Do lakes have tides?

Yes, but you will never see them

Every body of water on Earth feels the Moon, and a lake is no exception. The tide is there, it keeps lunar time, and it is far too small to notice. On the Great Lakes, NOAA measures a spring tide of less than five centimetres and describes the lakes as non-tidal — not because the tide is absent, but because nothing you would do on the water depends on it.

Five centimetres is the whole thing, crest to trough, on the biggest lakes in the world. Compare that with Boston, where the mean range is about 2.9 metres, or the Bay of Fundy at around 16 metres — more than three hundred times the Great Lakes’ entire tide. Meanwhile, on the same lake shore, air pressure alone moves the water level by roughly a centimetre for every hectopascal, so the difference between a deep low passing over and a settled high is worth several times the tide by itself. The Moon is not the thing moving that water.

Lakes have tides. The weather beats them every day.

Why lake tides are so small

What raises a tide is not the Moon’s pull but the difference in that pull across the water. The Moon is 384,400 kilometres away on average; a lake is tens or, at most, a few hundred kilometres across. Over that distance the Moon pulls one shore and the other almost exactly alike, so there is nearly no sideways force to slide water from one end to the other, and it is that sideways force — not the vertical tug — that builds a tide.

The second reason is the size of the wave. A tide is a wave whose crest is thousands of kilometres from the next one and which takes 12 hours 25 minutes to pass, and it needs an ocean basin to develop in. Ocean tides also grow by resonance and by co-oscillation: a gulf or a shelf sea is largely driven by the tide of the ocean it opens onto, which is why the Bay of Fundy reaches about 16 metres while Boston, on the same ocean, has a mean range of about 2.9 metres. A landlocked lake has no ocean next door to drive it, and no room to build a wave of its own. Our guide to tidal range works through what makes one coast enormous and another flat, and what is a tide covers the mechanism itself.

Seiches: the thing people mistake for a lake tide

Stand on a Great Lakes shore for a day and you may well watch the water go down and come back by far more than five centimetres. That is real, and it is not a tide. Wind blowing steadily along a lake pushes water towards the downwind end and holds it there; a difference in air pressure across the lake does the same. When the wind drops, the pile runs back — and then overshoots, and rocks to and fro at whatever period the basin happens to have. That oscillation is a seiche.

NOAA puts Great Lakes seiches at several feet, with four to seven hours between the high and the low. That interval is the trap: a semidiurnal tide puts about 6 hours 12 minutes between its high and its low, so a seiche keeps almost tidal time while having nothing to do with the Moon. The way to tell them apart is not the size or the timing but the cause — a tide is astronomy and can be printed years ahead; a seiche is weather, and can only be forecast the way weather is.

The sea does the same thing under the same forcing, and there it is called storm surge: about a centimetre of level per hectopascal of pressure, plus whatever an onshore wind heaps up, which in a real storm is half a metre or more on top of the predicted tide. A lake has no tide worth speaking of underneath it, so the weather signal is all there is.

Do the Great Lakes have tides?

They do, and NOAA has measured it: the spring tide — the biggest of the month, when the Sun and Moon line up — is less than five centimetres. Lake Michigan is no different from Lake Superior or Lake Erie in this. The same astronomy acts on all five, on the same lunar clock as the sea, and none of them is large enough to answer it with more than a few centimetres.

The reason you will not find that tide on a lakeside gauge trace is that it is buried. Averaged over a long record it separates out cleanly, because it keeps exact lunar time while the weather does not; on any given afternoon, wind set-up and a passing pressure system are simply bigger. That is what NOAA means by calling the Great Lakes non-tidal — a measurable tide that has no practical consequence.

Lakes vs seas vs oceans

Between a lake and an open ocean coast there is a middle ground, and it is occupied by seas that are almost enclosed. The Mediterranean is typically under half a metre — up to about a metre at Venice and around two metres in the Gulf of Gabès — because it meets the Atlantic through the single narrow gap at Gibraltar. The Baltic is smaller still, a few centimetres: at Helsinki the tide is closer to a lake’s than to an ocean coast’s, and there too the level you actually see is set by the wind.

The scale on that figure is logarithmic because an honest comparison has to span three decades. It is the same Moon and the same forcing all the way along it; the only thing that changes is how much ocean the water has to work with.

Why this site has no lake stations

The catalogue here is coastal by construction. Every place in it is measured against the ocean coastline and kept only if it is close enough to it to have a tide worth publishing, so a lake shore never enters in the first place. That is a deliberate limit rather than an oversight.

Underneath it is a second rule. Our heights come from the FES2022 global tide model’s harmonic constants, taken at the nearest ocean point to the station — how we compute them is set out in full. Where the model finds no tidal signal for a place, we mark it as having no tide and publish no tide page for it at all. We do not publish a tide for places where the model finds none, which is the only defensible thing to do with a flat line: a page of numbers that never changes is worse than no page.

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