The propagation of gravity currents over horizontal porous substrates which are initially saturated with a lighter fluid is analytically and experimentally studied. While the dense fluid flow through the bed is well described by Darcy’s law, the attention is focused on the gravity-driven flow dynamical changes generated by using an impermeable substrate under the porous layer. It is found that larger the thickness of the porous bed and the relative density difference between the current and environment fluids, quicker the current mass decreases, and the variation in time of the current lateral area, associated to the current mass, is a good experimental parameter to verify the model. In addition, the evolution of the pressure field and the stream function within the porous bed is provided for different thicknesses.