River Ice Dynamics and Ice Jam Flood Risk in Eastern Canada 

River Ice Dynamics and Ice Jam Flood Risk in Eastern Canada

An overview of river ice processes

Ice covers are a natural feature of rivers in northern climates, including Québec and the eastern provinces of Canada. They form through complex thermal exchanges between the river and atmosphere during prolonged cold periods. In most populated regions of Québec, ice formation begins in early December and continues to thicken into March. Then, as air temperatures and solar irradiance rise between mid-March and early April, the ice begins to degrade thermally. This thermal deterioration of the ice cover is preferred by municipalities because it structurally weakens the ice, allowing it to break apart gradually and drift downstream without jamming.

What causes ice jams?

However, occasionally in late winter once the ice cover has fully formed, sudden increases in discharge — caused by rain-on-snow events or rapid snowmelt triggered by warm temperatures and high solar irradiance — can fracture the ice cover. Large, intact blocks of ice are then mobilized downstream. These blocks may jam at intact ice fronts or in river reaches with morphological or hydraulic features that promote jamming (e.g., sharp bends, channel constrictions, sudden drops in slope or depth). Ice jams can also form against man-made structures such as quays, boat launches, or buildings constructed within or near the active floodplain.

Once a jam forms, upstream water is forced beneath the rough underside of the jammed ice, reducing the effective flow area for water to evacuate. To compensate, the water level upstream of the ice jam rises to generate more energy to force the water under the ice jam. This leads to a rapid rise in upstream water levels, often resulting in overbank flooding.

Impacts and Challenges of Ice Jam Flooding

Dynamic ice breakups pose serious risks to communities due to their sudden onset and unpredictable behavior. Water levels can rise by several meters in a matter of minutes, leaving little time for emergency response or evacuation.

Predicting where and when ice jams will occur remains a major challenge for authorities. Their location, severity, and associated flood extents are influenced by a complex interplay of climatic, hydrologic, and geomorphic factors. This unpredictability complicates the delineation of flood-prone zones and limits the usefulness of historical flood records. Moreover, with climate change expected to modify the timing and intensity of ice-related events, new methods are required to accurately account for these factors.

Our approach for reducing ice jam flood risk and assessing river ice circulation

The project to modernize the regulatory framework for aquatic environments takes ice jams into account by designating previously affected areas as being at a very high hazard level. However, it is difficult to determine the extent of past ice jam floods. That’s why Avizo is here—to help municipalities, regional county municipalities (MRCs), and private clients better understand the risks that ice jams pose in their territory.

To assist clients with ice jam flooding in their region and to help determine if any proposed projects may obstruct the circulation of river ice in accordance with provincial law (article 331, al. 1(4)a) du Règlement sur l’encadrement d’activités en fonction de leur impact sur l’environnement), Avizo offers expertise rooted in recent scientific advancement to characterize ice-related flood and circulation risks. Our approach integrates a suite of analyses, including:

• Compiling a comprehensive inventory of historically documented ice jams. 

• Surveying ice-scarred trees along the river corridor to detect the elevations of historical ice jams. 

• Analyzing climate records, discharge and water level datasets to understand the hydroclimatic conditions conducive to ice jam formation. 

• Examining the river’s morphology to identify features prone to causing ice jams to form. 

• Leveraging remote sensing tools, including satellite imagery and aerial drone observations to detect ice cover patterns. 

• Leveraging statistical hydraulic modeling techniques to delineate probable ice jam flood extents and corresponding water surface elevations. 

The results from our multidisciplinary analysis are synthesized together to develop a comprehensive understanding of the zones at highest risk of experiencing ice jam flooding and their associated water levels. Clients benefit from a clear picture of their territory’s vulnerability to ice jam flooding and enable them to identify mitigation options and implement effective flood risk reduction strategies and ensure the circulation of river ice.