Midrashic sources tell the Sambatyon is a miraculous river in which during the six week days only dirt and rocks flow, and on the Sabbath its rests. We must clarify why it is we are unaware of any such river in Damascus, Syria. An allusion to a possible answer can be found in the words of R’ Tzemach Gaon.
The personality of Eldad HaDani is a familiar one in Jewish history. He visited various Jewish communities in the ninth century CE and reported finding members of the tribe of Dan exiled in Africa. He tells enthralling stories of the Ten Tribes, describing them as remarkable warriors, and relates tales of the descendants of Moses who are imprisoned beyond the Sambatyon. The Jews of Kairouan requested R’ Tzemach Gaon’s opinion of these stories, and this is his reply (Otzar HaMidrashim [Eisenstein], Eldad HaDani, p. 21):
"[When he says] that the descendants of Moses are with them and the Sambatyon River encircles them, he tells the truth, for the sages of the Midrash teach that Nebuchadnezzar exiled 600,000 Jews and when they reached the rivers of Babylon, with their harps, they experienced what R’ Eldad has related to you. And before our forefathers came to the land of Canaan they engaged in wars and forgot the teachings which they had received from the mouth of Joshua. It is even written of Joshua, may he rest in peace, that he became uncertain about some matters after the death of Moses. And of all the tribes throughout the land, those of Judah and Benjamin clung to the Torah more than the rest."
When R’ Tzemach Gaon says that the descendants of Moses engaged in wars before entering the Land of Israel, he is not referring to Gershom and Eliezer, the sons of Moses, or their offspring, for they were members of the tribe of Levi, which did not participate in wars. Rather, he intends to hint that all of the Israelites who were in the army – they were 600,00 – are considered the (spiritual) sons of Moses. They ought to be versed in the Torah, but because they engaged in warfare they forgot what they had learned.
And when he says "and of all the tribes throughout the lands, those of Judah and Benjamin clung to the Torah more than the rest" he is insinuating that the Midrash about the Ten Tribes beyond the Sambatyon is a parable about those who chose a life of work rather than clinging to the Torah. Later, he says that because of the hardships that Eldad HaDani experienced, inaccuracies arose in his accounts, and one who is wise enough can deduce that due to these hardships he transformed the parables into a living reality.
Israel guarded the Sabbath (the Sambatyon), and the Sabbath guarded them. The Jewish people did not assimilate, "and the Sambatyon River encircles them." During the week days the Jewish people are busy with mundane matters, and the bustle and noise of these days is similar to the din of "rolling stones." Only with the onset of the Sabbath, when the Jewish people rest from their labor, does a spiritual cloud of sanctity descend upon them. The respite and joy and Torah study on Sabbath returns the souls to those who do not labor in Torah, and the Sambatyon "encircles them" and serves to prevent them from assimilating amongst the nations.