- 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:
- Dendritic: Resembling the branches of a tree. Eg. Northern Plain rivers
- Radial: Originate from a hill and flow in all directions. Eg. Rivers in Amarkantak
- Trellis: Primary tributaries parallel to each other and secondary tributaries join them at right angles.
- 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
| Jhelum | Origin: Verinag at foot of Pir Pinjal. Flows through Srinagar and the Wular lake. Joins Chenab near Jhang in Pakistan |
| Chenab | Origin: Two streams (Chandra and Bhaga) which join at Tandi near Keylong in HP. Largest tributary of Indus. Aka Chandrabhaga. Flows for 1180 KM before entering Pakistan |
| Ravi | Origin: Kullu hills of HP near Rohtang Pass. Enters Pakistan and joins Chenab near Sarai Sidhu |
| Beas | Origin: Beas Kund near Rohtang pass. Forms gorges at Kati and Largi in the Dhaoladhar range. Meets Satluj near Harike. |
| Satluj | Origin: Rakas lake near Mansarowar in Tibet. Known as Langchen Khambab in Tibet. Enters India at Ropar. Antecedent river. Bhakra Nangal Project is on this river. |
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
| Vishnu Ganga | Joshimath Dhauli and Vishnu Ganga meet to form Alaknanda |
| Alaknanda | Origin: Satopanth glacier above Badrinath. Consists of Dhauli and Vishnu Ganga. |
| Yamuna | Origin: Yamnotri glacier on Banderpunch range. Joins Ganga at Prayag (Allahabad). RBT: Chambal, Sind, Betwa, Ken LBT: Hindan, Rind, Sengar, Varuna. Agra canal |
| Chambal | Origin: Mhow in Malwa plateau. Gandhi Sagar dam, Rana Pratap Sagar dam and Jawahar Sagar dam. Famour for Chambal ravines. |
| Gandak | Origin: In Nepal between Dhaulgiri and Mt. