freeze–up

1 :  a freezing over of a body of water especially when marking the onset of winter goes on duty in April, just before the swans arrive, and stays with them until they leave at freeze-up — Lyn Harrington

2 :  a period during which the bodies of water in an area are frozen over the freeze-up lasted six weeks last year before the freeze-up sets in — V. E. Fuchs all their food for seven months had to be taken to camp before the winter freeze-up — Bill Wolf