Chris Caldicott joins the Fulani herdsmen's annual migration across one of Africa's greatest rivers
The day began with turbaned Fulani riders galloping out of the Saharan haze on camels and horses. This was my kind of day. I watched their robes billow behind them until, arriving at the far bank of the river in clouds of dust, they halted in preparation for driving their steeds into the water. Behind them thousands of long-horned cattle thundered towards the bank. One of Africa's most spectacular migrations was underway.

On our side of the river the huge crowd of expectant spectators erupted into excited shouts. Their husbands, fathers, sons and brothers were returning after months away in the harsh lands of the northern Sahel, and they were bringing the wealth of the tribe with them. Women made high-pitched rhythmic warbling sounds with their tongues. Young men thumped on an assortment of drums. Children were marshalled out of the way by old men with staffs, making way for the imminent stampede.

Every year the dry season heat of sub-Saharan Africa turns pastures on the edge of the desert to dust. This is the time for the semi-nomadic cattle herders of Mali to get moving: south across the Niger river to winter grasslands. Despite the devastating droughts of recent years this still involves driving many hundred thousands of cattle across one of the mighty rivers of Africa.

As rainfall eases in the mountain rainforests of Guinea - the source of the Niger - the water level drops. Eventually it is low enough to give the Fulani good enough odds to embark on their great exodus.

The normally quiet village of Diafarabe is the location of the first and largest of the annual crossings. For a few days its shabby inhabitants suddenly see their village transformed into the colourful stage for one of the most captivating events in West Africa.

A miracle? In more ways than one. As I had just found out, getting to Diafarabe from Bamako, Mali's capital, is not just a challenge for the Fulani. No nomadic trek has ever been harder for any tourist crazy enough to try to join them.

When the Niger is still full enough, passenger steamers provide a service from Bamako to the desert outpost of Gao, passing right by Diafarabe every 10 days or so. I had travelled to Mali with the intention of taking this steamer journey, including the cattle crossing as a highlight.

The only snag was that the river cannot be deep enough for the steamer to pass, and shallow enough for the cattle to cross at the same time. Timing mattered, even down here in the Sahel.

The first stage of the journey from Bamako down to Segou had been relatively straightforward, squeezed into a battered Peugeot 504 bush taxi grinding through the dust. But to travel beyond Segou without one's own vehicle required patience and luck. There was a dirt track as far as Ke Macina; but from there to Diafarabe only vague reports of passable terrain.

Mindful of the increasing demand among the Fulani for space on vehicles heading towards Diafarabe, the owners of trucks had been biding their time in expectation of premium fares. A Frenchman, inexplicably marooned in the middle of Mali, was running the auberge where we stayed. He helped us to find a man with a truck.

We spent our days waiting, eating peppery groundnut stews and barbecued brochettes in the restaurant of L'Auberge. We took long walks along the banks of the river and spent our evenings listening to local musicians in moonlit courtyards. Everything, in fact, felt good - until the morning of our appointed departure from Segou.

Eager for an early getaway, we rose before dawn and slunk through the dark streets in search of our truck driver. We found him. But it was 12 hours until he was satisfied that the number of people crammed into the back of his truck reached a threshold of discomfort not even his greed could exceed. We drove forward a few hundred metres. Hopes of progress rose, fractionally. Then we stopped again.

This was the cue for everyone to get out into the hot night to pay their fares. The driver had to buy petrol. An animated process of negotiation began. When at last petrol had been purchased, it had to be poured into the tank by hand from a watering can.

At last we resumed our journey, but minutes later a police checkpost loomed up out of the dark. The first. Everybody climbed out again, to have their papers examined by a gang of drunk and belligerent soldiers. Nobody's papers were in order. The driver announced we would sleep the night at the checkpost and start again in the morning.

I woke to find all the male passengers prostrate in morning prayers and a faint glow of dawn in the eastern sky. In the past 24 hours we had travelled three kilometres. But now the soldiers, too sleepy and grumpy to argue, agreed a sum of money which would allow them to overlook the irregularities in our papers. A whip-round took place to accumulate the fund. My contribution seemed to be larger than most, but I was keen to get going.

The rest of the day we bumped along at a miniscule pace in intolerable heat and dust, relieved by numerous stops for prayers, police, passengers and petrol. Each stop added hours to the journey. But at least - I consoled myself - they presented a chance to step outside my metal torture chamber, stretch my cramped limbs and see the country we were travelling through.

More and more northbound Fulani embarked during the day, until all of us were bound by a common destination. After some hours a finely dressed man strolled on board, and was promptly given the best seat, in the cab next to the driver.

During the next prayer stop this unexpectedly grand passenger came to talk to me. His French was fluent. He told me that he was a Fulani chief and that I would be his guest in Diafarabe. By the time the truck had deposited us in the middle of nowhere some time during the following night, the chief and I were great friends.

I was shattered from the conditions in the back. I was also considerably poorer, given that my perceived ability to contribute

to the communal fines had increased with every police checkpost. Not that this really mattered to me: my fellow passengers had been friendly, charming and good humoured through all the suffering.

And anyway, my selfless payment of other people's fines was about to be rewarded by the generosity of the chief. The night was dark and silent. I followed the other disembarking passengers, who, as we walked, began to sing. By the time we reached water they were yelping as if in imitation of wild animals.

Out of the darkness a group of pirogues paddled to the shore. We were to be carried in convoy across to Diafarabe, unseen on the far bank. Slowly I made out its dark shapes under a million stars.

My friend the chief led me through a maze of narrow alleyways between dried mud buildings to his compound. We slept on the roof. The village was completely still, silhouettes of mosques and palm trees towered above me and it felt good to be lying down. The river crossing was due to begin the next morning.

The event lived up to all my expectations. All day wave after wave of cattle crossed the Diaka, raced over the floodplain and into the narrow streets of the village, then down into the Niger and across it to fresh pastures. The herders shouted, whistled and wielded sticks against their confused livestock. Occasionally a bull would break free of the herd and run amok to the great delight of the crowd.

At nightfall there was music and dancing. An electric megaphone powered by a tractor battery was connected to a microphone attached to the end of a flute. The flautist played with such volume, speed and feedback the effect was like a rock guitar solo. He was backed by manic drummers performing with equal volume and energy. A reunion took place of all my fellow passengers from the journey, who insisted on supplying me with food and drink like old friends.

When it was finally time to leave, the chief arranged an escape for me on a motorised pirogue downriver to Mopti. I left with a head full of cattle, and would arrive in Mopti just in time to rent a cabin on the last boat out to Timbuktu.