The river that flows through Syria into Iraq and on to the Persian Gulf is the Euphrates River.
The river you are referring to is the Euphrates River. It originates in Turkey and flows through Syria and Iraq before emptying into the Persian Gulf. It is one of the two main rivers in Mesopotamia, the other being the Tigris River.
There is no river that flows into the gulf west of Euphrates.
The headwaters of the Tigris River is in Turkey, and the Tigris River flows through Iraq to the Persian Gulf.
The river flows south from the mountains of southeastern Turkey through Iraq
That is the Euphrates.
The confluence of the Tigris and Euphrates rivers forms the Shatt-al-Arab (Arabic, River of the Arabs) or the Arvand Rud (Persian, Swift River), a river of some 120 miles (200 km). The southern end divides Iran and Iraq, and flows into the Persian Gulf.
Euphrates river
The largest Iranian river that flows into the Persian Gulf is the KARUN RIVER which starts in the Zagros Mountains and winds its way, roughly parallel with the Iraqi Tigris River into the Persian Gulf via a delta in the Iranian portion of the Shatt al-Arab.
The controversial river you are referring to is the Tigris River. It originates in the Taurus Mountains of Turkey, flows through Syria and Iraq, and eventually empties into the Persian Gulf. The Tigris, along with the Euphrates River, is significant for its historical and cultural importance, but it has also been a source of tension among the countries it traverses due to water rights and management issues.
Euphrates River
The Euphrates river flows and ends in the Persian Gulf.