Earth Oddities • Strange Weather Phenomena • Ocean & Coastal Phenomena
Seiches are standing waves that slosh back and forth inside lakes, bays, harbors, reservoirs,
fjords and enclosed seas. They can make water suddenly rise, fall, surge, retreat or flood shorelines without
an ocean tsunami. This guide explains lake seiches, harbor seiches,
standing wave lakes, Great Lakes seiches, harbor oscillations
and why some people describe them as a “tsunami in a lake.”
Updated:
TL;DR: Seiches in Plain English
- A seiche is a standing wave that sloshes inside an enclosed or semi-enclosed body of water.
- Seiches can happen in lakes, harbors, bays, reservoirs, fjords and enclosed seas.
- They are caused by wind, pressure changes, storms, earthquakes, landslides or sudden water displacement.
- Lake seiches can make water pile up on one side of a lake, then swing back to the other side.
- Harbor seiches can damage boats, break moorings and create strong currents in marinas.
- Some seiches look like a tsunami in a lake, but the physics is usually basin sloshing rather than an ocean tsunami.
What Is a Seiche?
A seiche is a standing wave that oscillates inside a body of water. Instead of a single wave traveling
forward and disappearing, the water sloshes back and forth inside a basin, much like water moving inside a bathtub,
cup or swimming pool.
Seiches can occur in lakes, harbors, bays, reservoirs,
fjords, estuaries and semi-enclosed seas. They may last for minutes,
hours or even longer depending on the size and shape of the basin.
What Seiches Can Look Like
- water suddenly rising along one shoreline;
- water rapidly withdrawing from a harbor or lake edge;
- repeated surges every few minutes or hours;
- boats rocking, drifting or breaking moorings;
- strong currents inside harbors, channels or marinas;
- shoreline flooding without an ocean tsunami.
What Is a Standing Wave?
A standing wave is a wave pattern that appears to stay in place while oscillating.
It forms when waves reflect and interfere with themselves, creating repeated high and low points.
In water, a standing wave can develop when a basin is disturbed and then continues to oscillate at its natural rhythm.
One side of the basin rises while the other falls, then the motion reverses.
Standing Wave Terms
- Node: a point where water-level change is small or nearly zero.
- Antinode: a point where water-level change is greatest.
- Oscillation period: the time it takes for one full slosh cycle.
- Resonance: amplification when forcing matches the basin’s natural frequency.
How Do Seiches Form?
Seiches form when water inside a basin is disturbed and begins oscillating. The initial disturbance can come from
wind, atmospheric pressure changes, storms, seismic shaking, landslides, rapid inflow, outflow or passing long waves.
The Basic Seiche Recipe
- Disturbance: wind, pressure, storm forcing, earthquake shaking or displacement pushes water.
- Water setup: water piles up on one side of the basin or shifts away from equilibrium.
- Gravity response: gravity pulls the water back toward balance.
- Overshoot: water swings past equilibrium and piles up on the opposite side.
- Oscillation: the basin sloshes back and forth until friction damps the motion.
Lake Seiches
A lake seiche is a standing wave inside a lake. Wind can push water toward one end of the lake,
raising the water level there. When the wind weakens or shifts, the piled-up water flows back and may overshoot,
creating a repeated sloshing motion.
Lake seiches can be small and barely noticeable, or large enough to flood shorelines, expose lakebed, damage docks
and create dangerous currents.
Lake Seiche Effects
- rapid water-level rise on one shore;
- temporary water withdrawal on the opposite shore;
- flooding of beaches, roads or lakefront properties;
- boats grounding or pulling loose;
- strong currents near channels, piers and harbor entrances;
- repeated surges as the lake oscillates.
Great Lakes Seiches
The Great Lakes are famous for seiches because they are large, elongated freshwater basins exposed
to powerful storms, rapid pressure changes and strong winds. Water can pile up at one end of a lake and then swing back,
creating dramatic water-level changes.
Great Lakes seiches can affect harbors, beaches, shipping, marinas and lakefront communities. They are especially
important because people often underestimate how ocean-like large lakes can become during strong weather events.
Why the Great Lakes Are Seiche-Prone
- large lake size and long fetch;
- strong winds over broad water surfaces;
- rapid storm and pressure changes;
- elongated basin shapes;
- many harbors, bays and shoreline communities;
- water levels that can swing quickly during major events.
Harbor Seiches & Harbor Oscillation
A harbor seiche is an oscillation of water inside a harbor, marina, port or bay.
These events can be triggered by incoming long waves, atmospheric pressure disturbances, swell, storms,
earthquakes or changes in wind and water level.
Harbor seiches matter because even modest water-level changes can become damaging in confined spaces.
Boats can surge against docks, mooring lines can snap, currents can accelerate and floating structures can move violently.
Harbor Seiche Hazards
- boats breaking moorings;
- strong currents between docks;
- rapid water-level rise and fall;
- damage to marina infrastructure;
- dangerous conditions for people on docks;
- repeated surging after the initial trigger passes.
Harbor seiches often overlap with meteotsunamis,
because atmospheric waves can trigger water-level oscillations that are then amplified by harbor resonance.
Tsunami in a Lake?
People sometimes describe a sudden lake surge as a “tsunami in a lake”. In many cases, the better term
is lake seiche, especially when the water is oscillating back and forth inside the basin.
However, lakes can also experience true tsunami-like waves from landslides, volcanic activity, glacier collapse,
underwater slope failure or sudden displacement. That means the phrase “lake tsunami” can refer to different processes,
and the cause matters.
