Bas-Saint-Laurent

Our reservoirs in Bas-Saint-Laurent limit the impacts of spring runoff

In the Bas-Saint-Laurent region, Hydro-Québec has facilities on Lac Témiscouata and in the Rivière Mitis watershed. The Témiscouata dam is located 2.5 km north of Dégelis. The Mitis watershed covers an area of 1,820 km2.

Lac Témiscouata acts like a reservoir, holding back spring floodwaters

Thanks to the presence of the dam, Lac Témiscouata is managed like a reservoir: it is emptied gradually in the winter so that it can help limit the impact of the spring runoff. The dam is composed of a 25-gate spillway and a fish pass, which allows species to travel freely between upstream and downstream. Outside of spring runoff season, the level of the lake is controlled by three gates.

Carte Lac Témiscouata

Forecasting precipitation (snow and rain) plays an essential role in the management of Témiscouata dam. Before the spring thaw, Hydro-Québec drains the lake to a minimum level of 147 m, determined based on the water intake at Témiscouata-sur-le-Lac. This limits the impact of the spring runoff. During the spring runoff period, we open the spillway gates according to a specific sequence. When all 25 gates are open, the lake level depends on natural inflows, which are inherently unpredictable.

Lac Témiscouata Dam

During high flood periods, the level of Rivière Madawaska, at the foot of the dam, rises to meet the level of Lac Témiscouata. When this happens, the dam no longer restrains the flow, meaning that the maximum amount of water evacuated depends entirely on the Madawaska’s flow capacity. However, in some stretches of this river, bottlenecks are created, which have a funnel effect that considerably slows down the flow. These narrower stretches limit the quantity of water that can flow through Témiscouata dam.

Mitis watershed: Although the magnitude of spring runoff depends on the weather, it is reduced by our reservoirs

The Rivière Mitis watershed includes two headwater reservoirs (Mitis and Mistigougèche lakes) and two run-of-river generating stations (Mitis-1 and Mitis-2). In addition to the flow from the headwater reservoirs, Mitis-1 and Mitis-2 facilities receive significant inflows from Rivière Neigette and from the watersheds of Mitis and Mistigougèche rivers, over which Hydro-Québec has no influence.

Every year, Hydro-Québec gradually drains its reservoirs in the winter, to make room for as much water as possible during the spring flood. However, Mistigougèche and Mitis lakes cannot absorb all the spring runoff. The water that flows into the Mitis downstream of these structures is only about 30% of the volume of water coming from the many tributaries of this river.

When the spring runoff starts, Hydro-Québec closes the gates of its facilities to minimize the volume of water released into the Mitis, which in turn limits the impacts of the runoff. The amount of water released into the river is only increased again after the runoff has slowed and flood risks for neighboring communities have declined.

The intensity of the spring runoff depends entirely on weather conditions. For example, there is less runoff when accumulated snow absorbs less heat and there is little rain. On the other hand, warm, rainy springs accelerate snowmelt and increase flood-related risks.

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 RAM_Matapedia@hydro.qc.ca.