Nomad Tanzania updated it’s blog on the migration of the wildebeest - After their very early arrival in the North, the migration is now well spread out across the Northern Serengeti with plenty of animals in the Mara River and Sand River areas, a few towards Kichwa Tembo Rd and again big herds on the Bologonja side and on the Kogatende side. Some great crossings are being seen by our guides and guests on the Mara River, but also on the Sand River side.  

http://www.nomad-tanzania.com/blogs/serengeti-migration/