(Seliakha, Suliakha), a river in Iran and the Turkmen SSR, running along the Soviet-Iranian border. It is 669 km long, and its area in the mountain portion of the basin is 27,300 sq km. The source is the Turkmen-Khurasan mountains; within Iran the river flows in a narrow valley. The Atrek empties into the Caspian Sea, forming a swampy delta. In the mountains the river is fed by snow and rain. The river’s high-water period occurs in the spring and early summer, and the constant low-water period occurs in late summer and winter. The water reaches the Caspian Sea only during the high-water period and during the remaining time is diverted for irrigation. The main tributary of the Atrek is the Sumbar (on the right).