Amu Darya, a river in Central Asia known as the Oxus in ancient times. It is formed by the Vakhsh and Pyandzh rivers, which flow from the Pamirs. The Amu Darya is about 1,600 miles (2,580 km) long. It flows generally northwestward, forming a delta near the Aral Sea. The Amu Darya marks part of the Afghan-Tajik, Afghan-Turkmen, and Turkmen-Uzbek borders and all of the Afghan-Uzbek border. The river is navigable in its lower course and is important for irrigation.