Gravity currents are caused by density differences between two fluids which may be due to temperature, dissolved substances or the presence of particles in suspension. In this study saline currents, in which the higher density is produced by dissolved salt, are reproduced in laboratory with the aim to characterize the bed shear stress. Saline currents can in fact be responsible for high erosion rates and the bed shear stress is a quantification of this erosive capacity. The dynamics of buoyancy driven flows are complex and the effect of the initial density gravity current on the bed shear stress is not explored yet. The results herein showed confirm the importance of detailed velocity profile measurements for the determination of the friction velocity which is a key parameter for the currents propagation and for characterizing the momentum and mass exchanges between the current and the bed. The spatial evolution of the bed shear stress caused by the passage of a gravity current is here estimated using the logarithmic velocity profile method for, as a first attempt, a value of the von Kármán constant of k 0.405. The use of this constant is then verified and discussed.