The Indus ( IN-dəs) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The 3,180 km (1,980 mi) river rises in western China, flows northwest through the disputed Kashmir region, first through the Indian-administered Ladakh, and then the Pakistani-administered Gilgit-Baltistan, bends sharply to the left after the Nanga Parbat massif, and flows south-by-southwest through Pakistan, before bifurcating and emptying into the Arabian Sea, its main stem located near the port city of Karachi. The Indus River has a total drainage area of circa 1,120,000 km2 (430,000 mi2). Its estimated annual flow is around 175 km3/a (5,500 m3/s), making it one of the 50 largest rivers in the world in terms of average annual flow. Its left-bank tributary in Ladakh is the Zanskar River, and its left-bank tributary in the plains is the Panjnad River which is formed by the successive confluences of the five Punjab rivers, namely the Chenab, Jhelum, Ravi, Beas, and Sutlej rivers. Its principal right-bank tributaries are the Shyok, Gilgit, Kabul, Kurram, and Gomal rivers. Beginning in a mountain spring and fed with glaciers and rivers in the Himalayan, Karakoram, and Hindu Kush ranges, the river supports the ecosystems of temperate forests, plains, and arid countryside. Geologically, the headwaters of the Indus and to their east those of the Yarlung Tsangpo (later in its course, the Brahmaputra) flow along the Indus-Yarlung suture zone, which defines the boundary along which the Indian plate collided with the Eurasian plate in the Early Eocene (approximately 50 Million years ago). These two Eurasian rivers, whose courses were continually diverted by the rising Himalayas, define the western and eastern limits, respectively, of the mountain range. After the Indus debouches from its narrow Himalayan valley, it forms, along with its tributaries, the Punjab region of South Asia. The lower course of the river ends in a large delta in the Sindh province of Pakistan. Historically, the Indus was important to many cultures. The 3rd millennium BC saw the rise of Indus Valley Civilisation, a major urban civilization of the Bronze Age. During the 2nd millennium BC, the Punjab region was mentioned in the Rigveda hymns as Sapta Sindhu and in the Avesta religious texts as Hapta Həndu (both terms meaning "seven rivers"). Early historical kingdoms that arose in the Indus Valley include Gandhāra and Sindhu-Sauvīra. The Indus River came into the knowledge of the Western world early in the classical period, when King Darius of Persia sent his Greek subject Scylax of Caryanda to explore the river, c. 515 BC.
