The main body of water at the head of the Yamachiche River is the "Great Long Lake" located in the municipality of Saint-Élie. He obtains water from three landfills:
To the northeast: that of Lake Gareau, whose discharge flows south. This lake is located in the municipality of Saint-Mathieu-du-Parc;
To the north: that of the Little Lake Gareau on the north side.
To the northwest: that of a small lake whose discharge is flowing to the south and receiving the waters by:
* north of Bournival Lake whose discharge descends on ;
* the southwest of the Racquet Lake whose discharge is. This lake receives from the north the waters of the Second Racquet Lake whose discharge is also.
Riverside downstream from Grand Lac Long The Great Lake Long dump is located at the bottom of a bay south of the lake. From there, the Yamachiche River flows on: About 40 m in the "Petit lac Long", which flows through 340 m west; 240 m by crossing under a road bridge to go into the Lake Perchaud ; 860 m by crossing the lake at la Perchaud, from the north-west to the southeast. A dam has been erected at its mouth. The Lac à la Perchaude receives through its west point the waters of a series of lakes upstream on the north side: Lac des Chutes Lac Fer à Cheval, and Lake to the island. In short, the Second Lac Raquette is the highest lake of the watershed of the Yamachiche River; The second being the lake on the Island ; the third being Bournival Lake. The course of the river downstream from the lake to the perch From the perch lake, the river descends 260 meters to the south and flows into Lake Garand. Then the river descends 3.9 km southwest through the village of Saint-Élie to the mouth of the Paterson Lake Creek. From there, the river descends 4.7 km south to the limit of the municipality of Charette, Quebec. The river then descends to the south, then turns eastwards to cross the village of Charette. Then the river cuts the southwestern limit of Saint-Boniface-de-Shawinigan before crossing Saint-Étienne-des-Grès where it collects the waters of the Machiche River, where it branches off to the southwest. Then its route serves as the limit between Saint-Étienne-des-Grès and Saint-Barnabé. The river continues southward through the farmland west of the village of Saint-Thomas-de-Caxton. Downstream, the river passes east of the village of Yamachiche to its estuary in Lake Saint-Pierre, where there is also a point and a cove of the same name, 25 km upstream from Trois-Rivières. In addition, the "Petite rivière Yamachiche" runs for 16 km parallel to the west side; This river takes its source in the area of Saint-Barnabé-Nord. This twin river drains the area of the village of Ymachiche and then flows into Lake Saint-Pierre, 1.5 km upstream from the mouth of the main river.
Toponymy
In the past, this river was called "Rivière Grosbois" because it drains the territory of this former lordship, and "Grande Rivière" or "Rivière d'Yamachiche". The toponym "Yamachiche River" was officially registered on December 5, 1968, at the "Banque des noms de lieux" of the Commission de toponymie du Québec.
