Hiddekel

called by the Accadians id Idikla; i.e., "the river of Idikla", the third of the four rivers of Paradise (Gen 2:14). Gesenius interprets the word as meaning "the rapid Tigris." The Tigris rises in the mountains of Armenia, 15 miles south of the source of the Euphrates, which, after pursuing a south-east course, it joins at Kurnah, about 50 miles above Bassorah. Its whole length is about 1,150 miles.

EBD - Easton's Bible Dictionary