The question as written is unanswerable. The Euphrates River is already west of the Iranian border, so any point west of the Euphrates is not part of Iran. If the question meant to ask: What desert lies directly west of the Euphrates River in the southwestern part of Iraq? -- The answer would be the "Arabian Desert". North of the Arabian Desert and west of the Euphrates River, in northwestern Iraq is the "Syrian desert".
Bordering countries: Lebanon (west), Turkey (north), Iraq (east), Jordan (south), Israel (southwest) Bordering bodies of waters: the Mediterranean Sea (west) Bodies of water in Syria: Euphrates (river), Balikh and Khabur (left-side-rivers of Euphrates; on its other side there are just wadis), Lake Assad (created by a dam in river Euphrates) Barada (river) Khabur River Sinn (river) Yarmouk River
latitude is north to south as far as the Euphrates river is concerned, so that would vary as the river flowed southward. the river also flows east to west so the longitude changes also. Cjperry: It is around 38-30 degrees north. If you get Google Earth (free), you can figure any of those questions out answer 2 Latitude:* 39° 11.0' N Latitude (dec): 39.183