If streambeds and river bottoms were smooth, water would slide through with barely a rifﬂe. But the bed rises toward the surface in some places, disturbing the ﬂow. These shallow-water areas are known as shoals, as are the underwater sandbars, rock ledges, or debris that compose them, and which are often hazards to navigation. Shoals also occur at sea. Off Cape Hatteras, North Carolina, the collision of the Gulf Stream and the Labrador Current causes sediment to settle to the bottom, forming the treacherous Diamond Shoals. In The Outermost House, Henry Beston writes of a “wall of ocean dunes” on the forearm of Cape Cod: “Five miles long, this wall ends at a channel over whose entrance shoals the ocean sweeps daily into a great inlet or lagoon.” A large school of ﬁsh breaking the surface is also referred to as a shoal; the movement causes the water to shiver as it would running across a gravel bar.