Gullies on Martian sand dunes, like these in Matara Crater, have been very active, with many flows in the last ten years. The flows typically occur when seasonal frost is present.
Here, we see frost in and around two gullies, which have both been active before. (View this observation to see what these gullies looked like in 2010.) There are no fresh flows so far this year, but HiRISE will keep watching! [More at link]