In navigation, a bar is a shoaling—a shallowing—of water caused by any landform, from coral reef or barrier island to a ridge of sand. On rivers, the culprit is usually a sandbar. While making life difficult for river pilots, as Mark Twain recounts in Life on the Mississippi, sandbars also create pockets of calmer backwater where algae, invertebrates, and fish can flourish. Since water moves more slowly inside a curve or meander than on the outside, it tends to drop suspended sand and gravel there, creating a meander bar or point bar. A separation bar curls outward from the shore in the direction of the current, while a reattachment bar curls back against the current, a shape formed by eddies just downstream from a constriction, such as a narrows. A sandbank across the entrance to a harbor or bay is a bay-mouth bar, which sometimes takes the shape of a crescent moon. Then it is known as a lunate bar. In Adirondack Explorations, Verplanck Colvin gives us a sense of a purely terrestrial bar: “It was evening when the deep valley widened, and the mountains, parting to right and left, made space for a small plateau or upland prairie—a bar, in mountain parlance—then, circling and closing in darkly and gloomily, seemed to forbid further progress.”

Scott Russell Sanders