The Assomption River (in French Rivière l'Assomption, named after the Assumption of Mary) is the most important waterway in the Lanaudière region of Quebec, Canada, is over 200 km long, and has a drainage basin of 4220 km². Its source is the Mont Tremblant massif. Seven significant rivers flow into the Assomption (de la Boule, Versailles, Noire, la Chaloupe, Ouareau, Saint-Esprit, and Achigan) before it flows into the Rivière des Prairies at Repentigny, Quebec.
About 150000 people live in the drainage area of this river.
The L'Assomption River watershed overlaps two natural environments, the Laurentian Mountains and the St-Lawrence lowlands, which cover about a third of the watershed area. Its relief is a uniform plain with some hills (from 0 to 100 m) dominated by agricultural landuse. The texture of the soil is rather fine and rests on an impermeable argillaceous (clay) base.
The Laurentian Mountains form part of the Canadian Shield. They are separated from the lowlands by a well defined escarpment that rises about 150 metres. This escarpment has many waterfalls. The Laurentians Mountains have an average height of 230 metres in the south and 460 metres in the north. And there are even mountains of over 600 metres in the Mont Tremblant massif where the L'Assomption River forms.
From north to south the average slope of the river decreases. The steeper slopes of the highlands create a fast current whereas the lowland flats produces a slower flow. Moreover, the river meanders a lot which is especially evident at the town of L'Assomption, almost completely surrounded by a large loop of the river.
The L'Assomption River watershed has 490 lakes of which 24 have a surface area larger than 1 square kilometre. Most of the lakes are located in the Laurentian highlands.
Urban and industrial pollution
The water quality of the L'Assomption River is seriously affected by the area's social-economic conditions. Since 1950, the situation has been complicated by the urbanization of the river banks, by diversified agriculture, and by industrialisation along the southern portion of the river. Although the source of pollution has changed since 1950, because of strict environmental laws and the use of water purification equipment, water pollution is still not fully curbed.
Over 150000 people live in the river's watershed in 43 municipalities, of which the most important are Repentigny, Joliette, Le Gardeur, and L'Assomption. In addition, the northern portion of the river's watershed is a popular vacation area, swelling the population by about 45000 people during the vacation season.
The majority of the municipalities take their drinking water from the river and provide it to more than 100000 people. The drinking water supply to other residences and country cottages, distributed over approximately 90% of the watershed area, comes from water wells.
Besides the urban and industrial pollution, agricultural pollution also contributes greatly to the environment's contamination.
Agriculture has a dominant position along the river: in 1995 there were 1305 producers with 60221 ha (602.21 km²) under cultivation and possessing 73563 animals.
Agricultural pollution takes several forms: direct pollution (discharge of manure directly in the rivers) and diffused pollution (infiltration of fertilizer products in the groundwater). Diffused pollution is difficult to control because it has a multitude of sources throughout the whole region. Yet without a solution to this problem the water quality of the river will never have the desired quality.
List of towns crossed by L'Assomption River
- Saint-Côme, Quebec
- Saint-Alphonse-Rodriguez, Quebec
- Sainte-Béatrix, Quebec
- Saint-Jean-de-Matha, Quebec
- Sainte-Mélanie, Quebec
- Saint-Félix-de-Valois, Quebec
- Notre-Dame-des-Prairies, Quebec
- Saint-Charles-Borromée, Quebec
- Joliette, Quebec
- Saint-Paul, Quebec
- Saint-Gérard-Majella (merged with L'Assomption)
- L'Assomption, Quebec
- Repentigny, Quebec
- Le Gardeur, Quebec (merged with Repentigny)
- Charlemagne, Quebec