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From me looking at maps, excluding the Caspian sea and the Great Lakes, I see these in common for seas and lakes:

Seas

  • Most are saltwater
  • Connected to the rest of the ocean via either an isthmus, strait, river, or gulf
  • Bigger and deeper than lakes

Lakes

  • Most are freshwater
  • Landlocked
  • Not nearly as big or deep as any sea

So back to these 2 bodies of water in question. Some people consider the Caspian sea to be a lake and others consider it to be a sea. While historically, it might have been part of a sea, it is landlocked now so it would be more suited to call this the Caspian Salt Lake, thus making it the largest lake on Earth and also the largest saline lake.

The Great Lakes are also in question, to me at least. While the majority of the world considers them lakes and they were historically a lake (really, the rivers and small lakes in that area connecting them are so small in comparison that it is basically 1 body of water), the Erie canal has since then connected the lakes to the St. Lawrence River. So it would be more suited to call this the Great Freshwater Sea. I have no idea how this would rank up to other seas like the Mediterranean Sea in area but it would be one of the world's few freshwater seas (Joining the Sea of Galilee).

So why are the Great Lakes not considered a freshwater sea when other than the freshwater, they are essentially like any other sea?

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    $\begingroup$ You are wrong WRT the Great Lakes. The Erie Canal did not connect them to the St. Lawrence River (which was and is the natural outlet of the lakes to the sea). It connects the Hudson River drainage to Lake Erie (actually the Niagara River upstream of the Falls): en.wikipedia.org/wiki/Erie_Canal Also, while the upper lakes are effectively a single body, Ontario is quite distinct, being separated from Erie by a significant en.wikipedia.org/wiki/Niagara_Falls For the rest, it's a question more suited to the English Language & Usage site. $\endgroup$ – jamesqf May 20 '18 at 19:47
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    $\begingroup$ Just because the Caspian sea is named "sea" doesn't mean it's considered a sea. Don't get too hung up on the name - it's a very old name, long before modern definitions of what a sea is or isn't (and even today, some experts consider it a sea, while others consider it a lake). In many languages, the distinction doesn't even exist in the first place (e.g. the High German "See" means simply "large body of water", which is quite typical for cultures that didn't live close to the sea - they either used the same name, or imported/created a better name later, e.g. "Meer"). $\endgroup$ – Luaan May 21 '18 at 7:53
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    $\begingroup$ If they were in fact seas, wouldn't they then become Small Seas rather than Great Lakes? :) Sounds far less interesting. $\endgroup$ – Viktor Mellgren May 21 '18 at 13:24
  • $\begingroup$ As one "sea" they would share a common surface elevation, but they do not. The great lakes have 4 distinct surface elevations between them, although 3 of the 4 elevations are pretty close, so while they are one system, they are not really one lake (or sea). $\endgroup$ – Anthony X May 21 '18 at 15:18
  • $\begingroup$ @Luaan: Also consider such "seas" as the Dead Sea and the Sea of Galilee, both of which are equivalent to medium-sized lakes. $\endgroup$ – jamesqf May 21 '18 at 17:13
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"Sea" versus "lake" versus "just part of the ocean" is a bit fuzzy, but there are two things that are universal about seas:

  • They are saline.
  • They are either endorheic or have a sea-level (tidally-influenced) connection to the ocean.

Now, let's see how the Great Lakes look under these criteria.

Score: 0 out of 3. Not surprising that the Great Lakes are considered lakes, not seas.

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    $\begingroup$ I would say 0 out of 2, since 2 is the maximum score. $\endgroup$ – Communisty May 21 '18 at 10:00
  • $\begingroup$ Then there are endorheic seas? I have always think a lake is endorheic and a sea never does. $\endgroup$ – Leukocyte May 21 '18 at 15:44
  • $\begingroup$ The Sea of Galilee is not saline. Also note that Ontario is separated from Erie (and hence the upper lakes) by the Niagara River and a 167 ft/51 m waterfall. $\endgroup$ – jamesqf May 21 '18 at 17:19
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    $\begingroup$ Correction is in order here. The Great Lakes DO in fact experience tidal effects, though they are greatly diminished by barometric factors and wind. about 5 cm, depending on the positions of the sun and moon....oceanservice.noaa.gov/facts/gltides.html $\endgroup$ – htm11h May 21 '18 at 20:46
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    $\begingroup$ @htm11h, the Great Lakes experience tides, but it's the connection between them and the oceans that I'm referring to. Contrast the Saint Lawrence River with the Dardanelles. $\endgroup$ – Mark May 21 '18 at 21:15
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From a geological point of view, the two are very different.

