Tigris

Boyd’s Bible Dictionary:

(arrow). Great eastern tributary of the Euphrates, rising in the Armenian mountains and flowing southeastwardly 1146 miles. Between it and the Euphrates lay Mesopotamia. In the Septuagint version it stands for Hiddekel, one of the rivers of Eden (Gen. 2:1414And the name of the third river is Hiddekel: that is it which goeth toward the east of Assyria. And the fourth river is Euphrates. (Genesis 2:14)).