Fausset's Bible Dictionary
A river near Shushan, by the banks of which Daniel saw the vision of the ram and the he goat (Daniel 8:2; Daniel 8:16). The ancient Eulaeus or Choaspes, for these are two divisions of one river, bifurcating at Paipul, 20 miles N.W. of Shushan; the eastern branch Eulaeus, the western branch Choaspes (now Kerkhah) flowing S.W. into the Tigris. The eastern branch passes E. of Shushan and at Ahwaz falls into the Kuran (Pasitigris) which flows on to the Persian gulf. The undivided stream was sometimes called Eulaeus, but usually Choaspes.
In Pehlevi Eulaeus or Aw-Halesh means "pure water." Strabo (15:3, section 22) says the Persian kings drank only of this water at their table, and that it was lighter than ordinary water. The stream is now dry but the valley traceable, 900 ft. wide, 12 ft. to ft. 20 deep. A sculpture from Sennaeherib's palace at Koyunjik represents Shushan in the time of his grandson Asshur-bani-pal, its conqueror, and the stream bifurcated. In Daniel 8:16 Daniel says, "I heard a man's voice between the banks of Ulai," referring either to the bifurcation or to the river and one of its chief channels, for Eulaeus by artificial canals surrounded the Shushan citadel. The upper Kerkhah and the lower Kuran were anciently united and were viewed as one stream.
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Fausset, Andrew R. Entry for 'Ulai'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/u/ulai.html. 1949.