The response of a surface-trapped river plume to an upwelling favorable wind is studied using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross-shore Ekman transport. The thinned plume is susceptible to significant mixing because of the vertically sheared horizontal currents. The Ekman dynamics and shear-induced mixing cause the plume to evolve to a quasi-steady uniform thickness, which can be estimated by a critical Richardson number criterion. Although the mixing rate decreases slowly in time, mixing continues under a sustained upwelling wind until the plume is destroyed. Mixing persists at the seaward plume front because of an Ekman straining mechanism in which there is a balance between the advection of cross-shore salinity gradients and vertical mixing. The plume mixing rate observed is similar to the mixing law obtained by previous studies of one-dimensional mixing, although the river plume mixing is essentially two-dimensional. ¿ 2001 American Geophysical Union