The rivière du Castor (French pronunciation: [ʁivjɛʁ dy kastɔʁ], River of the Beaver) is a tributary of Rivière à l'Ours, flowing successively in the municipalities of Sainte-Hedwidge and Saint-Prime, in the Le Domaine-du-Roy Regional County Municipality, in the administrative region of Saguenay–Lac-Saint-Jean, in the province of Quebec, in Canada.

The surface of the Beaver River is usually frozen from the beginning of December to the end of March, except the rapids areas; however, safe traffic on the ice is generally from mid-December to mid-March.

The Beaver River originates at the confluence of two forest streams (altitude: 260 m (850 ft)), in Sainte-Hedwidge.

This source is located at: From its source, the Beaver river flows over 13 km (8.1 mi) with a drop of 150 m (490 ft) especially in forest areas, according to the following segments: The Beaver river flows into a bend on the south bank of the Rivière à l'Ours, on the southwest side of the 3rd range road.

This confluence is located at: From the mouth of the Beaver river, the current successively descends the course of the Rivière à l'Ours on 2.3 km (1.4 mi) toward the North-east; the Ashuapmushuan River towards the south-east on 7.3 km (4.5 mi); then crosses lake Saint-Jean east on 41.1 km (25.5 mi) (ie its full length), follows the course of the Saguenay River via the Petite Décharge on 172.3 km (107.1 mi) east to Tadoussac where it merges with the Saint Lawrence Estuary.