Ice Jams

According to FEMA, flooding is the number one natural disaster in the United States. Generally when it comes to floods, people imagine thunderstorms or hurricanes that happen during the summer. While these events obviously do cause flooding, flooding can also occur during the winter. One example of winter flooding would be ice jams.

An ice jam, also known as an ice dam, is as you might suspect a buildup of ice. Ice jams might occur in situations, such as the slope of a river changing, or moving ice meeting intact ice cover. This jam can be problematic in a few ways. First, once the ice jam blocks the path of moving water, it can cause flooding in the area behind the jam. Additionally, after the ice jam thaws, it releases a massive quantity of water, leading to severe flooding downstream.

The most common time for ice jams is late winter and early spring. During this time, temperatures usually start to rise above freezing. This leads to melting snow and ice, which in addition to rainfall creates additional runoff for rivers. In addition to runoff, there is also the added water from melting ice already in the river. Furthermore, the rising water level may break loose ice from the banks of the river. All these factors could contribute to an ice jam. Ice jams are possible during early winter as well, but generally aren’t as bad since not as much ice has developed at that point.

One major example of an ice jam occurred in Montpelier, Vermont on March 11, 1992. At approximately 7am, a large ice jam on the Winooksi River broke free and traveled towards the the city. Eventually, it created another ice jam just below the Bailey Avenue bridge, creating a dam on the river. Since the river was already at capacity due to recent snowmelt and rain, the banks began overflowing. Within minutes, parked cars and basements along Main Street and State Street of Montpelier were flooded. By 8am, some areas were reporting 2-3 feet of water with an estimated 100 people already stranded by the flooding. At this point evacuations began with a state of emergency being declared soon after. Finally at 3pm, after roughly 200 buildings were flooded, backhoes and a crane began work trying to dislodge the ice jam. The initial ice jam was broken up, but unfortunately, a second one occurred 2 hours later. Luckily it didn’t take long to break up the second jam. By 5:30pm, the last ice chunk cleared the Bailey Avenue bridge.

The ice jam in Montpelier caused severe damage to the city, especially the downtown area. When all was said and done, the ice jam created an estimated 4 million dollars in damage as water levels rose 3 feet above the main level floors of many businesses. Over 200 automobiles were damaged, in addition to 8,000 gallons of fuel being discharged into water.

With the right conditions, ice jams can actually happen on the roof of a house, as well. In regards to a house, an ice jam would be a line of ice that forms at the edge of a roof. This becomes problematic as it prevents melting snow from being able to drain off the roof. Water with nowhere to drain could lead to water entering the home through cracks and openings. If left untreated, this could cause damage to many areas of your home, including walls, ceilings, and insulation.

In order for this situation to occur, there needs to be some sections of the roof that are warmer than the edge of the roof. This will allow an ice dam to form at the edge while snow melts on the warm sections. Warmer areas of the roof could be caused by a variety of factors, ranging from heat conduction through the ceiling to heat transference from the chimney.

Ice dams can be quite damaging to your home, and may eventually lead to leaks, mold, and other moisture damage. For long term prevention of ice dams, it is helpful to make your home as air tight as possible. Once snow storms have occurred, the best short term option is to remove the snow as quickly as possible from your home.

Whether on a river or on the edge of a roof, ice dams can be very damaging to your home. Both types of ice dams can lead to extreme water damage and thus, are something to be aware of.

“25th Anniversary Montpelier Ice Jam March 11, 1992.” National Weather Service, National Weather Service,
Britannica, The Editors of Encyclopaedia. “Ice Jam.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 27 Mar. 2009,
“Dealing with and Preventing Ice Dams.” Extension at the University of Minnesota, University of Minnesota Extension,
“Winter Flooding.” FEMA, FEMA,