The Indus River Delta is where the Indus reaches the Arabian Sea. Most of it lies in Sindh province in southern Pakistan, and a small part extends into India’s Kutch region. Estimates of its total area range from about 29,500 to 41,440 km², of which an estimated 4,800 to 6,000 km² is the active delta still fed by the river. It meets the sea along about 210 km of coast. The landscape is an arid fan of mudflats, 17 major tidal creeks and grey mangrove forest, and it gets only 250 to 500 mm of rain in a normal year. The delta holds the largest arid-climate mangrove forests in the world. It was designated a Ramsar wetland of international importance on 5 November 2002, covering 472,800 hectares, and in December 2010 the Sindh government declared all of its mangroves Protected Forests. The landscape is the end of a story that began before the Himalayas existed, so this guide goes through it in order of age.
Older Than the Himalayas: The Proto-Indus and Its Fan
The Indus is an antecedent river. It was already flowing before the Himalayas rose, and it kept its course by cutting down as the mountains were pushed up around it. Sand and silt from western Tibet were reaching the Arabian Sea by about 45 million years ago, which means an early Indus existed by then. That river’s first delta has been identified in the Katawaz Basin on the border between Pakistan and Afghanistan, far inland from today’s coast. The Indus Plain itself was raised about 25 million years ago.
Not every river that now feeds the Indus belonged to it from the start. Sediment cores from the Arabian Sea show that five million years ago the Punjab rivers (the Jhelum, Chenab, Ravi, Beas and Sutlej) were not part of the Indus system. They flowed east towards the Ganges, and the Indus captured them only later. Further upstream, the river’s rerouting through the Nanga Parbat region caused so much erosion that it helped bring rocks from the middle and lower crust to the surface. The Indus has been reshaping the mountains that feed it for millions of years.
Most of what the Indus has carried away from the mountains never stayed on land. It went down the continental slope into the deep Arabian Sea and built the Indus Fan, an underwater apron of sediment that covers about 110,000 km². Near the top of the slope it is more than 9 km thick. The fan holds around 5 million cubic kilometres of material, which makes it the second-largest sediment body on Earth after the Bengal Fan in the Bay of Bengal. The fan began to build at the end of the Oligocene or the start of the Miocene, about 23 million years ago, when the Himalayas were being eroded faster, possibly because the monsoon was strengthening. It grew fastest during the middle Miocene.
The sediment that built it has been traced. The Karakoram, in northern Pakistan and India, is the single largest source of material in the modern river. The Himalayas come second, mostly through the Punjab rivers. Before people began damming the river, the Indus carried the fifth-largest sediment load of any river in the world. Most of that material was glacial and river debris from the rapidly rising western Tibetan Plateau and the Indus suture zone. The channel and levee systems buried in the upper fan are among the largest in the world, and the petroleum industry studies them closely.
Ice Ages and the Indus Canyon
The Pleistocene ice ages set the stage for the present delta. During the Last Glacial Maximum, about 20,000 years ago, so much water was locked in ice sheets that the sea fell and the continental shelf off Sindh lay above sea level. The Indus flowed about 130 km further across the dry shelf before it reached the Arabian Sea, and it poured its sediment straight to the shelf edge, about 135 m below today’s sea level. The Indus Canyon, a narrow, steep-sided trench cut into the seabed, probably began forming in the last interglacial period and was carved deepest during the glacial low stand. Turbidity currents, dense flows of sediment-laden water, ran down it and fed the channels on the fan below.
When the ice melted, the sea rose back across the shelf, and the river’s mouth moved back towards the land. The delta now visible on the Sindh coast is the youngest layer of all, a fan of mud and silt laid down on top of the drowned shelf. It is a broad alluvial fan of mudflats and channels, built from silt the river has been washing down for more than fifty million years. Its surface is the youngest layer of that long build-up, and the creeks, islands and mangroves on it are what the river has made most recently.