The Caspian Sea is thought to be the remnant of an ocean that got cut off from the ocean system (similar to how the Mediterranean might have been periodically cut off and re-connected to the oceans) due to tectonic uplift and a drop in sea levels. The Black Sea was part of the same ocean, but has a much lower inflow than it loses to evaporation - hence the Black Sea has a salinity much higher than the oceans, while the Caspian Sea has a salinity significantly lower than the oceans.

The Great Lakes could be considered a failed ocean. They are in a place where rifting started to create a new ocean, but it never got connected to the ocean system (and flooded), and that was still the case when the rifting eventually stopped. Those rifts were then further (much later) "excavated" by glaciers. The large depressions got filled with melt water (at the end of the last ice age) and rainwater over time, as is typical of many other large/deep lakes. Unsurprisingly, the glacier-sourced melt water wasn't very salty.

In the future, the Caspian Sea will probably get slightly saltier, and filled with sediment. While the influx of fresh water has driven the salinity relatively low compared to the oceans, the sea now only loses water through evaporation - so there's no way for the (tiny amounts) of incoming salts and (relatively large amounts) of sediment to go. As the depth decreases, there might be some expansion of the surface area, but that's mostly temporary - it increases the evaporation rate without increasing the inflow, so water is lost over time.

In contrast, the Great Lakes have an outflow into the ocean, which carries away a lot of the dissolved salts and sediment. Right now, the outflow, combined with evaporation both natural and man made, is larger than the inflow. Overall, this means it's not going to get saltier over time - it keeps pretty much constant salinity, which is pretty much zero compared to the oceans. As erosion continues over the Saint Lawrence River, the outflow will probably increase, resulting in a drop in the water levels of the lakes. However, unlike e.g. the Black Sea, this will not increase the salinity, since the salts are lost through the outflow, rather than being concentrated by evaporation.

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    $\begingroup$ @jamesqf We as a species are trying very hard to make that happen! $\endgroup$ – corsiKa May 21 '18 at 18:11
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    $\begingroup$ @jamesqf it would take melting the East Antarctic ice shelf (the big stable one). The low point for the blocking land between the two seas is 27m above sea level. The unstable West Antarctic ice cap and Greenland ice caps combined would only get us about halfway there. The East Antarctic Ice cap OTOH has about 60m of sea level rise in it; roughtly double what'd be needed to flood the Caspian basin. $\endgroup$ – Dan Neely May 21 '18 at 19:32
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    $\begingroup$ Also, strictly-speaking, the Caspian Sea is actually an ocean, as it's floored with oceanic crust. $\endgroup$ – Sean May 21 '18 at 19:58
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    $\begingroup$ @Dan Neely: As I said, barring serious changes in human behavior (and assuming it's not already too late for those changes to have effects), sea level rise will eventually join the Caspian to the Black Sea. $\endgroup$ – jamesqf May 22 '18 at 5:10
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    $\begingroup$ @Sean Humans might be able of doing a lot of things, yes. Remember that old German project to dry up the Mediterranean? The poor fools didn't quite consider the ecological and economical impact of that, and just imagined all the new land that would open to settlement. Fortunately, at that point, no country was powerful enough to force everyone to go ahead with that plan - and by the time they were, it was far easier (and faster) to just conquer the rest of Europe and start a massive genocide. $\endgroup$ – Luaan May 22 '18 at 5:57
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Other answers have suggested some ways of classifying things into seas or lakes, which are good guides, but inevitably there are exceptions to all of them.

Ultimately, the difference is not an oceanographic or hydrological one, but a semantic and cultural one. The names bodies of water have are historical, so what they are called is down to what somebody saw fit to call them in the past - not any modern taxonomy.

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