A tidal wave is not the same thing as a tsunami, and scientists avoid the term. NOAA says tsunamis are often called tidal waves, but oceanographers discourage the name because tides have little to do with these giant waves.
The confusion is worth sorting out, because four different water events get lumped together after a coastal flood: tsunamis, tides, storm surge and seiches. Each has a different cause, and only some of them reach the Great Lakes. This page separates them using NOAA’s own explainers.
Why “tidal wave” is the wrong name for a tsunami
NOAA’s National Ocean Service defines tsunamis as giant waves caused by earthquakes or volcanic eruptions under the sea. The Moon and Sun are not involved.
Tides are a separate process. NOAA describes them as very long-period waves that move through the ocean in response to the forces exerted by the Moon and Sun, appearing at the coast as the regular rise and fall of the sea surface. A tide is predictable and a tsunami is not.
The National Weather Service makes the same point about timing: a tsunami can strike any ocean coast at any time, there is no season for tsunamis, and nobody can predict where, when or how destructive the next one will be.
How a tsunami forms and why it grows near shore
A tsunami starts when an earthquake or volcanic eruption under the sea displaces water. Out in deep water, NOAA says, the waves do not dramatically increase in height, so a ship in the open ocean may barely notice one.
The speed of a tsunami depends on ocean depth rather than on the distance from its source. NOAA says the waves may travel as fast as jet planes over deep water and slow down only when they reach shallow water.
As the depth decreases near the coast, the waves build up to higher and higher heights. That is why a wave that was harmless offshore can flood a harbor.
Video: Today I Found Out
Tsunami, tide, storm surge and seiche compared
People reach for “tidal wave” whenever the sea or a lake rises unexpectedly. The table below shows what actually pushes the water in each case, using definitions from NOAA’s Ocean Service pages.
| Event | What causes it | Where it happens | Predictable? |
|---|---|---|---|
| Tsunami | Earthquake or volcanic eruption under the sea | Any ocean coast | The event is not; arrival times can be modeled once it starts |
| Tide | Gravitational pull of the Moon and Sun | Oceans and their coasts | Yes, tide tables |
| Storm surge | A storm’s winds pushing water onshore | Coasts during storms | Forecast with the storm |
| Seiche | Wind and air pressure changes pushing water from one end of a basin to the other | Lakes, bays and other enclosed or semi-enclosed water | Follows the weather |
Sources: NOAA National Ocean Service “What is a tsunami?”, “What are tides?”, “What is the difference between storm surge and storm tide?” and “What is a seiche?”.
Storm surge: the one that is most often mislabeled
When a hurricane floods a coastline, the water is usually storm surge, not a tsunami. NOAA defines storm surge as the abnormal rise in seawater level during a storm, caused primarily by the storm’s winds pushing water onshore.
Storm tide is the total observed water level, meaning storm surge plus the normal astronomical tide. NOAA notes that the highest storm tides are often seen when a storm coincides with a new or full moon.
So tides do matter here, but only as an add-on to wind. A hurricane is not a tidal wave and it is not a tsunami.
Seiches: the Great Lakes version of a sloshing wave
Lakes do not have ocean tides worth noting, yet they have their own hazard. NOAA describes a seiche as a standing wave in an enclosed or semi-enclosed body of water, much like water sloshing in a bathtub or a cup.
It starts when strong winds and rapid changes in atmospheric pressure push water to one end of a basin. When the wind stops, the water rebounds and keeps oscillating for hours or even days.
NOAA singles out Lake Erie, especially when strong winds blow from southwest to northeast. In 1844, a 22-foot seiche breached a 14-foot-high sea wall and killed 78 people, and in 2008 strong winds created waves 12 to 16 feet high that flooded areas near Buffalo, New York.
The timing adds to the confusion. In some of the Great Lakes the gap between the high and low of a seiche can be four to seven hours, close to the gap between high and low tide in the ocean, and NOAA says it is often mistaken for a tide.
Our explainer on the Lake Erie seiche covers how it plays out on the lake.
NOAA also describes a related event called a meteotsunami. Both seiches and meteotsunamis can be driven by wind and air pressure, but winds matter more for a seiche while pressure often plays the larger part in a meteotsunami.
The two can even occur at the same time. NOAA gives wave periods of three or more hours for seiches and two minutes to two hours for meteotsunamis.
None of these is the same as the slow rise and fall of lake levels over seasons and years.
That longer pattern is covered in our look at water level variability in the Great Lakes region. Water temperature is another way the basin’s water behaves differently, which is the subject of our guide to thermal pollution.
Frequently asked questions
Is a tidal wave the same as a tsunami?
No. NOAA says people often call tsunamis tidal waves, but oceanographers discourage the name because tides have little to do with tsunamis. Tsunamis come from earthquakes or volcanic eruptions under the sea.
Can a tsunami happen in the Great Lakes?
The lakes’ best-documented hazard is the seiche, which NOAA says Lake Erie is known for. NOAA also describes meteotsunamis, which are tsunami-like waves driven by weather rather than by an undersea earthquake.
What is the difference between storm surge and a tsunami?
Storm surge is the rise in seawater level caused primarily by a storm’s winds pushing water onshore. A tsunami is caused by an earthquake or volcanic eruption under the sea, so it can strike any ocean coast at any time.
