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Ocean water is flexible enough to allow water from each side of low tide to flow to the sides of high tide.

So for a high tide, there must be a low tide somewhere else from where water is drawn out. But if it's a small inland lake (not connected to any sea at all) that is in the part of earth closest to the moon, it cannot draw out water from any part to increase its water level even by a miniscule amount.

I feel I am missing some concept here. Any insight would be appreciated.

Curiosity
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    What small inland lakes do experience tides? – BowlOfRed Mar 10 '24 at 21:15
  • Tides are complicated. https://physics.stackexchange.com/a/121858/123208 – PM 2Ring Mar 10 '24 at 21:19
  • @BowlOfRed

    I read the same. https://www.theguardian.com/notesandqueries/query/0,,-199833,00.html

    To quote it, ' Theoretically lakes must experience tides but the tides would be so small that even in the largest lakes the effect is masked by river inflows...'

     – Curiosity Mar 10 '24 at 21:21
  • I wanted to understand the theoretical perspective itself. I get that the extent won't be much but that's not what I am after! – Curiosity Mar 10 '24 at 21:22
  • Multiple sources besides the link mention that theoretically tides occur even in the smallest of lakes that are not connected to any oceans or large bodies of water. – Curiosity Mar 10 '24 at 21:23
  • Related: https://physics.stackexchange.com/q/258706/123208 https://physics.stackexchange.com/q/202788/123208 – PM 2Ring Mar 10 '24 at 21:27
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    Lakes dont have significant tides but they do have seiches https://en.wikipedia.org/wiki/Seiche – mike stone Mar 10 '24 at 22:01
  • Water is a low viscosity liquid, not a flexible solid. – my2cts Mar 11 '24 at 05:38
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    ‘ water from the side where the moon is not located to flow to the other side where the moon is.’ The high tide is in principle both on the side of the moon and on the opposite side. This is why the frequency is 12, not 24, hours. – my2cts Mar 11 '24 at 05:42

2 Answers2

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Ocean water is flexible enough to allow water from the side where the moon is not located to flow to the other side where the moon is.

That's not how tides work. Ocean tides are complicated standing waves that slosh about in the world's major ocean basins.

https://www.youtube.com/watch?v=PSJRymZ5bJs

The oscillation is driven by the Earth's rotation and gravitational interaction with the Moon at the incredibly low-frequency of just a bit less than two cycles per day.

At such a low frequency, the wavelength of the tidal waves is huge. Ocean basins are large enough to contain waves of that length, and they can resonate with that two-cycle-per-day forcing function. Lakes just aren't big enough. The water in the lake feels the force, same as how ocean water feels it, but there's just no room in the lake for it to "slosh about" in the same way that it does in the oceans.

Solomon Slow
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  • I see. But how does one explain low tides in that case? – Curiosity Mar 10 '24 at 21:47
  • @Curiosity, Same as how one explains high tides? You can't have one without the other. Water level can't just keep going up forever. If it goes up twice per day, then it must also go back down twice per day. – Solomon Slow Mar 10 '24 at 22:13
  • Nice answer, +1. To stress the complexity of the phenomenon, one should take into account that there are places with just one high tide per day and also places without any tide at all (amphidromic points; https://en.wikipedia.org/wiki/Amphidromic_point ) – GiorgioP-DoomsdayClockIsAt-90 Mar 10 '24 at 22:24
  • Some lakes are large enough to have significant tides. Lac Leman in Switzerland, the Great Lakes in North America... – D. Halsey Mar 11 '24 at 00:11
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The largest gravitationally driven tides in lakes are not due to the Sun and Moon acting directly on the lake water, but instead are indirect effects due to solid Earth tides "tilting" the lake. If the lake is close enough to open sea (e.g. Lago Fagnano), then ocean tides can also change the local gravitational equipotential surface at the lake by the changing the ocean weight loading on the land mass containing the lake.

Tidal amplitudes in most lakes are only a few millimetres, much smaller than other water level oscillations such as wind driven seiches that may also have daily cycles that are hard to disentangle from semidiurnal and diurnal tides. Because it is shielded from wind and temperature swings, the 18 mm amplitude tides in the subglacial Antarctic Lake Vostok seem to be a particularly clean example of lake tides.

For more details, see (for example) Melchior (1978) "The tides of the planet earth", Section 8.2 "Lake Tides".

David Bailey
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