walk with perfect posture as they herd their animals. The women collect water in gallon jugs from a murky stream, about a 20-minute walk from the village.

In neighboring Sare A, the short, muscular farmers of the Pokomo tribe are dressed in ripped, secondhand T-shirts and pants from the United States. Some are barefoot.

Although many herders are aloof, the farmers are talkative, even effusive. Unlike the herders, who speak only their native language, the farmers speak English. They are Christian, and missionaries built a school for their boys and girls, and a health center for the farmers; the herders, who are building a mosque in their village, have no health center or schools for girls.

The farmers say the herders are unclean because they deal with animals. The herders say the farmers are too aggressive and loud, and the women are too chatty with men.

But with all their differences, "both groups want the same things: water and land," said the Rev. Sampson Maliwa, who has worked on a reconciliation project in the area.

Maliwa said that the weather played a part in exacerbating tensions here. A lack of rain has dried up smaller streams and forced herders to rely on the river more than ever. At the same time, the government started deeding land to the farmers, who had never held land titles. The herders complained that they were losing access to good grazing land.

On March 7, 2001, a 56-year-old schoolteacher named Ephriam Kiokomo went to farm his government-issued parcel of land along the Tana. A herder also showed up to graze his cattle. The two men argued, according to local officials, and Kiokomo was found dead, killed with a spear.

"That very day the violence spread all over the Tana River," said Mabombe, the chief. "The farmers came with machetes. The herders had bullets."

Now the herders say the fighting is over. They don't want to talk about it. "The only thing that bothers us is they got more help from the government rebuilding their huts then we did here," said Dende Wachu, a herdsman and village elder.

Hostility simmers on the farmers' side as well. "We will not buy their milk," said Peter San Umur, a farmer in Sare A. "We can't share anything with them."

Aid workers and a local government chief say they are hopeful that community talks and several projects will ease tensions. The Kenya Red Cross and the Swedish Red Cross are working on a project to build 35 wells to lessen both tribes' dependence on the river.

In the villages, the wells are welcome, because the resource that everyone is fighting over is in fact a murky brew of brown water that causes diarrhea and other waterborne illnesses.

"Clean water means health here," said Erik Pleijel, a well project manager with the Swedish Red Cross. "We are hoping the wells will really ease some of the tensions."

Aid workers said the two groups would not have to share any of the new wells.

Hundreds of cattle swim across the Tana River in search of better grazing lands near the village of Ngao, Kenya, leading to conflict with local farmers.Farmers in Ngao are rebuilding houses burned in last year's clashes with herders. There have been no recent outbreaks of violence, but says the area's chief, "the tensions are here and will be here."