Hiddekel was the name of a river created by God in the pre-Flood world, it is also associated with another river of the same name in the post-Flood world.
Matthew G. Easton writes:
Hiddekel was called by the Accadians id Idikla; i.e., “the river of Idikla”, the third of the four rivers of Paradise (Genesis 2:14).
Gesenius interprets the word as meaning “the rapid Tigris.”
The [modern, post-Flood] Tigris river rises in the mountains of Armenia, 15 miles south of the source of the Euphrates, which, after pursuing a southeast course, it joins at Kurnah, about 50 miles above Bassorah. Its whole length is about 1,150 miles.