A laboratory study of a river discharge to a stratified receiving basin has been made to control a conjecture that mixing occurs predominantly in a frontal region in the absence of wind and tidal currents. Buoyant surface discharges with a densimetric Froude number of ca. 1 and various aspect ratios were studied in a simulated fjord basin with a seaward narrows. The results show that river plume mixing can be simulated with reasonable accuracy in distorted Froude models as long as the width-to-depth aspect ratio is large in the laboratory. The salinity of the river plume reduces with reduced width of the seaward constriction, supporting the above conjecture. Surface tension is a crucial limiting factor for laboratory simulation of plume mixing.