The hydrodynamics of the St. Jean River estuary, particularly the flow dynamics, was evaluated and the mixing mechanisms were studied for fluvial water – marine water. In order to realize this objective, two series of measurements were undertaken on the solid and liquid discharges: the first took place during the spring flood (May 1980), the latter in conditions of lowest level spring tide (August 1980).The average stream discharge is 1035 m3/s during floods and 85 m3/s at periods of lowest water level. When the stream discharges exceed 160 m3/s, the wedge of salt water is pushed out of the estuary and no reversed currents are present; there is only a reduction of current velocity at the mouth of the river.The quantities of solid particles in suspension vary from 96 mg/L during floods (65–95% mineral matter) to 4.6 mg/L for lowest water levels (58% mineral matter).The general behaviour of the St. Jean River estuary suggests a new type of environment, the subboreal estuaries. They are characterized by a Qm/Qf ratio close to the one observed for highly stratified estuaries, but the regime of the material in suspension does not fit with that of micro- or mesotidal estuaries of New England or with that of microtidal estuaries of western Europe. [Journal Translation]

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