River plume fronts are the locus of strong mixing between plume and ambient coastal waters, contribute to coastal productivity, and exert a major impact on coastal ecosystems. The frontal Froude number Fr is an important parameter characterizing the frontal status with respect to both propagation and vertical mixing. In this study, we examine azimuthal variations in Fr using a new remote sensing method. We derive Fr from SAR image data on the basis of the SAR imaging theory and the mechanism of internal wave fission at front. This method is applied to a SAR image showing a front off the Columbia River (CR) mouth taken on 31 May 2003 at 14:33:19 UTC under weak wind conditions. Fr increases from south to north along the front. This variation is consistent with potential vorticity conservation and the influence of tidal currents on the plume. This calculation confirms arguments based on vessel observations by Jay et al. (2009).