The New York City section of the Hudson River almost never freezes completely. But, with high temperatures only in the teens the past few days, the river is currently churning with ice.
Near the city, the Hudson is an estuary. Fresh water flows down from the north and salt water moves in from the Atlantic Ocean to the south. Since salt water has a lower freezing point than fresh water, it is usually only the fresh water, floating on top of the denser salt water, which freezes. This process forms ice floes – sheets of floating ice that oscillate with the tide.
The last time the Hudson River froze completely, according to historical records, was January 25, 1821. The temperature in the city that day was a frigid -14°F.