The hydrographic slopes adjacent to the Saint-Cyr River are:

• north side: Poussière River, Dontenwill Lake, Bronson Lake;
• east side: Boom River, Marie-Jeanne Creek;
• south side: Ottawa River;
• west side: Penniseault Creek.

"Lac la Truite" (English: Troot Lake) (length: 2.4 km, altitude: 355 m) is the headwaters of the Saint-Cyr River.

From "Lac la Truite", the Saint-Cyr River flows over:

• 2.6 km to the southwest, to the outlet (coming from the north-west) of Fontaine Lake (altitude: 314 m);
• 2.0 km southerly to the outlet (from the east) of Nuage Lake (elevation: 312 m);
• 3.3 km to the southwest, to the outlet (coming from the west) of Braun Lake (altitude: 300 m);
• 2.2 km southerly to the Bory Lake outlet (from the northwest) (elevation 288 m);
• 360 m to the south, to the mouth of the outlet (coming from the southeast) of Lac Sérien (altitude: 267 m;
• 2.5 km to the southwest, to the outlet (coming from the north) of Bonpland Lake (altitude: 301 m);
• 2.3 km southerly to the north shore of Aberdeen Lake;
• 1.2 km crossing Aberdeen Lake to the south (length: 0.4 km, altitude: 212 m);
• 5.0 km to the south, to the mouth of the Boom river (coming from the north) which in particular drains the "Lac de la Touche" (altitude: 168 m);
• 1.0 km southeast across rapids to the outlet (from the north-east) of Walls Lake (elevation 190 m) and Lac des Amis (elevation 274 m);
• 2.6 km (or 1.4 km direct line) southerly passing west of the James Little Little Ecological Reserve and zigzagging to the mouth.

The Saint-Cyr River is a tributary of the northeastern shore of the Ottawa River. The mouth of the Saint-Cyr River is located almost opposite the mouth of "Tee Creek" (Tee Creek - Ontario), 4.1 km downstream of Rapides-des-Joachims, Quebec and 1.1 km downstream of "Point Stewart" (Ontario).