Fully developed saline gravity currents with specific gravity of 1.003 intruding a fresh water body are generated with the lock-exchange scheme in a 9.0 m long, 65 cm wide and 60 cm deep tank for investigations. The laser-induced fluorescence (LIF) flow visualization technique is utilized to observe the detailed flow structures with special emphasis on the current head. Continuous fluid motions in the longitudinal and transverse planes starting at 2.2 m downstream from the gate opening are recorded by a professional digital video camera for frame-by-frame visual analyses and measurements. It is shown that the continuous generation of eddies is originated at the current front. The flow in the head is very complex and it has structures varying both in the longitudinal and transverse directions. The eddies generated at the front are then manifested, stretched and bended by the shear flow developed behind and between the lighter and heavier fluids to form large-scale rollers similar to Kelvin-Helmholtz vortexes developed in free shear layers but much short-crested. It is believed that the cross-stream ambient fluid entrainment actions at the foremost leading edge of the current initiate intense mixing and it may provide the needed instabilities at the flow interface, hence feeding the growth as well as creating the three-dimensionality of the subsequent large-scale rollers exhibiting the lobe-and-cleft features.