A seiche is a standing oscillation of a body of water. Its natural frequencies depend on the basin geometry, water depth and the boundary conditions at its closed walls or open connections.
An ideal uniform shallow basin, closed at one end and held at zero displacement at its open end, has its lowest standing wave with a displacement node at the opening and an antinode at the closed end. Its wavelength is four times its length. Real mouths need not satisfy this perfectly reflecting idealization.
