The Great Trek of the Wildebeest in East Africa, also known as "The Great Migration" takes place between the Masai Mara in Kenya and Serengeti in Tanzania. Thousands of wildebeest and zebras migrate between these natures, constantly driven by their quest for fresh grass, they follow the rains. The huge herds, a necessary and very tasty meal for predators such as lions and cheetahs, are to be monitored closely by the carnivores. The exact timing of the annual wildebeest migration is unpredictable and remains a spontaneous natural event. During June the animals begin the western corridor of the Serengeti (Tanzania) to leave to go further into the northern plains and forests. The fresh, mineral-rich, lush meadows on the other side of the human border, in Masai Mara, are a magnet for the animals and they eventually penetrate massively into the reserve in Kenya.
The wondrous event usually starts around July. The herds withdraw from the south and meet here another migration: the fixed wildebeest herds and residents of the Mara region. These animals stay in the Loita Hills and Plains, north-east of the Mara, until the dry season arrives and it is time to search for the ever-green Mara basin. Throughout the months of July and August, stampeding herds of Wildebeest cross the Sand River, a predominantly dry tributary of the Mara River, which roughly marks the border between Kenya and Tanzania. Whereupon the herds go to the biggest challenge of their quest for food: the crossing of the Mara River and its tributary often, the Talek. By that time it rains on the Mau Escarpment, and the river water rises. The currents can be very strong and the river has steep slopes, but the biggest obstacles are the enormous Nile crocodiles! More than one million wildebeest and two hundred thousand zebras on this endless quest for fresh pastures. The herds are usually around November to return to the Serengeti where the journey to continue, but this depends on rainfall only.
Many travelers have put seeing the Great Migration and the crossing of the river on the wish list. Unfortunately we cannot say exactly where the animals are located at a given time. By monitoring the animals for years it know can be estimated. When you are visiting the Masai Mara in August and September you have a fair chance that you can witness the hundreds of animals and also experience a crossing of one of the rivers. July gives a minimum chance and October a reasonable chance.