Abstract

In angled river plumes, the river flow is oriented obliquely to the coast, resulting in a lateral expansion into the sea rather than a radial one. However, the frontal structure and dynamics in these systems are not straightforward, especially under the influence of coastal currents. Here, we present detailed observations of an angled river plume and its fronts in the Macao waters. During the ebb tide, the river plume transitions from subcritical to supercritical due to the acceleration caused by lateral contraction. Despite these changes, the plume front remains nearly stationary, forming a steady gravity current that meets the critical conditions in a coordinate system normal to the front. Furthermore, the adjustment of the cross-flow changes the Froude angle, leading the frontal shape to a parabolic planform. Beneath the surface front, internal bores have observed, predominantly driving frontogenesis processes via vertical straining rather than horizontal advection. These findings offer novel understanding of stationary surface fronts, which may trap the river-borne materials and pose significant challenges to coastal environments and ecosystems.