The Sault Plat River leaves Lake Delaunay at an elevation of above sea level and flows south from there to the Gulf of Saint Lawrence. This lake between the mountains is fed in particular by nine discharges of lakes or streams. In particular, it receives the waters of Grace and Boutereau lakes. The mouth of Lac Delaunay is located at:
south-west of the center of the village of Rivière-au-Tonnerre.
From Lake Delaunay, the course of the Sault Plat River descends on, with a drop of, according to the following segments:
to the southeast in a widening of the river on, then crossing the Tête de Loon lake on turning south in the middle of the lake after having bypassed a peninsula attached to the west shore, until its mouth;
towards the south, in particular by crossing Lac Plat, up to a bend river corresponding to the discharge of a set of lakes ;
south, collecting three streams on the east side, descending the mountain in the last to go under the route 138 bridge at the end of the segment, up to its mouth.
The mouth of the river is in the municipality of Rivière-au-Tonnerre in Minganie Regional County Municipality. The mouth is southwest of the center of village of Rivière-au-Tonnerre, north-east of down-town of city of Sept-Îles and south-west of the center of the village of Havre-Saint-Pierre. The elongated river basin runs from NNW to SSE with a length of and maximum width of. In addition, 7.9% of the basin is in the unorganized territory of Rivière-Nipissis in the Sept-Rivières Regional County Municipality. The remainder is in the Minganie Regional County Municipality, divided between the unorganized territory of Lac-Jérôme and the municipality of Rivière-au-Tonnerre. It is to the west of the Tortue River watershed and to the east of the Bouleau River watershed.
Name
The river was not named on the maps by Gustave Rinfret or Edgar Rochette. The name first appeared officially on a map in 1969 in the Répertoire géographique du Québec. The toponym "rivière du Sault Plat" was made official on December 5, 1968 in the place name bank of the Commission de toponymie du Québec..
Terrain
The upper part of the Sault Plat watershed is in a piedmont region that falls from an elevation of around in the north down to in the south. The piedmont region has rounded rocky hills, the highest being in elevation. Below the piedmont is a coastal plain wide that falls from an elevationof in the north to an escarpment almost high along the sea. The bedrock is magmatic, mostly an undeformed assembly of granite and pegmatite, with a band of migmatite more than wide that runs from east to west across the center of the watershed. A thin and discontinuous layer of glacial till no more than thick covers the bedrock. On the coastal plain the post-glacial Goldthwait Sea left deep marine clay and silt sediments, later covered by deltaic sandy sediments. A series of huge glacial flutes on the Sault Plat river bed near its mouth are visible from Quebec Route 138. The Quebec government has designated this section of the river as an exceptional geological site. The river cascades through the large flutes and troughs dug out by glaciers during the last ice age, about 18,000 years ago. At one time the ice sheet was thick. As the climate warmed the ice gradually withdrew to central Quebec, where it completely disappeared about 6,500 years ago. The flutes and grooves are elongated in the direction of the ice flow.
Hydrology
The streams in the Sault Plat basin follow angular courses with straight stretches and sharp bends dictated by fractures in the bedrock. In the widest valleys the streams sometimes meander. The Sault Plat River, counting the section above Lake Delaunay, has a length of and vertical drop of. The annual average flow at its mouth is estimated at, with monthly averages that range from. Lake Delauney, in the center of the watershed, is in an old U-shaped glacial valley. It is long but narrow, covering. In the north of the watershed Lake Boutereau covers and Lake Grace covers. Both are irregular in shape. In total, water bodies cover 12.2% of the basin's area. Wetlands, mainly ombrotrophic bog on the coastal plain, cover 1.63% of the area.
Environment
The Matamec weather station, west of the Sault Plat River mouth, reports annual average temperature of and annual average rainfall of. A map of the ecological regions of Quebec shows the river in sub-regions 6j-T and 6m-T of the east spruce/moss subdomain. Forest cover consists of black spruce and balsam fir, with black spruce dominant on the coastal plain and mixed spruce/fir in the piedmont region. The only recorded fish species is brook trout. Waterfowl congregate on the banks of the south of the river. Reported species at the river mouth by a visitor in August 2019 included double-crested cormorant, northern gannet and various gulls and ducks.