Lake Seiche vs Lake Tsunami
- Lake seiche: basin-wide sloshing and standing-wave oscillation.
- Lake tsunami: sudden displacement wave from landslide, collapse, impact or slope failure.
- Meteotsunami on a lake: tsunami-like wave generated by atmospheric forcing.
Main Seiche Triggers
Seiches can be triggered by several different forces. What matters is that the water is displaced and the basin
is able to oscillate afterward.
Common Triggers
- Strong winds: push water toward one side of a lake or bay.
- Rapid pressure changes: disturb the water surface and help trigger oscillations.
- Thunderstorms and squall lines: combine wind shifts and pressure jumps.
- Meteotsunamis: incoming weather-generated long waves can excite harbor seiches.
- Earthquakes: shaking can disturb enclosed water bodies.
- Landslides or slope failures: sudden displacement may start oscillation.
- Rapid inflow or outflow: changes in water volume can disturb reservoirs or basins.
Basin Resonance & Natural Frequency
Every lake, harbor or basin has natural oscillation periods controlled by its size, depth and shape.
If a disturbance pushes the water at or near that rhythm, the motion can amplify. This is called
resonance.
Resonance explains why one harbor may experience a damaging seiche while another nearby harbor sees only minor motion.
The same incoming wave can affect different basins very differently.
What Controls a Basin’s Response?
- basin length;
- average water depth;
- shoreline shape;
- harbor entrance width;
- seafloor or lakebed slope;
- friction and bottom roughness;
- whether the forcing matches the natural period.
Warning Signs of a Seiche or Harbor Oscillation
Seiches can begin suddenly, especially during storms or rapid pressure changes. In some cases, the first sign is
water behaving strangely along the shoreline or inside a harbor.
- water level rapidly rises or falls without an obvious tide explanation;
- boats suddenly surge against docks;
- currents strengthen inside harbors or channels;
- shoreline water withdraws and then returns;
- repeated water-level oscillations occur every few minutes or hours;
- strong winds or pressure jumps have just passed through;
- official lake, coastal flood, high wind or marine warnings are active.
Seiche Comparison Table
| Phenomenon | Main Cause | Where It Happens | Key Difference |
|---|---|---|---|
| Seiche | Basin oscillation after water displacement | Lakes, bays, harbors, reservoirs | Standing wave sloshing inside a basin |
| Lake seiche | Wind setup, pressure change or storm forcing | Large lakes and reservoirs | Water piles up on one side, then swings back |
| Harbor seiche | Harbor resonance from incoming long waves or pressure forcing | Harbors, marinas, ports and bays | Confined water oscillation that can damage boats and docks |
| Meteotsunami | Atmospheric pressure jump or fast storm disturbance | Bays, harbors, lakes, enclosed seas | Weather-generated tsunami-like wave that may trigger seiches |
| Storm surge | Storm winds and low pressure raising water level | Coasts, estuaries, deltas | Broad water-level rise, not mainly basin sloshing |
| Lake tsunami | Landslide, collapse, impact or sudden displacement | Lakes and reservoirs | Displacement wave, not necessarily a standing oscillation |
Seiche Hazards
Seiches can be harmless background oscillations, but larger events can become dangerous for shorelines, harbors,
boats, docks and low-lying areas.
Main Hazards
- Shoreline flooding: water piles up and overtops low areas.
- Sudden drawdown: water withdraws from one side of a basin.
- Strong currents: flow accelerates through channels, harbor entrances and docks.
- Boat damage: vessels surge, collide or break moorings.
- Dock hazards: people can be knocked down or trapped by moving boats and water.
- Repeated impacts: oscillations may continue after the trigger has passed.
Seiche Safety
- Stay away from docks, piers and harbor edges during sudden water-level oscillations.
- Move back from shorelines if water rapidly rises or withdraws.
- Do not walk onto exposed lakebed or harbor floor during sudden drawdown.
- Secure boats only if it is safe to do so; do not risk personal safety for property.
- Watch for repeated surges after the first water-level change.
- Follow local weather, marine, flood and shoreline warnings.
FAQs About Seiches and Standing Waves
What is a seiche?
A seiche is a standing wave that sloshes back and forth inside a lake, bay, harbor, reservoir or other enclosed basin.
What causes a lake seiche?
Lake seiches are commonly caused by strong winds, rapid pressure changes, storms or other disturbances that push water
toward one side of a lake and then allow it to oscillate back.
What is a harbor seiche?
A harbor seiche is a water-level oscillation inside a harbor, marina or port. It can cause boats to surge,
moorings to break, currents to strengthen and water levels to rise and fall rapidly.
Can a seiche happen in the Great Lakes?
Yes. The Great Lakes are well known for seiches because their large size, long fetch, strong storms and basin shapes
allow water levels to oscillate significantly.
Is a seiche the same as a tsunami?
No. A seiche is a standing-wave oscillation inside a basin. A tsunami is usually a traveling long wave caused by
earthquake, landslide, volcanic activity or sudden displacement. Some seiches can look tsunami-like near shore.
Can there be a tsunami in a lake?
Yes, a lake can experience tsunami-like waves from landslides, collapses or sudden displacement. However, many sudden
lake water-level changes are better described as seiches when the basin is sloshing back and forth.
Are seiches dangerous?
They can be. Large seiches can cause shoreline flooding, sudden drawdown, strong currents, boat damage, dock hazards
and repeated water-level surges.
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