In a sandfall, dry sand either falls vertically over suitable drops or cascades down hard slopes, creating stunning visual effects. The process has been described as “dry sandflows cascading down the escarpment face, where the grain concentration decreases dramatically and the streaming component of stress greatly exceeds the collisional component”
Sometimes water enters the mix and makes sand rivers possible too. When it rains in the desert and the water isn’t absorbed quickly, liquid can mix with the top layer of sand … and this wet sand can form moving rivers and waterfalls that you have to see to believe.
Sand generally can’t flow on its own like water as it quickly reaches a stable point where all the sides are at the angle of repose. If you have a very steep bank or sand dune then sand can flow down the side (which can actually be pretty dangerous if you are under it) but it self-limits and stops relatively quickly.