The Mara River originates in the Mau Escarpment, along the western edge of the Great Rift Valley in southern Kenya, meandering southwest some two hundred and forty miles through Kenya's Masai Mara and Tanzania's northern Serengeti before emptying into Lake Victoria. It is a defining feature of the great annual migration of wildebeest and zebra in the Serengeti plains of Tanzania.

The migration is a huge, clockwise movement of thousands of wildebeest and zebra, starting in the southern Serengeti plains to the west of Ngorongoro Crater, where the wildebeest birthing grounds are located. Over the year, they move north towards Kenya's Maasai Mara, following the pattern of the rains and the fresh grasses that follow. Later in the year, the rains move back south, with the wildebeest and zebra following, completing the circle.

Exactly where and when a crossing will occur is a bit of a guessing game, but there are typically used crossing points. When large numbers of wildebeest begin to gather along the riverbanks, it might signal that a crossing is developing. What triggers an actual crossing remains mysterious, but it's not dissimilar to watching a large flock of geese in the early morning, where presumably one bird decides to go first, and suddenly thousands take to the air. Here, it seems obvious that a) they all want to get to the other side of the river, and b) they are terrified at the prospect of actually crossing, since dozens of crocodiles likely lie in wait.

Witnessing a wildebeest crossing at the Mara River during the Great Migration can overload the senses—a raucous cacophony of sound and motion within which are embedded multiple smaller, but poignant stories. Some become marooned on large rocks in the middle of the river, unsure or afraid of moving forward or backward. A mother zebra, after crossing once, becomes separated from her foal who didn't cross. She crosses back to retrieve it, then re-crosses a third time to bring the foal to safety, somehow avoiding the many crocodiles lurking in the waters. Crocodiles stockpile dead wildebeest in the river's shallows, returning to the maelstrom for more. Once the crossing is largely over, groups of wildebeest calves gather along the riverbank, plaintively bleating for their lost mothers, some of whom may have perished in the river, others not having made the crossing at all.

In the midst of the cacophony, we watched a wildebeest, separated from the majority of those crossing, get attacked by a crocodile mid-stream. At first, it seemed this story would end quickly. But as the minutes passed, the wildebeest slowly fought to make it to shore, its back sheared open, horribly mutilated, dragging the crocodile who seemed to be slowly losing its grip. How did any of this make sense? All of us in the gathered vehicles were cheering the wildebeest on, the children in the vehicle next to us screaming and crying in terror. Yet the wildebeest, so slowly, stepped onto shore, finally shaking the crocodile loose.

The sun was setting. We were exhausted from just watching. Those that had successfully made it across the Mara gathered in the plains, seemingly congratulating each other on their success. The lost calves continued their plaintive bleating for their missing mothers. And our hero wildebeest slowly made his way out onto the plains, where waiting for him, as night fell, were the lions and hyenas.