This study focuses on the formation and physical properties of the gullies observed over large Martian dunes, especially those of the Russell crater (54¿S, 347¿W). Geomorphic features like sinuosities and connections of the channels show that gullies over dunes involve flows with a significant proportion of liquid. The occurrence of levees implies that these flows are debris flows with a yield strength characteristics of Bingham plastic materials. We apply terrestrial methods to estimate viscosity and velocity of these flows from levee size and sinuosities. We obtain average velocities in the range of 1 to 7 m s-1 and apparent viscosities of 2.8 to 46,000 Pa s, with an average at 740 Pa s, compared with the 0.001 Pa s of pure water. These viscosities and velocities are in the range of terrestrial debris flows with a proportion of 10 to 40% of H2O. These properties are typical of water-holding debris flows but not of pure water surface runoff or CO2 driven flows. The debris flows over dunes are oriented on south-facing slopes like other recent gullies. Meltwater from ground ice formed during a recent period of high obliquity is the more likely explanation for the formation of such flows over dunes.