Abstract: Saline current with specific gravity of 1.003 intruding into a fresh water body of 45 cm deep was generated in a 9.0 m long, 65 cm wide and 60 cm deep tank with a partial-depth lock release scheme. The flow structures of the fully developed current and its front propagation speed at a distance 11.6 to 17.0 times of the partial lock depth were observed and measured, respectively with the laser-induced fluorescence (LIF) flow visualization technique. The results show that the velocity of the advancing front fluctuated as the current moved along. It was caused by the changing shape of the front as well as the three dimensional action of the cross-stream ambient fluid entrainment to the current front. These processes affected the position of the nose of the current hence its propagation speed. Meanwhile, it also complicated the mixing behavior of the gravity current.