Fausset's Bible Dictionary
(See HIDDEKEL.) Genesis 2:14, "running eastward to Assyria." Daniel 10:4, "the great river." Rising in the Armenian mountains, not far from the sources of Euphrates, it flows N.E. of the latter for 1,100 miles, when at last they join and flow as one river into the Persian gulf. Its greatest breadth is more than 200 yards. For the last two hundred, miles before its confluence with the Euphrates the country was intersected with artificial watercourses and adapted river beds, such as the Shat-el-Hie, or river of Hie; and in this district are the ruins of old towns; some scarcely known, as Zirgul, "the city of the brilliant light"; others better known, as Ur (Mugheir). (See UR.) It ran through Armenia and Assyria, and then separated Babylonia from Susiana. Subsequently it was the boundary between the Roman and Parthian empires.
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Fausset, Andrew R. Entry for 'Tigris'. Fausset's Bible Dictionary. https://www.studylight.org/​dictionaries/​eng/​fbd/​t/tigris.html. 1949.