is used by the LXX. as the Greek equivalent of the Hebrew Hiddekel, and occurs also in several of the apocryphal books, as in Tobit, ch. 6:1, Judith, ch. 1:6, and Ecclesiasticus, ch. 24:25. The Tigris, like the Euphrates, rises from two principal sources in the Armenian mountains, and flows into the Euphrates. Its length, exclusive of windings, is reckoned at 1146 miles. It receives, along its middle and lower course no fewer than five important tributaries. These are the river of Zakko or eastern Khabour, the Great Zab (Zab Ala), the Lesser Zab (Zab Asfal), the Adhem, and the Diyaleh or ancient Gyndes. All these rivers flow from the high range of Zagros. We find but little mention of the Tigris in Scripture. It appears, indeed, under the name of Hiddekel, among the rivers of Eden, (Genesis 2:14) and is there correctly described as "running eastward to Assyria;" but after this we hear no more of it, if we accept one doubtful allusion in Nahum (Nahum 2:6) until the captivity, when it becomes well known to the prophet Daniel. With him it is "the Great River." The Tigris, in its upper course, anciently ran through Armenia and Assyria.
Smith's Bible Dictionary (1884) , by William Smith.