The Tigris River
The Tigris River was known by the Hebrews as "Hiddekel" and is one of the two large rivers of Mesopotamia, which the Bible says, flowed from the Garden of Eden (Gen 2:14).
It is formed by the confluence of two rivers that flow from the mountains of Armenia. In ancient times the courses of the Tigris and Euphrates were separate. Their confluence before they flow into the Persian Gulf is very recent.
The Tigris has a greater volume of water than the Euphrates and flows faster, making upstream navigating impossible. The powerful and prosperous cities of Nineveh, Calah and Ashur flourished along its shores. [Ancient Babylonia] Bible History Online