What you need to know about freshet

As warmer temperatures set in, OPG’s water management teams across the province are managing the impacts of freshet - the annual spring runoff that occurs from melted snow and rain.

It’s an important, but often misunderstood, process that can impact Ontario’s hydroelectric operations and the people who use the affected river systems. OPG water management staff must balance many factors, including water management plans and agreements that help to determine allowable limits.

On the Ottawa and Madawaska rivers, OPG prepares for the spring melt as early as October with the start of the drawdown at Bark Lake.

In the winter months, larger hydroelectric stations on those rivers will begin to release water from their reservoirs, lowering levels in anticipation of the spring deluge. This includes Mountain Chute Generating Station (GS), Otto Holden GS and Des Joachims GS.

The reservoirs for these “annual-cycle” stations should be entirely empty by the end of March in an average year. As flows pick up due to the thaw and spring rainfall, the reservoirs can be refilled while the stations continue to operate.

How much water enters these reservoirs depends on a number of factors, including addressing flooding issues if they occur, and other environmental conditions, like fish spawning activities.

The water levels in the reservoirs are generally kept in the top-range from summer to fall. Drawing down water levels begins between Thanksgiving and the winter months, and the cycle starts all over again.

But predicting water levels can be difficult, especially if there are many freeze-thaw cycles in a winter season.

An abundance of snow on the ground also doesn’t necessarily mean there will be an increase in runoff as not all snow makes its way into the rivers and snow can evaporate without melting depending on weather conditions.

Alternatively, increased rainfall and high humidity can lead to higher flows. On average, spring freshet can take up to several weeks on large river systems.

Regardless of what the water levels may look like, residents should remain vigilant around rivers and dams at all times. Even if it may look safe, water can rise at any minute around OPG’s facilities.