The rivers flow in a south-easterly direction through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf.

[8][9] Since the 1960s and in the 1970s, when Turkey began the GAP project in earnest, water disputes have regularly occurred in addition to the associated dam's effects on the environment.

Annual snow melt from the mountains brings spring floods, and sustains permanent and seasonal marshes in the lowlands.

From ancient times empires arose and fell in the river basin, including Sumer, Akkad, Babylonia, Assyria, and the Abbasid Caliphate.

[11] As a result of its invention of the qanat system thousands of years ago which uses gravity to transport water through subterranean tunnels, Iran has a history as an agricultural nation, despite its aridity.

The Hilsa shad (Tenualosa ilisha) is an important food fish that lives in the coastal waters and spawns in the lower reaches of the basin.

Other ocean species occasionally visit the lower reaches of the rivers; bull sharks (Carcharhinus leucas) used to swim up the Tigris as far as Baghdad.

Marshy land is home to water birds, some stopping here while migrating, and some spending the winter in these marshes living off the lizards, snakes, frogs, and fish.

The wetland birds Basra reed warbler (Acrocephalus griseldis) and Iraq babbler (Argya altirostris) are endemic to the Mesopotamian Marshes.

Saddam Hussein extended this work in the late 1980s and early 1990s, as part of ecological warfare against the Marsh Arabs, a rebellious group of people in Baathist Iraq.

However, with the breaching of the dikes by local communities after the 2003 invasion of Iraq and the ending of a four-year drought that same year, the process has been reversed and the marshes have experienced a substantial rate of recovery.

Saddam Hussein's government water-control projects drained the inhabited marsh areas east of An Nasiriyah by drying up or diverting streams and rivers.

After the 1991 Gulf War, Iraq's President Saddam Hussein initiated a drainage project on these marshes, leading to degradation of ecosystem services that caused economic and social issues for civilians.

Although they had started to recover after the fall of Ba'athist Iraq in 2003, drought, intensive dam construction and irrigation schemes upstream have caused them to dry up once more.

In 1994, 60 percent of the wetlands were destroyed by Hussein's regime – drained to permit military access and greater political control of the native Marsh Arabs.

In 1983, the Joint Technical Committee (JTC) was established to solve ongoing data controversies between Turkey, Syria, and Iraq but stopped meeting in the early 1990s after only passing two bilateral agreements.

Turkey, Iraq and Syria signed a memorandum of understanding on September 3, 2009, in order to strengthen communication within the Tigris–Euphrates Basin and to develop joint water-flow-monitoring stations.

In exchange, Iraq agreed to trade petroleum with Turkey and help curb Kurdish militant activity in their border region.