Bars may form wherever energetic bodies of water meet: at the meeting of two rivers or where a river meets the sea. Here at the mouth of the Russian River, the river's current meets the onshore-pushing surf, and in the endless battle between the two, the sediment they carry is deposited in this graceful pile. Larger storms or high river flows may push the bar one way or the other. In the meantime, the river gets its business done through the small channel that cuts across the bar.
A bar is often also a barrier to navigation. Thus a sailor may use the word "bar" for a ridge of bedrock, but the geologist reserves the word for a pile of alluvium—the material carried by streams—under the influence of water.