Reducing spring flooding on the Saint-Laurent and Outaouais rivers: A team effort

In the Montérégie region, Hydro-Québec operates hydroelectric generating stations in two watersheds: the Outaouais (Ottawa) river system and the Saint-Laurent (St. Lawrence) river system. These watersheds are home to Carillon, Beauharnois and Les Cèdres generating stations, which are all run-of-river stations. Because they do not have reservoirs, these generating stations cannot store water the way reservoir generating stations can.

Role of the Ottawa River Regulation Planning Board

Several large rivers flow into the Outaouais, which drains a watershed of 146,000 km2 and covers a distance of 1,120 km, from Abitibi-Témiscamingue to Montréal. These inflows make the Outaouais a significant tributary of the Saint-Laurent near Montréal. Hydro-Québec is not the only operator of reservoirs and generating stations in the Outaouais watershed. Managing the water levels and flows in the Outaouais is a collaborative effort between partners. Every drop of water in the river is monitored by the Ottawa River Regulation Planning Board (ORRPB).

The ORRPB is made up of all the agencies that participate in managing the Outaouais watershed: the Ministère de l’Environnement et de la Lutte contre les changements climatiques du Québec, the Ministry of Natural Resources and Forestry of Ontario, Environment and Climate Change Canada, Public Services and Procurement Canada, the Canadian Coast Guard, Ontario Power Generation and Hydro-Québec.

Visit the ORRPB website to track the hydraulic conditions of the Rivière des Outaouais.

We reduce spring flooding thanks to our reservoirs. Find out how.

From December to March, Hydro-Québec gradually empties its reservoirs located in the northern Outaouais and Abitibi-Témiscamingue regions. When the spring thaw begins, they contain almost no water. From early April to early June, we fill the reservoirs to almost capacity and store the water as long as possible to limit inflows to the Outaouais and the Saint-Laurent rivers, which are already swollen with water from the surrounding watersheds. However, the reservoirs only have access to 40% of the water inflows. The remaining 60% are from watercourses that do not run through our reservoirs; they flow freely and cannot be retained.

All the generating stations located on these watercourses are run-of-river and do not have reservoirs to hold water. The only facilities that impact the magnitude of the spring flood are the reservoirs located north of the watershed, which we manage in collaboration with our partners.

For more information on the Rivière des Outaouais watershed, visit the Ministère de l’Environnement et de la Lutte contre les changements climatiques (web page in French only).

Duration : 3 minutes 29 seconds [In French Only]

Watch our expert explain how Hydro-Québec contributes to managing spring runoff along the Fleuve Saint-Laurent.

Did you know?

Carillon generating station, which is located at the foot of the Rivière des Outaouais watershed, is a run-of-river generating station.

It does not have a reservoir and therefore cannot store spring flood water. During flood periods, the spillway discharges the excess water.

If the spillway gates were closed at the peak of the high water levels due to spring flooding, the water would spill over the facility within a few hours!

Carillon generating station.
Carillon generating station.

Hydro-Québec does not manage the level of the Saint-Laurent. The ILO‑SLRB does.

The inflows to the Saint-Laurent are managed in conjunction with the International Lake Ontario – Saint Lawrence River Board (ILO-SLRB), which ensures that the regulation of the levels and flows in Lake Ontario and the Saint-Laurent complies with the legal requirements of the International Joint Commission (IJC). The mandate of the ILO-SLRB covers the entire watershed, from the Great Lakes to Lac Saint-Pierre and to the Montréal archipelago area.

The ILO-SLRB works daily with numerous stakeholders, including the St. Lawrence Seaway Management Corporation, the Canadian Coast Guard, U.S. and Ontario hydropower producers and Hydro-Québec.

Hydro-Québec has a direct role in managing the Beauharnois–Les Cèdres hydropower complex, located in the west of the Montérégie region, but does not manage the level of the Saint-Laurent. We work with the ILO-SLRB to safely manage the ice near the complex, which consists of run-of-river generating stations and therefore cannot retain water. For example, Beauharnois generating station receives the water that comes through the Beauharnois canal, runs it through turbines and then redirects it toward Lac Saint-Louis.

During the spring thaw, the members of the Ottawa River Regulation Planning Board (ORRPB) and the ILO-SLRB work together to reduce the inflows to the Montréal archipelago area. Since Hydro-Québec operates the two generating stations closest to the archipelago, our experts play a key role in this aspect of flood management every spring.

Great Lakes system profile
Great Lakes system profile
Beauharnois generating station
Beauharnois generating station
Les Cèdres generating station
Les Cèdres generating station

A tool that identifies flows and water levels

We are installing measuring instruments on rivers and reservoirs where we operate dams and generating stations. They provide flow, water level and meteorological data. This data is available to you through a simple tool, which can provide information about flows on rivers and water levels in reservoirs.

Learn more about the Tool

Important reminder: In the event of a flood, contact your municipality. They will advise the Ministère de la Sécurité publique du Québec, which coordinates all flood-related operations.

Questions on how Hydro‑Québec facilities are managed? Contact us at

Frequently asked questions

Why can’t Hydro-Québec hold back water upstream of the Coteau dams on the Saint‑Laurent?

The hydraulic and hydropower facilities in the Montérégie region, including the Coteau dams, are run-of-river and have no way of storing water. The dams and generating stations must allow the water to flow through, without holding it back. Some of the water is used to generate electricity, and the rest is discharged through the gates of the dam or spillway.

Why has the flow in the Saint-Laurent been higher since 2017?

Inflows to the Saint-Laurent come mainly from Lake Ontario, the most downstream of the Great Lakes. Throughout the decades, these lakes have gone through wet cycles and dry cycles. The latest wet cycle, which began in 2015, led to a gradual rise in the water levels of the Great Lakes, and a subsequent increase in the flow of the Saint-Laurent. Since 2017, the levels of the Great Lakes have been higher than average and these lakes have also been receiving above-normal precipitation. It is therefore necessary to accept higher flows in the Saint-Laurent to ensure the safe management of this major river. Similar conditions were observed in the 1970s and 1990s.

How does the emptying and filling of basins along the Saint-Laurent work? Would it be possible to have water year‑round?

Following the reduction in flow that occurred in the natural riverbed of the Saint-Laurent from 1930 to 1960, structures were built to maintain water levels that met the community’s needs, including vacationing needs. In addition, because managing the ice on the Saint-Laurent is a challenge every winter, some structures have been designed to stay open during the winter to reduce ice-related problems. Due to their design, these structures cannot be operated in the winter. Water levels are therefore always low during this season.

On the Rivière des Outaouais (Ottawa River), could more water be held back by the facilities located in the north so that less water flows to the south?

In spring, Hydro-Québec and its partners fill their reservoirs to almost capacity and store the water to reduce the impact of spring runoff on communities. However, the reservoirs located in the northern part of the Outaouais watershed have access to only 40% of the water inflows. The remaining 60% come from the southern part of the watershed, downstream of the facilities that can hold back water. As a result, the water flowing south of the reservoirs cannot be managed or held back. Although the collaborative efforts of Hydro-Québec and its partners can reduce flows during spring runoff periods, they cannot prevent floods in certain areas when the inflows are too high, since only four drops of water out of ten flow through control structures