The mass evacuations underway below the Oroville Dam capped a week of frantic efforts to prevent flooding as America’s tallest dam reached capacity and its main spillway was severely damaged. Here is a brief explanation of the events so far. On Saturday, water levels reached so high that an emergency spillway was used for the first time. Officials initially believed the measure worked. But on Sunday afternoon, as more water from record storms flowed into Lake Oroville, officials detected a hole in the emergency spillway. That prompted the evacuation order.