that would be indicative of viscous flow, as seen here

While I cannot tell for sure without contour lines, or altimetry data, I presume that the longitudinal profile of these gullies is concave. In that case, in the lower "run out" zone, the steadily decreasing gradient should cause deceleration of the fluid that will lead to sinuous flow. In this case, the almost total lack of sinousity argues that the fluid had very low viscosity, too low to be water or the even more viscous brine or acid solutions, and more likely to be a gas/dust fluid. Only in the rarer geomorphic cases of steady gradient (smooth slope), or convex (increasing) gradient, would a viscous flow remain linear. Knowledge of the actual gradient here would allow direct calculation, or at least estimation of the fluid viscosity. I believe that these equations are known, but I do not have my Geomorphology text handy for reference.