What are seiches in the Great Lakes?

A seiche is a rhythmic oscillation of water within a basin, such as the Great Lakes, caused by the tilting of the lake's surface. This tilting, or denivellation, is typically initiated by persistent wind stress or gradients in atmospheric pressure.12

When the external forcing mechanism ceases, gravity acts to restore the water to a horizontal level, causing the water to flow back and forth in a periodic motion. This oscillation continues until it is leveled by damping effects, such as lake-bottom friction and internal turbulence, or until new wind and pressure forces create another tilt.12

In the Great Lakes, intense storms can produce significant water level changes, sometimes reaching as high as 2 metres (7 feet). If a disturbance moves across a lake at a speed near that of the shallow-water wave, the resulting amplification can be profound. These oscillations can disrupt shipping by creating strong currents at harbor entrances or by causing moored vessels to break free from their cables.21