Theoretical fomulas of narrowing the breadth of the river mouth (i.e. side gate) in the finite depth estuary were figuredout. The enlargement effect of the entrainment rate by the side gate was also investigated. The results indicated that potential of the entrainment rate was related to the four parameters, i.e. the densimetric Froude number at the outlet of river water, the depth of river mouth relatives to bay, the relative opening rate of the gate and the supply of sea water at the bay mouth. Moreover, the decrease of the relative opening rate of the gate influenced to the increasing of l) the outlet densimetric Froude number of river water, 2) the relative breadth of the river mouth, and 3) the potential of the entrainment rate.
