The Gulf of Lion's hydrodynamics are investigated with a numerical simulation of the 1998--1999 winter, performed with a three-dimensional (3-D) free-surface model and real forcings. The model initial state derives from the large-scale outputs of the MOM (1/8¿) using a specific initialization method. The model results are validated with an oceanographic data set collected in the gulf during the MOOGLI3 cruise (11--21 January 1999). The modeled outputs agree well with the main hydrological and circulation patterns observed during MOOGLI3. The study focuses on a strong and peculiar event of dense water (Western Mediterranean Intermediate Water (WIW)) formation on the continental shelf and its cascading over the shelf break. This latter was detected by a steep temperature decrease in the time series of a mooring line in the nearby Lacaze-Duthiers canyon which is well reproduced in the simulation. Modeled dense plumes appear during the MOOGLI3 period but more especially during late winter. Shapiro's theory of dense plume propagating on a sloping bottom is applied and provides information of the plume driving mechanisms. In both cases of plume studied, friction effects and Ekman drainage by the ambient current explain only a part of the plume motion. The complementary forcing is probably the steep local bathymetry irregularities. This 3-D simulation permits also to evaluate the total amount of WIW formed during the 1998--1999 winter over the Gulf of Lion's shelf to 500 km3.