India's 90 per cent total surface water drained into the Bay of Bengal and rest are in the Arabian Sea. The drainage systems flowing into the Arabian Sea and the Bay of Bengal are separated by a water divide extending approximately along the Western Ghats, Aravallis and Yamuna Sutlej divide. The Indian drainage system consists of a large number of small and big rivers. It is the outcome of the evolutionary process of the three major physiographic units and the nature and characteristics of precipitation.
The Himalayan Drainage System
The Himalayan drainage system has evolved through a long geological history. Since these are fed both by melting of snow and precipitation, rivers of this system are perennial. There are differences of opinion about the evolution of the Himalayan Rivers. However, geologists believe that a mighty river called Shiwalik or Indo-Brahma traversed the entire longitudinal extent of the Himalaya from Assam to Punjab and onwards to Sind, and finally discharged into the Gulf of Sind near lower Punjab during the Miocene period some 5-24 million years ago. The remarkable continuity of the Shiwalik and its lacustrine origin and alluvial deposits consisting of sands, silt, clay, boulders and conglomerates support this viewpoint. It can be divided into three main river-systems:
The river Ganga is the combined stream of Bhagirathi and Alkhnanda, which meets near Devprayag. It originates from Gongotri Glacier in Uttarakhand.
This system includes the Indus and its tributaries like Jhelum, Chenab, Rabi, Beas, Sutluj (The Panchnad) etc. It originates from a glacier near Bokhar Chu in the Tibetan region at an altitude of 4,164 m in the Kailash Mountain range. It is 3600 km long and one of the largest rivers of the world.
It is one of the longest rivers of the world and with regard to the volume of water it is among the four largest rivers of the world. Its drainage system is spread in three countries- China (Tibet), India and Bangladesh. It originate from Chemayungdung glacier of the Kailash range near the Mansarovar Lake.
The Peninsular Drainage System
The peninsular plateau is drained by several rivers. The Narmada and the Tapi rise in the hills of central India. They flow westward and join the Arabian Sea. The Narmada flows through a narrow valley between the Vindhyas in the north and the Satpura ranges in the south. The Tapi flows south of Satpura ranges in the south. All other major rivers- the Mahanadi, the Godavari, the Krishna and the Kaveri flow eastward and join the Bay of Bengal. The Godavari is the longest peninsular river.
Three major geological events in the distant past have shaped the present drainage systems of Peninsular India:
1. Subsidence of the western flank of the Peninsula leading to its submergence below the sea during the early tertiary period. Generally, it has disturbed the symmetrical plan of the river on either side of the original watershed.
2. Upheaval of the Himalayas when the northern flank of the peninsular block was subjected to subsidence and the consequent trough faulting. The Narmada and The Tapi flow in trough faults and fill the original cracks with their detritus materials. Hence, there is a lack of alluvial and deltaic deposits in these rivers.
3. Slight tilting of the peninsular block from northwest to the South-eastern direction gave orientation to the entire drainage system towards the Bay of Bengal during the same period.
The Peninsular Rivers are broad, stable and flow through shallow valleys. These are more ancient then the Himalayan Rivers and have almost attained the old age. Hence, the slope-gradient of these rivers are very slow. Only those areas are its exceptions where new rifts have been created. Most of the peninsular rivers flows eastward because their main water divide is the Western Ghats. Only Narmada and Tapi are the exceptions, which flow from east to west in the rift valley. The rivers of peninsula are less important for irrigation because they are rain-fed. Hence, Peninsular rivers flows through hard peninsullar rocks, their courses are straight and linear. Since, allucial deposits are almost absent.