Chris Caldicott follows the herd through Mali
Within the next few days, an official announcement will be made that is crucial for the nomads on the fringes of the world's biggest desert. At Bamako, the capital of Mali, the government declares the date for the annual crossing of cattle over the Niger River to their winter grazing ground. This decision is also critical for travellers seeking a magnificent December spectacle.

As the annual dry season begins in sub-Saharan Africa, pastures on the edge of the desert turn to dust. This is the time for the nomadic herdsmen of Mali to drive their cattle southward. Despite the devastating droughts of the last decade, this still involves many hundred thousand head of cattle. Even at the beginning of the dry season the Niger is a substantial river. At Diafarabe the river narrows where it meets the Diaka, this is the location of the first and largest of the annual crossings.

The cattle herders spend months in the northern Sahel up to and beyond Tombouctou. Their emergence from the desert means a reunion with their wives and families.

Hundreds turn up at Diafarabe on the day of the crossing to welcome the returning men. For one day a remote, timeless village on the bank of the Niger is transformed into a colourful stage for one of the most captivating events in West Africa.

The date of the crossing changes every year, determined by the level of the Niger. To prevent complete destruction of grazing grounds the cattle must remain spread out. The crossing at Diafarabe is the first of many which take place over the period of a month, moving down river as the water level drops. A council of elders determines when and in what order the herds will cross and where they will graze.

Getting to Diafarabe to witness this spectacle is not a simple matter. When the water is high enough the village can be reached by the river steamers which sail between Bamako and Mopti. By the time the water is low enough to permit the cattle to cross, it is by definition too low to allow the passage of the steamers. Other river transport is very slow and infrequent.

Mali is one of the poorest countries in the world, the annual income per capita is not much over pounds 100 per person. Beyond Bamako, and the few towns linked to it by road, there is very little development. Electricity and running water are very rare.

Diafarabe has neither, nor does it have anything approaching a public transport system linking it with the rest of Mali.

To travel without one's own vehicle beyond the town of Segou requires much patience and luck. There is a dirt track as far as Ke Macina; the last section from there to Diafarabe is increasingly vague. The vehicle I eventually found going in this direction after three days in Segou was an old Parisian delivery van, sold off after a trip across the Sahara a decade earlier. The 200-kilometre drive took 16 hours.

The journey began, as so many do in Africa, after much delay. Once the driver was satisfied that 29 people in the back of the van reached a threshold of discomfort not even his greed could exceed, we drove a hundred metres and spent a further half an hour pouring petrol into the van. Another hundred metres to the police check post for another stop of half an hour, then we left Segou. We continued in a similar vein until, in what appeared to be the middle of nowhere, in the middle of the night, myself, a man who claimed to be the chief of Diafarabe, and half a dozen other passengers disembarked.

The "chief" invited me to be his guest once we reached his village. This involved waiting until two pirogues (dug-out canoes) emerged out of the darkness, after much shouting to summon them. We waded out to them and were paddled across the Diaka, under a million stars, to the dark shape of Diafarabe. No electric light or unnatural sound challenged the medieval atmosphere as I was led down a maze of narrow alleyways between mud buildings to the "chief's" compound. I slept on a flat roof, under the desert sky.

The fresh cool air of dawn brought more delights: a rising sun revealed a village of flat roofed mud houses between palm trees, and a dramatic towering mosque. Fishermen were casting their nets into the Niger and breakfast fires were being lit - the place was alive with the sounds and smells of ancient Africa.

As Diafarabe is located on a peninsula between the Niger and the Diaka, the cattle cross both rivers to pass through the village. The Diaka is crossed first and this is where most of the activity takes place. It was rumoured that the crossing would start at 9am. By 10am there were hundreds of people gathered along the cliff above the river. A handful of dignitaries, ministers from Bamako, high ranking military and the ambassador of Burkina Faso were seated under a makeshift shelter. And there was a film crew from Mali Television - that night the event was to be lead story on the national news. By 11am there were thousands of cattle gathered on the far bank marshalled by men on camels and horses, dressed in desert robes and turbans, Half an hour later the crossing began.

For the rest of the day, wave after wave of cattle came across in small groups with their herders. Sometimes swimming, mostly wading through the water, then stampeding over the floodplain towards the cliff. There is only one break in the cliff where the cattle may ascend to the village. This creates a bottleneck of converging beasts - and a cacophony of noise and dust as they jostle to pass. It is the most popular place to watch the crossing from, especially among the children.

The herders shouted, whistled and wielded sticks against their confused livestock. Occasionally some of the cattle would break away from the main group and run off out of control, to the great delight of the crowd. As the day passed, most of the audience drifted back into Diafarabe where a small stage had been erected from which the dignitaries were to make speeches. After that there was a presentation of the finest bulls. Over excited, with their horns painted in bright colours, they charged into the crowd, causing a great deal of chaos and laughter.

At nightfall there was music and dancing. A megaphone powered by a truck battery was connected up to a microphone which was attached to the end of a flute. The flautist played at such volume and speed, and with so much feedback, that his instrument sounded more like a heavy rock guitar than a flute. He was accompanied by some manic drumming, performed with equal volume and energy. By comparison, the dancing was tame. Three girls, with their backs to the audience, shuffled about while small boys followed them around, shining torches on their shoulders. Meanwhile, the musicians became more and more animated, rolling about on the ground and leaping in the air as they played. Some women from the audience joined in the dancing. With sudden bursts of energy they bent down to the ground, then whipped their torsos through the air, arms waving until they were bent over backwards.

Getting away from Diafarabe is no easier than getting there. Occasionally motorised pirogues call in on their way to Mopti; no one knew when there might be one. There was no more certainty about finding a vehicle back to Segou. There was, however, a truck leaving the next day, in the same direction as the cattle, to Djenne. So I followed the herd.

The road to Djenne, via Sai, was as slow and non-existent as the one between Ke Macina and Diafarabe. Djenne was worth all the hardship of the journey. A 15th-century settlement of the same vintage as Tombouctou, yet much better preserved and more lively. The Sudanese mud architecture is the finest in the Sahel. The mosque is particularly impressive. The Monday market is held in its shadow: a collection of people as colourful in their attire as those at the cattle crossing, in from the villages, and from as far as Mopti gathering to sell their wares. I found a country boat, a pinasse, leaving that night. So I climbed aboard and set out for Mopti - if you're flexible about arrangements you can go far in Mali.

Mali travel essentials

Getting there: the Niger river is not the easiest place to reach from Britain. You can travel to the capital of Mali, Bamako, via Paris on Air France. Until the 15 December, the Africa Travel Centre (0171-387 1211) has a fare of pounds 505 including tax from several UK departure points to Bamako. You must stay away at least 10 days, but no longer than one month.

Getting around: the Thomas Cook Overseas Timetable contains some details of travel within Mali but there is a lot to be said for being flexible.

Tours: Explore Worldwide (01252 319448) has an 18-day tour of Mali departing on 10 January, costing pounds 1,365 including flights, accommodation and some meals.

Red tape: British passport holders must obtain a visa in advance through the Embassy of Mali in Belgium, at 487 avenue Moliere, 1060 Brussels (00 322 345 7589).