Estuary plumes on the western Louisiana continental shelf in the Gulf of Mexico, outside of constricted channels of the Sabine and Calcasieu Lakes were observed on two RADARSAT-1 synthetic aperture radar (SAR) images. The in situ data showed a change in salinity of similar to 2-3 PSUs across the front on the northwest edge of the plume, corresponding to locations of vertical salinity variation of 5.5-7.5 PSU. Velocity magnitude outside of the inlet reached more than 2.3 ms(-1) seaward and a brackish lens in the upper 4.5 m of the water column, the spread of which contributed to the development of a plume. The volume of brackish water within the plume was estimated an order of magnitude larger than the volume output from the Calcasieu River within an entire ebb tidal period. This implies that the plume was formed during multiple tidal cycles and it sustained during the flood tides because only weak diurnal tides exist. Both satellite and in situ observations showed an interesting western intensification of the plume. An analysis to the momentum/vorticity equations suggests that the western intensification is due to the asymmetry of the vorticity-divergence relationship under the Coriolis force. This can also be enhanced by an ambient coastal current that strengthens the convergence at the front. Estuary plumes are shown as bright and dark features in two SAR images. Results from a radar model simulation and in situ water temperature data showed that the SAR observed plume features were dominated by the water temperature differences.