The hydrographic slopes adjacent to the Assinica River are:

• north side: Broadback River, Deriares Lake, Camousichouane Lake;
• east side: Broadback River, Assinica Lake, Labeau Lake, Troïlus Lake, Frotet Lake, Mistassini Lake;
• south side: Assinica Lake, Comencho Lake, Capichigamau Lake;
• west side: Lucky Strike Creek, Broadback River, Evans Lake, Nipukatasi River.

The Assinica Lake (elevation: 359 metres (1,178 ft)) is the head lake of the Assinica River. The mouth of the lake is located to the northwest.

From the mouth of this lake, the Assinica River flows for 59.2 kilometres (36.8 mi) along the following segments:

• 25.1 kilometres (15.6 mi) westerly crossing several rapids, then lakes Trépezet (altitude: 342 m), Thiballier (altitude: 339 metres (1,112 ft)) and Boissy (altitude: 319 metres (1,047 ft)); in this segment, the river gathers on the south side the outlet of Triat Lake, as well as the outlet of Chevardière Lake and Lafargue Lake;
• 15.8 kilometres (9.8 mi) northerly across an unnamed lake and collecting the waters of the Sirmac Lake outlet from the east;
• 5.9 kilometres (3.7 mi) westerly across several rapids to the outlet of Lucky Strike Creek;
• 12.4 kilometres (7.7 mi) to the northeast crossing several rapids at the beginning of this segment, to its mouth.

The Assinica River flows on the south bank of the Broadback River, in a bend of the river, into an archipelago area. This mouth is located at:

• 98.6 kilometres (61.3 mi) east of the mouth of Evans Lake, which is crossed by the Broadback River;
• 241.4 kilometres (150.0 mi) East of the confluence of the Broadback River, with the Rupert Bay;
• 122.4 kilometres (76.1 mi) north-west of downtown Chibougamau;
• 108.6 kilometres (67.5 mi) west of Mistassini Lake;
• 24.5 kilometres (15.2 mi) northwest of Assinica Lake.[2]