This is the only mention of this river in the Bible. (See: rc://en/ta/man/translate/translate-names)
flows throughout the whole land of Cush
The river did not cover the whole land, but wound through various parts of the land.
the whole land of Cush
"the entire land called Cush"
which flows east of Ashur
"which flows in the land east of the city of Ashur." The Tigris River flows from north to south. The phrase "which flows east of Ashur" gives information about where the Tigris River is. Some languages would translate it as a separate sentence. AT: "It flows east of Ashur" (See: rc://en/ta/man/translate/figs-distinguish)