Rainfall moves the herds
The Great Migration is not a single march from one destination to another. Wildebeest, zebras and other grazing animals move through the Serengeti–Mara ecosystem in a broad seasonal loop, tracking fresh grass and water as rains shift across Tanzania and Kenya. Wildebeest make up most of the migrating animals; zebras often travel alongside them, grazing different parts of the vegetation and benefiting from the same pasture.
Because rain is local and variable, the route and timing can change from year to year. A herd may spread out across open grassland, then gather where a river or a patch of suitable grazing narrows its choices.
Why a river crossing becomes a bottleneck
At the Mara River, reaching the opposite bank can lead to better grazing, but the crossing is not a coordinated group decision. Animals bunch at the edge, hesitate, and sometimes turn back. When some finally enter, others may follow in a rush. Steep banks and strong currents make it hard to keep footing, while crowding can cause animals to fall or be pushed under.
Nile crocodiles lie in wait in the river, making the crossing dangerous, but they are only one part of the risk. Drowning, exhaustion and the crush of the herd can also be fatal. The animals are not choosing danger for its own sake: staying put can mean running short of the fresh grass and water they need.
A border does not stop the seasonal circuit
The migrating herds range across a connected landscape that includes protected areas on both sides of the Tanzania–Kenya border. That political line does not divide the grazing system the animals follow. For travelers, this means a river crossing is a brief, unpredictable episode within a much longer cycle—not a scheduled performance. The next movement depends on where rain has made the grass grow.
