• Drainage: Flow of water through well-defined channels. Network of such channels is called a drainage system .
• Drainage basin: An area drained by a river and its tributaries.
• Watershed: Boundary line separating one drainage basin from other.
• River basins are larger watersheds.
• Drainage pattern of an area depends on the geological time period, nature and structure of rocks, topography, rocks, slope, amount of water and periodicity of flow.

Important drainage patterns:

1. Dendritic: Resembling the branches of a tree. Eg. Northern Plain rivers
2. Radial: Originate from a hill and flow in all directions. Eg. Rivers in Amarkantak
3. Trellis: Primary tributaries parallel to each other and secondary tributaries join them at right angles.
4. Centripetal: Rivers discharge waters from all directions in a lake or depression

A river drains the water collected from a specific area, which is called its catchment area.

Nearly 77 pc of drainage is towards the Bay of Bengal while about 23 pc is towards the Arabian Sea.

The Himalayan Drainage System

Mainly includes the Ganga, Indus and Brahmaputra river basins.

Over the plains, rivers of this system change the course often. River Kosi is also known as the ‘sorrow of Bihar’ due to flooding by its frequent change of course by deposition of sediments.

Evolution

Geologists believe that a mighty river called Shivalik or Indo-Brahma traversed the entire length of the Himalayas some 5-24 million years ago. Over time this got dismembered into the present three major river systems.

The Indus System

Indus river originates from a glacier near Bokhar Chu in Tibet in the Kailash Mountain range. Known as Singi Khamban (Lion’s mouth) in Tibet. It forms a spectacular gorge near Gilgit in J&K. Enters Pakistan near Chillar in the Dardistan region. Flows only through the Leh distt of J&K.

Smaller tributaries: Shyok, Gilgit, Zaskar, Nubra, Hunza, Shigar, Gasting, Dras. On right bank: Kabul river, Khurram, Tochi, Gomal, Viboa and hte Sangar.

Major tributaries: Satluj, Beas, Ravi, Chenab, Jhelum

The Ganga System

It is the largest river system in India.

Ganga rises in the Gangotri glacier near Gaumukh in the Uttarkashi district. Here it is known as the Bhagirathi. At Devprayag, Bhagirathi meets Alaknanda and is known as Ganga hereafter.

Panchprayag

• Ganga enters the plains at Haridwar.
• Left Bank tributaries (LBT): Ramganga, Gomati, Ghaghara, Gandak, Kosi and Mahananda.
• Right Bank tributaries (RBT): Son
• Discharges into Bay of Bengal near Sagar island.