The Hiddekel (Heb: Chiddeqel; Persian: Tigra) was one of four rivers that flowed from the region of the Garden of Eden. This also known as the Tigris. It is said to have been the one that ran towards the east of Assyria (Asshur). It branched off from a mighty river that came "out of" the Garden. Most translations favor using the Tigris, based on both the Greek and Latin forms of the original.
In the years following the Great Flood, Noah and his descendants would find a plain between two rivers, they would name the one on the east "the swift river" (Sumerian id igina, which became digna, then tigra.) The one on the west, being wider, ran slower. They called it "Phrat," interpreted "fruitfulness" from a root meaning "to break forth" (from the ground, as in plant life?). Other languages added a prefix and the "Euphrates" came into common usage.
Between these two rivers, civilization would be restarted. Out from their banks would go seventy people groups, each looking for their own rivers to help them survive. Sons of Japheth would find the Indus to the east and the Rhine to the west. Sons of Ham would find the Nile and the Jordan. Semites would cling to the Euphrates before Abraham was called to Canaan, where his descendants would eventually claim the Jordan valley.
After the Babylonians had taken the Jews captive, and themselves fallen to the Persians, Daniel would find himself outside on the banks of the Hiddekel. There he would be visited by an angel from God whose voice both scared away all others and put him in a trance.