[5][6] After crossing the Gulf of St. Lawrence, the itinerary of the first Europeans: whalers, fishermen, explorers and navigators enters the waters of the Estuary, where the most difficult manoeuvres of the voyage begin: reefs, shoals, diagonal currents, fog and the presence of shallow channels, often narrow and winding, not to mention winter and ice and its unpredictable and fearsome storms.

[7] From Les Escoumins[8] to the Great Lakes, St. Lawrence maritime pilots experts in navigating specific sectors of the River[9] ensure the safe efficient passage of the vessels under their care.

The slow and continuous advance of the great mass of fresh water gradually gives way to the regime of the upflow tide, where twice every twenty-four hours the shores undergo a short alternation of emersion and immersion.

[13][14] Between Saint-Augustin-de-Desmaures and Île aux Grues, from 2008 to 2012, three plants characteristic of the St. Lawrence freshwater estuary and in a precarious situation were monitored annually: Victorin's gentian (Gentianopsis virgata ssp.

[15] Running for about 195 km, the middle estuary of the St. Lawrence extends, according to the authors, from the eastern tip of Île d'Orléans[17] or Saint-Jean-Port-Joli, to the mouth of the Saguenay River.

The hydrodynamic conditions, mainly the reversal of tidal currents and the increase in the salinity gradient, are such that a large amount of suspended solids is trapped and forms a muddy plug (turbidity zone).

The large layers of sediment in the territory are the legacy of tectonic movements combined whit glacial and interglacial periods.

The mechanisms of estuarine circulation associated with this environment make it a privileged site of primary and secondary production which shelters many fish nurseries.

High environmental turbidity provides shelter against predators while larvae are maintained under optimum temperature and salinity conditions.