Winter drawdown helps OPG prepare for spring freshet
At its reservoirs across Ontario, OPG is drawing down water levels this winter to prepare for spring freshet, the annual influx of water that comes with increased rainfall and melting snow.
The process of releasing water from reservoirs, referred to as winter drawdown, has multiple benefits, including helping to mitigate flooding risks near the province’s river systems. The additional water released also provides affordable and sustainable hydropower during the late fall and early winter. In the spring, the space created by reservoir drawdown is used to absorb some freshet water and reduce the peak flow.
“We need to start drawdown of our reservoirs before freshet occurs depending on the size of the reservoir. If we wait, we simply won’t have enough time to safely create storage,” said Kurt Kornelsen, Senior Manager of Water Resources at OPG.
With that said, OPG is just one of many entities in the province acting to mitigate the impacts of freshet. While OPG continually prepares its stations and equipment, and operates dams to manage high flows and flooding, the company’s storage capabilities are not infinite and can be exceeded during a high-water season.
Drawdowns are conducted every winter, starting as early as October, according to water management plans on provincial rivers. Depending on the size, it can take as little as a week or several months to draw down a reservoir.
“We are dealing with a number of factors that can change quickly. At the end of the day, we’re contending with Mother Nature.”Kurt Kornelsen, Senior Manager of Water Resources at OPG
There are several factors that can influence just how much water is released. This includes historic and current snow levels, weather forecasts, precipitation, and other data.
To get a more accurate picture, OPG staff conducts bi-weekly snow surveys at fixed sampling locations to measure the depth of accumulation of snow during the winter months. A sampler is used to cut and extract a core sample of snow from the pack, which is then weighed and recorded in water equivalent centimetres.
This data, along with other information, such as weather forecasts, is valuable in estimating the spring run-off potential and dictates how much water is released. Based on predicted conditions, drawdowns at this time of year may be full or partial to ensure the reservoir can be refilled to meet the summertime obligations for fisheries and recreation.
While the amount of snow certainly factors into drawdowns, snow usually only accounts for a fraction of the water that ends up in area lakes in the spring. “Snow in the ground doesn’t equal water in the reservoir,” Kornelsen emphasizes.
Periods of sunny weather with warm winds may cause snow to skip the melting stage entirely and directly evaporate into the atmosphere.
On the flip side, sudden warming and spring rainfall can quickly melt the snow and significantly increase the water available in the watershed, which was the case in many parts of the province in 2019. That spring’s record-setting flooding was caused by a combination of weather conditions, most significantly rapid snow melt and heavy rainfall, according to the Special Advisor’s Report on the 2019 flooding events.
All of this makes predicting freshet so challenging. OPG’s water management staff make decisions based on the best information available, but forecasts are not 100 per cent reliable.
“The bottom line is that we are dealing with a number of factors that can change quickly,” Kornelsen said. “At the end of the day, we’re contending with Mother Nature.”
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