The Euphrates River is the most important water source and the only navigable river in Syria. It originates in Turkey and flows southeastward across the eastern part of Syria (see Tigris-Euphrates river system). The Euphrates Dam, constructed on the river at Ṭabaqah, was completed in the 1970s. The reservoir behind the dam, Lake Al-Asad, began to fill in 1973.

 

The Orontes is the principal river of the mountainous region. It rises in Lebanon, flows northward through the mountains and the Ghāb Depression, and enters the Mediterranean near Antioch, Turkey. The Yarmūk River, a tributary of the Jordan River, drains the Jabal Al-Durūz and Ḥawrān regions and forms part of the border with Jordan in the southwest.

Scattered lakes are found in Syria. The largest is Al-Jabbūl, a seasonal saline lake that permanently covers a minimum area of about 60 square miles (155 square km) southeast of Aleppo. Other major salt lakes are Jayrūd to the northeast of Damascus and Khātūniyyah to the northeast of Al-Ḥasakah. Lake Muzayrīb, a small body of fresh water, is located northwest of Darʿā; the larger Lake Qaṭṭīnah (Lake Ḥimṣ), a reservoir, is west of Ḥimṣ.

Most of the country's drainage flows underground. On the surface, impervious rocks—consisting of clay, marl (clay, sand, or silt), and greensand—cover a relatively small area. Porous rocks cover about half of the country and are mainly sandstone or chalk. Highly porous rocks consist of basalt and limestone. Water penetrates the porous rocks, forming underground springs, rivers, or subterranean water sheets close to the surface. Although the springs are profuse, the water sheets are quickly exhausted and may turn saline in areas of low precipitation.