We saw hundreds of zebras crossing, or at least trying to cross, the Grumeti River, which is one of the 2 big river crossings on the Northern migration into Kenya.
Whilst having "sun downer drinks" within walking distance of our tented camp, we saw hundreds of zebras coming up to the Grumeti River, honking away to each other loudly. As they had selected a spot away from waiting crocs, scores made it across, and we saw them exiting the water safely, and then calling to their friends to come across. But perhaps because of the presence of our jeep, the rest of the zebras decided NOT to cross the river, and ultimately stayed on the South side for another night.
It would've been great to see more cross, but at one stage it looked like our presence could have forced them to cross at a different point - directly into the waiting mouths of crocs, some people were saying "this will be great, we see them cross and see the crocodiles in action"! But as they had changed location because of us, really we were glad no more went for it.