Kommentar zu Bereschit 2:14

חדקל TIGRIS — It is called חדקל because its waters are pungent in taste (חד) and light in weight (קל) (Berakhot 59b).

ושם הנהר השלישי...קדמת אשור, it flows more or less in a continuous direction to the eastern region of what was the land of Ashur at the time the Torah was given. It does not circumvent that country but flows in a northerly direction, seeing that both Babylon and Ashur are situated north of Gan Eden. [In order to make even partial sense of all this, it helps to understand such definitions as “northerly, southerly, easterly,” as relative to the location of Eretz Yisrael, not as relative to the north and south poles of our globe. Rashi already found it necessary to describe the river Nile as running underground for a considerable distance in order to make the Biblical report compatible with the facts as known in his time, much earlier than Kimchi. Ed.] חידקל, this river is so called because its waters are clear and flow rapidly (Berachot 59).

But the Torah writes [these names]... Why did Rashi not say about Pishon, regarding which it is written (v. 11), “It surrounds all the land of Chavilah,” [that the name Chavilah is based on the future]? For a man named Chavilah is mentioned [twice] in Parshas Noach (10:7, 29). The answer is: Here, Chavilah is simply the name of a territory, not named after a person. [Rashi knows this] because Pishon flows in the land of Egypt, so how could it be named after the man Chavilah [who was from a different place]? Furthermore, since it is written ארץ החוילה, with a modifying ה it must merely be the name of a territory. And this answers the question that people ask: How does Rashi know that Kush and Ashur are based on the future? Perhaps they are simply names of territories, just like Chavilah. The answer is: If so, when they are mentioned, their names should begin with a ה, like החוילה is written.