What river is XXX a tributary of?
Euphrates
The Euphrates (i/juːˈfreɪtiːz/; Sumerian: 𒌓𒄒𒉣: Buranuna, Akkadian: 𒌓𒄒𒉣: Purattu, Arabic: الفرات‎: al-Furāt, Syriac: ̇ܦܪܬ‎: Pǝrāt, Armenian: Եփրատ: Yeprat, Hebrew: פרת‎: Perat, Turkish: Fırat, Kurdish: Firat‎) is the longest and one of the most historically important rivers of Western Asia.