[1] When the river's flow is low, salt waters from the Gulf can reach over 1 km (0.62 mi) from the mouth.

[2] The river enters the Saint Lawrence opposite the Île du Havre de Mingan.

[2] A footpath runs up the river's east bank from the Quebec Route 138 bridge for 5 km (3.1 mi) to a relaxation and picnic area.

According to the surveyor C.-E. Forgues (1885) it is navigable by light canoe to the foot of the large rapid about nine miles from its mouth.

The banks, from the sea to the foot of the first falls, for about five miles, are hills of clay on which there is a layer of sand mixed with black soil.

[7] The bulk of the watershed is on a high plateau that is slightly inclined towards the south and is deeply incised by alluvial valleys.

[10] The coastal plain has large amounts of clay and silt deposited by the Goldthwait Sea after the glaciers retreated.

[8] The valleys of the streams and rivers conform to fractures in the hard bedrock, with straight-line sections intersecting at right angles.

The larger rivers in the center flow in more winding courses through U-shaped glacial valleys, and in places meander.

In its last section the Mingan River again follows a rectilinear course cut through the loose coastal sediments, then makes two large meanders before entering the Saint Lawrence.

The estuary is a submerged delta with multiple channels and shoals fed by erosion from the banks of the downstream part of the river.

When flow is low the salt waters can travel up the estuary 1.5 km (0.93 mi) to the Quebec Route 138 bridge.

Other tree species include white spruce (Picea glauca), jack pine (Pinus banksiana), paper birch (Betula papyrifera) and trembling aspen (Populus tremuloides).

The forest is generally mature and virgin, with few fires and little forestry in recent decades, but there was a large infestation of hemlock looper moths (Lambdina fiscellaria) in the late 1980s, 1990s and early 2000s that caused considerable defoliation of the fir trees in the center of the basin.

[2] The Pourvoirie du Lac Allard et Rivière Mingan, which does not have exclusive rights, manages fishing on part of the river.

[2] The river is known for Atlantic salmon (Salmo salar), and also has rainbow smelt (Osmerus mordax), brook trout (Salvelinus fontinalis), Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus), northern pike (Esox lucius), lake whitefish (Coregonus clupeaformis) and brown trout (Salmo trutta).

[2] Other species include round whitefish (Prosopium cylinraceum), lake trout (Salveninus namaycush), burbot (Lota lota), alewife (Alosa pseudoharengus), American shad (Alosa sapidissima), Atlantic tomcod (Microgadus tomcod) and American eel (Anguilla rostrata).