Seiche, standing wave that occurs in a lake, bay, or similar basin. A seiche is most often started by wind or a change in air pressure, and sometimes by an earthquake or ocean swell, that suddenly alters the basin's surface. While the surface regains stability, a simple harmonic wave is set in motion along the length of the basin. This wave is reflected by the basin's walls, creating interference. The repeated reflections produce a standing wave, a wave having fixed points with no vertical motion, with a wavelength equal to twice the length of the basin. Lasting a few minutes to an hour, seiches can interfere with shipping, damage property, and imperil lives.