What was it that made them think it was water rather than grain flows in the first place, on this dry grainy planet?
The main driver is that the flows seem to be associated with seasonally higher temperatures, and therefore the flows ("RSL") may be caused by a melting brine. RSLs have been controversial and they have gotten a lot of attention from planetary scientists.
also an isotope in the sand would have allowed water to stay flowing at the below freezing temperatures that mars has, so if you were grasping for straws at why there would be water, this would support it, but if you weren't wishing there was water there as the presupposition then this new answer would have been more immediately obvious
There's nothing more lowering of discussion quality otherwise; it's like a snake in snakes and ladders.