How Hydro‑Québec installations reduce the impact of spring runoff

Hydro‑Québec’s facilities can actually limit the magnitude of spring flooding. The company has two types of facilities – some have large reservoirs and others are run-of-river facilities.

Those with large reservoirs can retain significant amounts of water in the spring, and they are emptied during the winter months, when the water is used to supply hydropower generating stations to meet the population’s high demand for electricity. Hydro‑Québec ensures that most of its reservoirs are practically empty in the spring. This way, the reservoirs can retain a large amount of water resulting from snowmelt and rainfall, which would otherwise flow directly into rivers. Hydro‑Québec’s reservoirs thus help to reduce the impact of spring runoff.

Run-of-river stations do not have reservoirs and thus cannot retain spring floodwaters. In these facilities, water is simply left to run through the turbines to generate electricity. When water inflows become too large, the spillway gates are open for water to run through. Even if we tried to use such generating stations to retain the excess water, doing so would not reduce the magnitude of the flooding at all because the water would overflow in at most a few hours and would only damage the station.

The water level is low because, a few days before the spring flood period starts, we lower the water level in order to reduce the effect of the facilities on the river flow and the effects of high flow backing up in the river. You could say that we try to make the river behave as it would in its natural state, that is, before the generating station was built.

The water level upstream of the generating station can go down over a distance of several kilometres, depending on the river’s natural topography.

The river flow is then naturally determined by restrictions such as islands, narrows, shoals or rapids, not by the hydropower facilities. These natural restrictions are what cause water levels to rise further upstream. The stronger the river flow, the greater the backup will be.

When water levels are lowered, the effect downstream of facilities is negligible. To reduce water levels upstream, we temporary increase the flow through the turbines for a few hours or a few days, before the water inflows increase. Once the upstream level has stabilized, a run-of-river generating station lets the river’s natural flow pass through, no more, no less. The return to normal operating levels takes place when the spring runoff subsides.

The level of a river is affected by its whole watershed, that is, the entire territory that feeds it. When two rivers meet, their watersheds combine, resulting in a huge network of watersheds and sub-watersheds that end up flowing into large rivers such as the Rivière Outaouais (Ottawa River), the Saint-Maurice and, ultimately, the Saint-Laurent. It can take a few hours to a few weeks for water to travel from the highest point, upstream, to the lowest point, downstream, depending on the nature or characteristics of the terrain and the size of the watershed.

No. Hydro‑Québec employees monitor waterways and precipitation daily, throughout the year.

A network of hydrometeorological stations make it possible to constantly monitor river levels and flows as well as rainfall and snowpack.

In winter, employees also analyze the snow cover to determine its density and water content and thus better predict how much water will flow when it melts.

For most watersheds, and in particular for the Rivière Outaouais and Saint-Laurent watersheds, water is managed through the concerted effort of all organizations involved in water management in Québec, such as the Ministère de l’Environnement et de la Lutte contre les changements climatiques, the Ottawa River Regulation Planning Board and the International Lake Ontario–St. Lawrence River Board. All generating facility operators are also involved, including Hydro‑Québec.

The Ministère de l’Environnement et de la Lutte contre les changements climatiques provides information on Québec river water level and flow rates on its website.

Other relevant websites:

• Ministère de la Sécurité publique – Vigilance Website [In French only]
• Ottawa River Regulation Planning Board
• International Lake Ontario – St. Lawrence River Board (Board’s Facebook page)

No. The Ministère de la Sécurité publique is responsible for managing all communications during floods. It works in concert with municipalities, which act as the intermediary with residents.

During the spring melt, Hydro‑Québec’s responsibility involves managing its facilities to lower river levels as much as possible and communicating any relevant information to the Ministère de la Sécurité publique and affected municipalities.

The water levels upstream and downstream of hydroelectric facilities can change rapidly based on water inflows, electricity needs and the operating constraints of facilities. This makes it hard for Hydro‑Québec to notify residents in advance.

Please remember to always be extra cautious near a generating station or spillway and to respect all safety warnings.

To learn more, visit the Hydro‑Québec web page Safety measures near hydropower facilities.