Ice Jams

The Nottawasaga Valley Conservation Authority (NVCA) is one of 36 conservation authorities in Ontario. Conservation authorities help manage flood risks on a watershed scale—because water doesn’t stop at municipal boundaries. Protecting people and property from flooding is a shared responsibility between conservation authorities, municipalities, and the Ministry of Natural Resources.

What is an Ice Jam?

Ice jams form when chunks of ice pile up in a river or stream, blocking the flow of water. As water backs up behind the jam, it can quickly overflow the banks and flood nearby areas—even when river flows are relatively low. These jams tend to form in shallow or winding sections of rivers, and can develop in just minutes, often with little or no warning. In the Nottawasaga Watershed, communities like Creemore, Avening, and Collingwood have experienced ice jam flooding in the past.

Why do ice jams cause flooding?

Ice jams can block the natural flow of a river or stream, causing water to back up behind the jam. When this happens, water levels can rise quickly, increasing the risk of flooding in nearby areas.

The severity of flooding depends on how much water is flowing and how significantly the ice is blocking the channel. In some cases, multiple ice jams may form and release in sequence, sending ice downstream and creating new blockages. As a result, flooding can occur both upstream and downstream of the original jam.

Because ice jams can form suddenly, there is often little or no time to issue warnings.

Who is responsible for dealing with ice jams?

Under the Emergency Management and Civil Protection Act (Ontario Regulation 380/04), municipalities hold the primary responsibility and authority for responding to flooding and flood emergencies. They are also responsible for the welfare of residents and protection of property.

Conservation authorities, municipal staff, and emergency responders work together to monitor conditions, coordinate responses, and share vital information. Residents also play an important role by staying informed, communicating with neighbours, and reporting ice jams or flooding when they see them.

While Conservation Authorities do not directly manage ice control, they operate flood forecasting and warning programs and provide expert advice on ice jam prevention and mitigation.

Where do ice jams occur?

Certain natural features of waterways increase the likelihood of ice buildup in specific spots, including:

Bends, meanders, and river mouths, where water flow slows down
Shallow riverbeds, narrow channels, or tight bends where ice can get stuck
Areas with anchor ice, ice that freezes to the riverbed, which can block ice movement and redirect water
Man-made structures such as bridges, culverts, dams, reservoir entrances, fencing, or construction debris that obstruct the passage of ice

Understanding ice jam formation?

Every ice jam is unique, but there are common factors and conditions that often lead to their formation. Historically, ice jam floods have been preceded by rapid weather changes and colder-than-average winters.

Ice jams are more likely to occur in areas where the river channel widens or deepens, causing water to slow down suddenly. They also tend to form at sharp bends, constrictions in the river, or near bridges and their approaches—places where the flow direction changes quickly.

The start of ice breakup depends on two main factors: the accumulated degree-days of melting and the amount of precipitation during the melt. If rising water levels are high enough, they can loosen the ice cover from the riverbanks. However, this doesn’t always mean the ice will move downstream or break apart immediately.

Whether the ice breaks up depends on the river’s flow velocity and its physical features—like islands, bends, or ice control structures (ice booms)—which can either slow or restrict the movement of ice.

Ice jams cannot be predicted with accuracy. A variety of factors affect when and where ice jams may form, including weather conditions, river channel features, and streamflow. At this time, current river ice science does not allow for reliable predictions of ice jam formation.

Winter safety information

Ice around rivers, streams, and dam reservoirs is unpredictable and can be very unstable. Warm weather and rainfall can quickly raise water levels, making ice conditions even more dangerous. Always keep a safe distance from frozen water bodies, especially near dams and channels.

Stay clear of ice jams, as water levels can change rapidly and riverbanks may become unstable. Large chunks of ice can suddenly jam up or break loose without warning, often within minutes. Your safety is the top priority—please exercise caution around all ice-covered waterways.

Responding to ice jams

Ice jams can form and break up naturally without causing significant flooding. Any decision to remove an ice jam that poses a risk to public safety requires careful assessment of all potential risks.

Conservation authorities work closely with municipal staff to address flood risks related to ice jams. Using tools like drones, remote cameras, and stream gauges, conservation authorities provide enhanced monitoring and technical advice to emergency responders when needed.

If an ice jam forms and threatens flooding in nearby areas, removal options are limited and typically managed by the local municipality and emergency services.

Ice jam mitigation

Intervention to address ice jams should only be considered if it effectively reduces flood risk without causing new problems elsewhere. Mechanical removal using heavy equipment like excavators may be used to clear an ice jam.

Other methods, such as channel straightening and ice control structures are generally not recommended due to potential environmental impacts and safety concerns.

What is a Conservation Authority’s role in flooding?

Ontario’s Flood Forecasting and Warning program aims to reduce risks to life and property from river flooding. Working alongside the Province, conservation authorities across Ontario maintain systems that track weather and river conditions to provide timely flood forecasts and warnings.

Flood Duty Officers monitor rainfall and streamflow using a network of gauges, helping identify potential flooding during severe weather events. Conservation authorities then share alerts, information, and advice with municipal governments, emergency services, police, school boards, media, and others involved in community safety.

Residents can also sign up to receive these flood messages directly, staying informed and prepared when flooding risks arise.

Frequently Asked Question: Is ice jam flooding covered by insurance?

Most standard insurance policies do not include coverage for overland flooding. Residents are encouraged to speak with their insurance representatives to understand their coverage options and any additional protection they may need.

For more information, visit the Insurance Bureau of Canada.