Samarra Dam or Barrage and Thar Thar Dam or Barrage is a multi-purpose dam

Dam

A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...

 on the Tigris River adjacent (west) of Samarra

Samarra

Sāmarrā is a city in Iraq. It stands on the east bank of the Tigris in the Salah ad-Din Governorate, north of Baghdad and, in 2003, had an estimated population of 348,700....

 and north of Baghdad, Iraq. The main purpose of the dam is to divert floodwater in the Tigris River to Lake Tharthar

Lake Tharthar

Lake Tharthar, known in Iraq as Buhayrat ath Tharthar , is a lake situated 120 kilometers north of Baghdad between the Tigris and the Euphrates rivers. It is the largest lake in Iraq. The construction of several dams has made it an important source for irrigation. Its size shrinks and expands...

 through the Thar Thar depression along with irrigation and a 75 MW hydro-electricity station.

It had been hoped that stored water from this dam and its counter-part project, the Ramadi Barrage

Ramadi Barrage

Ramadi Barrage is a two-section diversion dam on the Euphrates River adjacent of Ramadi, Iraq. The main purpose of the northern barrage is to slow or stop water if needed, allowing it to be diverted through the southern barrage into a canal...

might be used for irrigation during the summer months, but it was discovered that the very large evaporation losses, together with the dissolution of salts from the soils of the depressions, seriously diminished water quality and rendered it unsuitable for irrigation purposes.