The Talmud brings a dispute between a Jewish sage and a sage from the nations of the world, and cites the Sambatyon as proof…
That seems to imply that even non-Jews at that time knew there was such a thing?
And also the story about Akdamut on Shavuot — that story didn’t happen either?

I have no idea. Not regarding the Sambatyon (it could be aggadic literature, and maybe even in their time it was already such a legend), and also not regarding Akdamut. See Maimonides in his introduction to the Commentary on the Mishnah about the three groups in understanding the aggadot.

The story about Akdamut is strange and far-fetched for many reasons, not just because of the Sambatyon.

With God’s help, 23 Sivan 5781

In our lifetime we’ve already come across stories far more bizarre than a river that rests on the Sabbath. They tell of a country in Southwest Asia where general elections were held once every six months! And in that same country there were, according to the stories, politicians who thought the prime minister was not right-wing enough, and so they set up a government with the far left and the Arabs! Apparently the heat in the Middle East affects the natives and makes them imagine bizarre things 🙂

As for the Sambatyon, we also find it in Roman literature from the first century CE. Pliny the Elder tells of a river in Judea that rages on weekdays and rests on the Sabbath. By contrast, Josephus tells of a river in the Lebanon mountains that rests all week and flows turbulently on the Sabbath day (see Wikipedia and the “Jewish Encyclopedia – Daat,” entry “Sambatyon,” where reports of this kind are also brought from other places and periods).

The river described by Josephus — which rests all week and flows on the Sabbath — could fit the phenomenon of a “pulsing spring,” whose source is a karst cavity in which water collects until it passes the “pulse level,” and then bursts out all at once. Springs of this kind include Ein Po’em in Nahal Amud and Ein Mabu’a in Nahal Prat. The Gihon too was “pulsing” until close to our own times, until it became a continuous spring (because of hydrological changes and perhaps also tectonic ones).

So it is possible that the river Josephus describes in the Lebanon mountains emerged from a pulsing spring, which would fill up over six days and burst out on the seventh, so there is natural plausibility to the description. Pliny operated in Rome, Germany, and Gaul, so his testimony about Judea is a case of “distant hearsay,” whereas Josephus was a native of the land, and his report about Titus passing by such a river while going through the Lebanon mountains is “close hearsay.”

In the opinion of the researcher Yechiel Shavi, the Sambatyon River was in western Afghanistan, where the members of the Ten Tribes were exiled. According to his hypothesis, the river’s resting on the Sabbath stemmed from the control exercised by the exiles, who observed the Sabbath and therefore closed the river crossings on the Sabbath day.

With blessings,
Nehorai Shraga Agmi-Psisovich

And perhaps the Sambatyon is right here in this place, where on weekdays it roars and foams, while on the Sabbath it “rests like a treasured people; they cease and are quiet” 🙂