The great Serengeti wildebeest migration is the movement of vast numbers of the Serengeti's wildebeest, accompanied by large numbers of zebra, and smaller numbers of Grant's gazelle, Thompson's gazelle, eland and impala. These move in an annual pattern which is fairly predictable. They migrating throughout the year, constantly seeking fresh grazing and, it's now thought, better quality water. The precise timing of the Serengeti wildebeest migration is entirely dependent upon the rainfall patterns each year.
Around the month of June, the wildebeest migration is often halted on the south side of the Grumeti River, which has some channels which block or slow their migration north. The wildebeest then congregate there, in the Western Corridor, often building up to a high density before crossing the river. The river here is normally a series of pools and channels, but it's not continuous – and so whilst they always represent an annual feast for the Grumeti River's large crocodiles, these aren't usually quite as spectacular as the crossings of the Mara River, further north.
The wildebeest migration continues moving northwards during July and August, often spreading out across a broad front: some heading through Grumeti Reserve and Ikorongo, others north through the heart of the Serengeti National Park.
September sees the herds spread out across the northern Serengeti, where the Mara River provides the migration with its most serious obstacle. This river gushes through the northern Serengeti from Kenya's adjacent Maasai Mara Game Reserve. Watching the frantic herds of the wildebeest migration crossing the Mara River can be very spectacular; there are often scenes of great panic and confusion. It's common to see herds cross the Mara River north on one day, and then back south a few days later.
By October, the wildebeest herds are migrating again with more accord: all are heading south, through western Loliondo and the Serengeti National Park's Lobo area, returning to the green shoots which follow the rains on the short-grass plains of the southern Serengeti in November.