Seiche

A seiche (pronounced “saysh”) is a standing wave: water oscillates within a bounded or partly bounded basin rather than travelling onward as a single wave. A common trigger is wind that pushes water toward one end of a lake or harbor. A rapid change in atmospheric pressure can also alter water levels. When the forcing eases, gravity draws the displaced water back, and it can continue sloshing between the basin's boundaries. Earthquakes can generate seismic seiches in lakes, rivers, reservoirs, and ponds by setting water bodies oscillating with passing seismic waves.

A basin's geometry and depth influence its natural oscillation periods. The time between high and low water can be several hours; NOAA notes that the interval in some large lakes is roughly four to seven hours, which can make a seiche seem like a tide. The strength of an event depends on the forcing and how the basin responds. Water may rise at one end while falling at another, causing flooding, strong currents, or temporary exposure of shore areas. In ordinary conditions, oscillations may be modest; strong storms can produce hazardous fluctuations. A seiche can continue for hours or, in some cases, days.

A seiche is a standing oscillation within a basin. A meteotsunami is a progressive, tsunami-like wave generated by atmospheric disturbances; it can travel along a coastline or through a basin. The two phenomena can occur together during a storm, and local reports may reflect a combination of pressure-driven waves, wind-driven setup, and subsequent seiche oscillations. Neither should be confused with astronomical tides, which result primarily from the gravitational effects of the Moon and Sun. Weather-driven water-level changes are particularly important on the Great Lakes.

The U.S. Geological Survey documents seismic seiches associated with major earthquakes. Following the 1950 Assam earthquake, oscillations were studied in Norway and England; the 1964 Alaska earthquake produced recorded seiches at numerous water-level gauges across North America and even observations in Australia. Weather-driven seiches are also well documented in the Great Lakes. NOAA's Great Lakes Environmental Research Laboratory describes a complex June 21, 2025 Lake Superior event in which a meteotsunami, wind-driven storm surge, and seiche activity interacted; at its Point Iroquois station, water level rose 45 inches in under two and a half hours. These examples show why identifying the forcing and measuring water levels at multiple sites matter when interpreting an event.