There are several rivers in or near Israel, whose Hebrew names begin with the syllable "Yar". These include Yarden, Yarkon, and Yarmuk. Yarden and Yarkon are mentioned in Tanach; the first recorded mention of Yarmuk is in the Roman period. Two more rivers - Yabok and Arnon - might hypothetically begin with a distorted version of "Yar". Israel is semi-arid and only a small number of rivers exist there. Of this small number, it’s surprising that so many begin with the same or similar syllable.
It is often proposed to explain this by saying that before the Hebrew/Canaanite language was spoken in Israel, a different language was spoken, and in this language "yar" was the word for river. So the "Den" river was called Yar Den, and when Hebrew/Canaanite became the local language, "Yarden" was retained as a name. Similarly for the other rivers.
What language could it be that was spoken in Israel before Hebrew?
Look around online, and you will find claims that "yar" means river in ancient Egyptian or Akkadian. If you look in online dictionaries for these two languages, in both the main word for "river" does not resemble "yar". But in the Bible "ye'or" is used to refer to Egyptian rivers (the Nile or one of its branches), and according to the Brown-Driver-Briggs Lexicon, this is derived from the Egyptian "iotr" which can be shortened to "io'r". So Egyptian seems like the most likely source for a word "yar". Based on this, we can hypothesize that before Hebrew/Canaanite was spoken in Israel, a language related to Egyptian was. We don't know if this language became extinct through assimilation or violent conquest, but either way it left only a handful of traces, perhaps including our river names.
Thinking about this last night, I thought of an entirely different line of evidence that Egyptian was once spoken in Israel. In Breishit 10, the genealogy of the 70 nations descended from Noach, Canaan is mentioned as a son of Ham, along with Mitzraim, Kush (Ethiopia), and Put (?). This is even though the Hebrew/Canaanite language is Semitic, so one would expect Canaan to be descended from Shem! Evidently the Torah sees something Hamitic about Canaan, even though the local language at the time of the Torah was Semitic.
To be fair, everything I have said so far is speculation, rather than clear-cut proofs. But when you take two "puzzle pieces" from completely different places (hypotheses based on geography and on genealogies), and find that the pieces "match", each of the two hypotheses looks much stronger than it did before.