Wandering Mouths: Debal, Thatta and the 1819 Earthquake
A delta built of soft silt cannot keep one channel for long. The Indus has repeatedly switched its course, which is why geographers separate the active delta, still reached by the river, from the far larger area that was delta at some point in the past. Ports on the coast show the shifts. According to the Chach Nama, a port called Debal stood in the western delta by the 6th century AD. Raids by pirates based at Debal helped prompt Muhammad bin Qasim’s conquest of Sindh around 710. The last record of Debal is from 1223, and by the time Ibn Battuta arrived, silt had cut it off from the sea and it had been abandoned. Thatta, further inland, later became the capital of the Samma dynasty, which ruled the whole delta from 1333. The current delta stretches from the sea to just south of Thatta, about 100 km.
Tectonics changed the delta again in 1819. On 16 June that year an earthquake of magnitude 7.7 to 8.2 struck the Rann of Kutch. Land to the south sank and flooded to form Sindri Lake. To the north a ridge about 80 km long, 6 km wide and 6 m high was pushed up and dammed the Kori (Puran) channel. It became known as the Allah Bund, the “Dam of God”. The quake killed at least 1,543 people, and there is evidence that the Indus shifted its course westward afterwards. Kori Creek is now the easternmost creek of the delta. Its neighbour, Sir Creek, is the subject of a boundary dispute between India and Pakistan: Pakistan controls Kajhar Bet to the west of it, and India holds the Vighakot marshes to the east. In November 2011, two centuries after the 1819 shift, satellite images showed Indus floodwater feeding the Great Rann and Little Rann of Kutch again after heavy rain.
What the River Built: Seventeen Creeks and Grey Mangroves
All this history produced the delta’s shape. Seventeen major creeks, and many minor ones, cut the coast from Korangi Creek on the south-eastern edge of Karachi to the disputed Sir Creek, with Kori Creek, the easternmost, on the Indian side. Khobar Creek is treated as the river’s main mouth. Hajamro Creek leads to Keti Bandar, a historic port town in Thatta District. The water is unusually salty for an estuary because the river carries salt down from the Thar Desert, so few plants can survive. A 2003 survey put the delta’s mangrove forest at about 820 km², and 95 percent of the trees were grey mangrove, Avicennia marina. Red mangrove grew only on small patches of higher ground. The mangrove belt protects the coast against storm surges and shelters the young fish and crustaceans that support the delta’s fishing villages. The population of the active delta was estimated at about 900,000 in 2003.
Since 1955: A Delta Starved Below Kotri Barrage
The newest stage in the delta’s history is the work of engineers. Since the 1940s, dams, barrages and canals across the Indus Basin Irrigation System have taken more and more of the river before it reaches the sea. The lowest of them on the river, Kotri Barrage between Jamshoro and Hyderabad, was completed in 1955. The river once delivered about 180 billion m³ of water and 400 million tonnes of silt a year to its mouth. By 2018 the average flow below Kotri had fallen to 33 billion m³, carrying about 100 million tonnes of silt. Active flow now lasts only about two months of the year.
Without that silt, the sea has been winning. Sedimentation has fallen by about 80 percent since 1950 in the parts of the delta cut off by infrastructure, and erosion has cut into the delta front by up to about 50 m in places. Through land erosion and sea intrusion, the delta has shrunk by about 92 percent since the 19th century, from about 12,900 km² to nearly 1,000 km² in 2018 (sources give the base year as 1833 or 1883). Seawater now pushes about 80 km up the river channels, spoiling farmland and stressing mangroves. Mangroves covered 16 percent of the delta in 1990, and that cover had shrunk by about 10 percent by 2017, and many villages have been abandoned. Damming has also cut off the delta’s Indus river dolphins from the populations upstream. The 2010 floods were even welcomed as good news in the delta, because for once they brought fresh water all the way to the sea.
To trace the same water from Tibet through Punjab, read our complete guide to the Indus River. To compare its history with the way other deltas build up, see how deltas are formed. And to see how Sir Creek fits into the wider frontier, read our guide to the India–Pakistan border.