Seiche

A seiche (pronounced say'sh) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, bays and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing natural phenomena to form a standing wave.

The term was first promoted by the Swiss hydrologist François-Alphonse Forel in 1890, who had observed the effect in Lake Geneva, Switzerland. The word originates in a Swiss French dialect word that means "to sway back and forth", which had apparently long been used in the region to describe oscillations in alpine lakes.

Small rhythmic seiches are almost always present on larger lakes. On the North American Great Lakes, seiche is often called slosh. It is always present, but is usually unnoticeable, except during periods of unusual calm. Harbours, bays, and estuaries are often prone to small seiches with amplitudes of a few centimeters and periods of a few minutes. Seiches can also form in semi-enclosed seas; the North Sea often experiences a lengthwise seiche with a period of about 36 hours.