Rashi on Genesis 2:14/רש"י על בראשית ב׳:י״ד

1חדקל TIGRIS — It is called חדקל because its waters are pungent in taste (חד) and light in weight (קל) (Berakhot 59b).

2פרת EUPHRATES — It is called פרת because its waters grow (פרה) and increase and make men healthy (Berakhot 59b). כוש ואשור CUSH AND ASHUR did not then exist as countries, but Scripture writes with reference to the names which those districts would bear in the future (Ketubot 10b; Genesis Rabbah 16).

3קדמת אשור means AT THE EAST OF ASHUR.

4הוא פרת IS THE EUPHRATES — the most important of all, being mentioned in connection with the Land of Israel (Genesis Rabbah 16:3).