Which waterway forms the border between Iran and Iraq?

The Shatt al-Arab forms the waterway section of the border between Iran and Iraq.

Known as the Arvand Rud in Iran, the Shatt al-Arab is formed by the meeting of the Tigris and Euphrates rivers near al-Qurnah in Iraq. It flows south toward the Persian Gulf, passing the important port cities of Basra and Abadan before reaching the sea.

The waterway has long been strategically important because it provides access to the Gulf and carries substantial commercial shipping. Its exact boundary line has been contested, especially over whether the frontier should follow the Iranian or Iraqi bank or the channel’s thalweg, the deepest navigable line.

The 1975 Algiers Agreement sought to settle the dispute by placing the boundary along the thalweg. Tensions over the waterway contributed to the Iran–Iraq War, which lasted from 1980 to 1988. The Tigris and Euphrates are tempting wrong answers because both feed the waterway, but the border waterway itself is the Shatt al-Arab.