The migration took a dramatic change at the end of the past week.  All the excitement at the crossing died out when most of the herds from the eastern side of the Mara River crossed west. 

Once on the Mara triangle, most herds headed south into northern Serengeti.  This was a quick exit, since they were expected to stay in the Mara till November as is always the case.  The rain pattern has played a role in this change of events.  However, all is not lost because the movement of the herds inside Serengeti shows they are bound for the Mara again. 

The prolonged dry spell in the Mara is believed to have been the cause of the changes in the concentration and duration of the stay in Mara.  Even after the wildebeest leave, the lions and cheetahs are still there to photograph. (summarized from paul-kirui.blogspot.ca)