the.com/sand bar

means A ridge of sand built up just offshore or at a river mouth by currents and waves, often shallow enough to break the surface or snag a boat.

from Straightforward compound: 'sand' plus 'bar' in its old sense of a barrier or obstruction — the same 'bar' that gives us 'harbor bar' and 'sandbar across a river mouth,' meaning anything that blocks or impedes passage. Sailors and river pilots used 'bar' for underwater ridges long before it meant a place to get a drink; the navigational sense is the older one, tracing back to the general idea of a rod or barrier laid across a path.