Physiography
The state of Punjab forms a part of Indo-Gangetic alluvial plain and is composed of sediments of Shiwalik hills and Himalayas brought down and laid by the rivers of Indus system. The exact depth of the alluvium has not been ascertained, though it varies from a few metres to over 2000 metres. The state can be divided into the following major physiographic units:
The Siwalik hills in the north-east are steeply sloping. Number of choes originate in the Shiwalik zone and drain the excess storm water. The Shiwalik hills occupy nearly 2.6 per cent area of the state and cover sizeable area of Gurdaspur, Hoshiarpur, S.B.S. Nagar, Rupnagar and S.A.S Nagar districts of the state. The hills have dense to open scrub forest. The piedmont area forms a transitional zone between the Shiwalik hills and alluvial terraces. It is about 10 to 15 km wide and comprises of Gurdaspur, Hoshiarpur, S.B.S. Nagar, Rupnagar and S.A.S Nagar districts. The elevation of this zone varies from 300 to 375 m above MSL.
The piedmont area is gently sloping to undulating and is dissected by number of seasonal rivulets (choes) which transport storm water with sediments from their catchment. The coarsest of these sediments are deposited in the form of alluvial fans at the foot hills and finer fractions are deposited aling the choes within the piedmont area.
The sand dunes are low ridges along the present and old courses of rivers and choes. They are formed as a result of reworking of sand bar deposits of rivers. The deposits are sandy in texture and dominated by quartz and feldspar minerals. The sand dunes covered nearly 9.0 per cent area of the state during 1987, however, as a result of levelling and clearing by the farmers in the recent past, the area of sand dunes has been reduced to barely 0.56 per cent during 2004. The areas in and around the sand dunes are moderately sloping whereas interdunal areas are nearly level to gently sloping.
The alluvial plain/terraces are the old flood plains of the rivers, the remnants of which lie above the level of the present river beds. They are separated from flood plains at their bases by broken chains of sand dunes and cliffs. The deposits of terraces vary with respect to texture, depth of carbonate leaching and translocation of other mobile soil constituents. Some parts of these terraces are affected by water logging and/or salinity and alkalinity. The unit occupies nearly 76.9 percent of the total geographical area of the state. Three major alluvial plains/ terraces are recognised in the state. They are popularity known as Uppar-Bari Doab covering most parts of Tarn Taran, Amritsar and Gurdaspur districts. Bist Doab covering area between Beas and Satluj rivers and Malwa plain, area south of river Satluj.
The flood plains of Ravi, Beas, Satluj and Ghaggar rivers and many seasonal rivulets cover nearly 10.0 per cent area of the state. The flood plain soils are young and stratified without appreciable alteration of sediments. The continuous erosion cum deposition keeps the soils young as time becomes a limiting factor for the consolidation of sediments into pedogenic horizons.
The palaeo-channels are believed to be the remanants of the old active channels. The origin of these channels may be due to the frequent changes in the courses of Ravi, Beas, Satluj and Ghaggar rivers and their tributaries, which became defunct and silted up. These areas occupy a low-lying topographic position on the landscape.