We studied the viscosity of ferric sulfate Fe₂(SO₄)₃ solutions as a model for low-temperature liquids on the surface of Mars and their implication in the formation of gullies. Viscosity varies with temperature and concentration, ranging from 7.0 × 10⁻³ Pa s for 38.8 wt % at 285.15 K to 4.6 Pa s for 58.2 wt % at 260.15 K. Using the experimental results, we built a semiempirical equation of viscosity as a function of temperature and salt concentration, which was combined with a numerical model to estimate the effect of these solutions on the formation of gullies. Calculated fluid velocities ranged from 0.5 to 14 m s⁻¹, in accordance with estimates from image analyses. Turbulent flow occurs in the majority of the conditions and is characterized by a constant velocity (∼8.5 m s⁻¹). At very low temperature and high concentration, the laminar regime shows reduced velocities (down to ∼0.5 m s⁻¹). In between, a transitional regime presents high velocities, up to 14 m s⁻¹. Using the velocities, we determined the size threshold for boulders to be moved by the liquid flow. Depending on the regime, boulders of diameter inferior to 3 m (turbulent), 4 m (transition), and down to 0.5 m (laminar) are displaced. Since laminar flow occurs only in an extremely limited range of conditions, for low temperatures (<240 K) and supersaturated solutions, the abundance of small boulders (∼0.5 m) in gully channels requires lower velocities and higher viscosities than ferric sulfate solution or any other water-based liquid can reach. This suggests an important participation of debris mixed with the liquid phase.