When a river plume encounters a coastal valley, local dynamics are modified relative to those with seawater. However, the underlying dynamical processes remain poorly understood. In this study, an idealized numerical model is employed to investigate how the presence of a river plume alters the flow behavior over a coastal valley under downwelling-favorable winds. The arrival of the plume intensifies the alongshore current, weakens the cross-shore flow, and induces another upwelling–downwelling dipole within the valley. Simultaneously, the net vertical and cross-shore transport reverse from a downward–offshore pattern under seawater conditions to an upward–onshore pattern with a plume. The river plume not only amplifies the contribution of vertical vorticity diffusion but also reshapes the spatial distribution of relative vorticity changes that govern vertical motion in the valley. Compared with the dominance of relative potential vorticity advection in the no-plume case, the primary driver of cross-shore transport within the valley shifts to the joint effect of baroclinicity and topographic relief when the plume is present. These dynamical changes arise from the sea level anomaly associated with valley-induced coastal trapped lee waves and the spatial density variations within the river plume. After wind relaxation, mesoscale instabilities develop throughout the water column, featuring longer wavelengths on the downstream side of the valley. During this stage, eddy potential energy is converted into eddy kinetic energy, part of which is subsequently transferred to the mean kinetic energy.

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