There are fundamental differences between the mechanics of entrainment in river plumes and two-layer channel flow. Entrainment of seawater to a river plume is seen to occur predominantly in a frontal region. A simple theory for the energetics of frontal entrainment is applied to river discharge in a wide fjord with a seaward narrows. The salinity of the plume depends onW/BandB/Ho, whereWis the width of the narrows,Bis the river width andHois the river thickness. The theory contains two constants which are determined from laboratory experiments. From laboratory observations, active plume entrainment is estimated to extend to 7 (H + B) seaward. The theory supports the conjecture that river plume entrainment can be simulated in distorted laboratory models as long asB/Ho≫ 1 in the laboratory.