Salt water becomes heavier as it freezes. This leaves a greasy soup of ice washing in and out with the tide, each wave leaving just a little more ice clinging to the shore. In Churchill, high tide returns every 12.5 hours and it does not take long for the shore ice to extend well out into the tidal zone.
As well, Hudson Bay’s watershed extends west to the Canadian Rockies and south to Minnesota. This means that a tremendous amount of fresh water pours into the bay from several northern rivers. This inflow results in brackish water (a mix of salt and
fresh water) along the coast and surface of Hudson Bay. Since freshwater begins to freeze at a higher temperature than salt water, this further contributes to the speed of freeze up.
All the while, the ice builds along the northwestern coast of Hudson Bay. Soon, the ‘grease ice’ forms into little ice floes called pancake ice. A strong north wind and consistently cold temperatures of -20C (-4F) or lower will push this ice together and pack it onto the coast of Cape Churchill.
Once these sheets have frozen together, it signals the bears’ departure. They will venture out to hunt seals even with only a few kilometres of ice. As winter progresses, the ice continues to encroach eastward until the bay is completely frozen, usually occurring in early December.
Almost every year, initial freeze up occurs around mid-November. However, in both 1991, 2002, 2012 and 2013, conditions prevailed for an early freeze. The freeze up was so sudden in 1991 that the bears departed near Halloween night. In other years, winter takes its time – 1999, 2003 and 2009 saw the bears remain ashore well into December. While a late freezeup is not as critical to the bears’ health as an early breakup, it does result in an extreme increase in polar bear occurrences within the community of Churchill.