From the Weather Center by John Simon
An ice jam can happen when chunks of ice clump together to block the flow of a river or stream. Ice jams are usually caused by melting snow and ice in the springtime. Warm temperatures and spring rains cause snow and ice to melt very rapidly. All this extra water causes frozen rivers and streams to swell up and the layer of ice on top of the river or stream begins to break up.
Ice jams can form almost anywhere that has winters cold enough for rivers to freeze, such as areas in the northeast, the midwest and Alaska.
There have been serious ice jams from time to time in the Delaware River, and they have impacted parts of Sullivan and Orange County, with the city of Port Jervis being particularly hard hit several times in the late 1970s and early ’80s.