The scale of it is hard to comprehend. Even standing on the dented roof of an old jeep in the Serengeti and seeing hoofed animals stretching out to the horizon in all directions only affords you a glimpse of it. The great wildebeest migration is underway and more than a million of them, together with hundreds of thousands of zebras, are heading north from the endless plains of Tanzania's Serengeti region into the pastures of the Masai Mara in Kenya, seeking water and grass.
The quintessential image of the migration is the crossing of the Mara river: the surprising vulnerability of the wildebeest horde as they scramble down one bank and up the other, their fierce horns and grey-bearded heads turning nervously in search of predators. It's a spectacle routinely referred to as the seventh wonder of the world and one thatdraws tens of thousands of top-dollar tourists to both banks of the Mara every year.
But this year there is something missing – the water. "This is the first year we've ever seen the river this low," says Will Deed, who works with the Mara Conservancy, a not-for-profit group which manages one third of this huge reserve.
"In parts there's just small channel, one or one-and-a-half feet deep." In the same stretches of the river last year the water was as deep as five feet and "the wildebeest and zebra were up to their chests or necks, or even swimming," he adds.
"Normally it's an incredible ordeal for the wildebeest, with crocodiles waiting for them in the river."
At this point of the animals' migration north – which begins in earnest in early August – the ordeal has been reduced to a short hop and the grateful wildebeest can move on to the rich, green grazing of the reserve itself. "What we are seeing is not necessarily the end of the migration but it is the end of the spectacle of the crossing," says Mr Deed.
The drying-up of the river, which should be at its highest point at the end East Africa's long rainy season, is one of a series of ominous signs that conservationists believe could add up to an ecological disaster.
The sun-scorched boulders that ring the shore of Kenya's Lake Baringo are cut by a sharp brown line, running horizontally, that shows the watermark of the past. Beneath the dark divide is an expanse of white stone freshly bared to the elements as the lake has receded dramatically.
A report released last week by Kenya's Water Resource Management Authority has dismissed any hopes that these phenomena could be unrelated.
In the report, Simon Mwangi, the authority's Rift Valley regional technical manager, said that the River Perkerra, which feeds Lake Baringo, and the Malewa which drains into Lake Naivasha, were at their lowest levels on record.
The picture was similarly bleak, he reported, with the Ewaso Nyiro and Mara rivers. The country's great lakes from Turkana in the north to Nakuru, and the economically vital Naivasha, home to Kenya's flower industry, were also alarmingly low.
A similarly bleak report on endangered species and mass migrations from the American Museum of Natural History last month has even stirred fears of a repeat of the Emutai ("wipe-out") -– the ecological shock that struck the region inhabited by the Maasai ethnic group at the end of the 19th century, hitting people livestock and wildlife. The results were described in 1891 by the Austrian explorer Oscar Baumann: "There were women wasted to skeletons from whose eyes the madness of starvation glared ... warriors scarcely able to crawl on all fours, and apathetic, languishing elders."
The answer to the riddle of the Mara's dwindling waters, and the general drought conditions, lies upstream, in the Mau forest.
The Mara river originates on the Mau escarpment, eventually draining into Lake Victoria. The largest remaining forest in Kenya, Mau functions as a water tower for the East African country, feeding rivers and helping to regulate rainfall.
However shocking reports this year have a revealed that the eco-system is under siege from illegal loggers and land-grabbing farmers, as well as large and small recipients of political patronage. Many of those are smallholders receiving parcels of the forest as "land for votes", while Kenya's Daily Nation newspaper named two of the children of former president Daniel arap Moi as among the bigger owners.
There effect is a devastating fragmentation of what environmentalists call an ecological utility whose services stretch from watering Kenya's tea estates to feeding the rivers powering its hydroelectric plants, and regulating temperature and rainfall throughout an often arid land. Despite being home to the headquarters of the United Nations Environment Programme, Kenya has systematically ignored warnings over the importance of conserving the Mau forest. While the troubled coalition government in Nairobi has belatedly begun to recognise the problem, it has so far done next to nothing about it.
Wangari Maathai, the veteran Kenyan green campaigner and Nobel laureate, says the destruction of the Mau and other forests is possibly more damaging to the region than climate change."Life is unsustainable in East Africa without these environmental services from forests," she says. The former MP has launched a campaign called "Enough is Enough", urging the government to reclaim grabbed forests, rivers and wetlands. The American Natural History Museum study also outlined the effects of serious water loss. "Herbivore populations would almost certainly decline, carnivore populations would follow" and the Serengeti would lose tourism dollars, it said.
While there is scientific agreement on the importance of restoring the Mau for both Kenya's economy and environment, vested interests have so far managed to block the feeble power-sharing government, as land grabbers angle for lucrative compensation for the resources they hold.
Downstream, amid the luxury tented camps and lodges of the Masai Mara reserve, the extraordinary appeal of the great migration has helped to insulate revenue from the global recession. Occupancy rates for August have been close to 100 per cent. But the squadrons of Safari Land Cruisers on the river banks will not get quite the spectacle they came to see this year "During the migration we are always packed " says Mr Deed. "But you can go on much cheaper safaris in South Africa, and high-end ones in Namibia."
The portends are not good to the north of the Mara, where a smaller wildebeest migration from the Loita plains into the wetlands of the reserve has now stopped. A mixture of farming, fencing and water shortages are to blame.
For now the continued rainfall within the reserve means that the estimated 1.3 million Connochaetus taurinus, or common wildebeest will keep coming crossing from the Serengeti, even if there's not much of a river to cross. But in the longer term, there are profound choices to be made, scientists warn, if these great natural life cycles are to be preserved. As the US report concludes: "Eradicating migrations and relegating migrants to zoos or fenced parks represents one of the worst examples of destructive human impact."