The Indus River plain

The Indus River plain is a vast expanse of fertile land, covering about 200,000 square miles (518,000 square km), with a gentle slope from the Himalayan piedmont in the north to the Arabian Sea in the south. The average gradient of the slope is no more than 1 foot per mile (1 metre per 5 km). Except for the micro relief, the plain is featureless. It is divisible into two sections, the upper and lower Indus plains, on account of their differing physiographic features. The upper Indus plain is drained by the Indus together with its tributaries, the Jhelum, Chenab, Ravi, Beas, and Sutlej rivers, forming a developed system of interfluves, known locally as doabs, in Punjab province (Persian panj āb, “five waters,” in reference to the five rivers). In the lower plain the Indus River has a Nilotic character; i.e., it forms a single large river with no significant tributaries. The plain narrows to form a corridor near Mithankot, where the Sulaiman Range comes close to the plain and the Indus merges with its last major tributary, the Panjnad River (which is itself merely the confluence of the five Punjab rivers). Flooding is a perennial problem, especially along the Indus, as a consequence of heavy rains (usually in July and August).

The upper Indus plain consists of three subdivisions: the Himalayan piedmont, the doabs, and the Sulaiman piedmont (referred to locally as the Derajat). The Himalayan piedmont, or the sub-Shiwalik zone, is a narrow strip of land where the rivers enter into the plain from their mountain stage, thereby giving each a somewhat steeper gradient. The zone is characterized by numerous rivulets, which have produced a broken topography in parts of the zone. These streams remain dry except in the rainy season, when they swell into gushing streams with considerable erosive power.

The doabs between the various rivers display similar micro relief, which comprises four distinct landforms—active floodplains, meander floodplains, cover floodplains, and scalloped interfluves. An active floodplain (known locally as a khaddar or bet), which lies adjacent to a river, is often called “the summer bed of rivers,” as it is inundated almost every rainy season. It is the scene of changing river channels, though protective bunds (levees) have been built at many places on the outer margin of the bet to contain the river water in the rainy season. Adjoining the active floodplain is the meander floodplain, which occupies higher ground away from the river and is littered with bars, oxbow lakes, extinct channels, and levees. The cover floodplain is an expanse of geologically recent alluvium, the result of sheet flooding, in which alluvium covers the former riverine features. The scalloped interfluves, or bars, are the central, higher parts of the doab, with old alluvium of relatively uniform texture. The boundaries of the scalloped features are formed by river-cut scarps at places over 20 feet (6 metres) high. The generally level surface of this section of the plain is broken into small pockets in Chiniot and at Sangla Hill, near the much denuded Kirana Hills, which stand out in jagged pinnacles. These hills are considered to be the outliers of the Aravali Range of India.

The largest but poorest of the doabs is the Sindh Sagar Doab, which is mostly desert and is situated between the Indus and Jhelum rivers. The doabs that lie to the east of it, however, constitute the richest agricultural lands in the country. Until the advent of irrigation, at the end of the 19th century, much of the area was a desolate waste, because of the low amount of precipitation. But irrigation has been a mixed blessing; it has also caused waterlogging and salinity in some places. In an attempt to correct this problem, the Pakistan government, with the financial support of such international agencies as the World Bank, constructed the Left Bank Outfall Drain (LBOD) in the 1980s and ’90s. The intent was to build a large artificial waterway roughly east of and parallel to the Indus to carry salt water from the plains of Punjab and Sindh provinces to the Arabian Sea coast in the Badin region of southeastern Sindh. The final segment of the LBOD consisted of building a “tidal drain” 26 miles (42 km) to the sea. However, instead of draining salt water away, the improperly designed tidal drain produced an environmental disaster in southeastern Sindh: large portions of the land and freshwater lakes and ponds were flooded by salt water, crops were ruined, and freshwater fisheries were destroyed. The tidal drain issue was further complicated by instances of severe weather in the coastal region, including a destructive tropical cyclone in 1999 and torrential rains there and in Balochistan in 2007—both of which caused many deaths and forced the evacuation of tens of thousands of people. After the 2007 storms, the people of Badin called on the government to cease using the LBOD.

The Sulaiman piedmont is different from the Himalayan piedmont in that it is generally dry. Seamed with numerous streams and wadis, the surface is undulating. The gradient of the streams is comparatively steep, the floodplains are narrow, and the right bank of the Indus sometimes rises just above the main channel.

The lower Indus plain, the course of which goes through Sindh province, is flat, with a gradient as slight as 1 foot per 3 miles (1 metre per 10 km). The micro relief is quite similar to that of the upper Indus plain. The valley of the Indus and its banks have risen higher than the surrounding land as a result of the aggradational work of the river; and though the river is lined with flood-protecting bunds along its course, the alluvial sands and clays of the soil tend to give way before floods, leading the river to change course frequently. The level surface of the plain is disturbed at Sukkur and Hyderabad, where there are random outcroppings of limestone. The Indus delta has its apex near Thatta, below which distributaries of the river spread out to form the deltaic plain. To the southeast of that point is the Rann of Kachchh (Kutch), which is an expanse of saline marsh. The coastal tract is low and flat, except where the Pabbi Hills meet the coast between Karachi and Ras Muari (Cape Monze).

Manchhar, a marshy lake west of the Indus, has an area of 14 square miles (36 square km) at low water but extends for no less than 200 square miles (500 square km) when full; on such occasions it is one of the largest freshwater lakes in South Asia. The quality of groundwater in the Indus plain varies, that in the southern zone (Sindh) being mostly saline and unfit for agricultural use. Extensive areas in both the northern and southern zones of the plain have been affected by waterlogging and salinity. In the south the Indus delta (in marked contrast to the Ganges-Brahmaputra delta) is a wild waste. When high tides and Indus floods coincide, the littoral is flooded for some 20 miles (30 km) inland.