breakwater, offshore structure to protect a harbor from wave energy or deflect currents. When it also serves as a pier, it is called a quay; when covered by a roadway it is called a mole. In the United States a breakwater commonly consists of a long mound of stone rubble and of cheaper materials like rubber tires and oil drums. The flow of waves up its slope, and the formation of swirls by its rough surface dissipate wave energy. A pneumatic breakwater consists of perforated pipes discharging air bubbles; another type has underwater pipes that direct streams of water against approaching waves to cause them to break. Breakwaters are also used to promote sedimentation, which, depending on the breakwater's alignment, will infill to produce a stable beach. The Chesapeake breakwater was the first built in the United States. See coast protection.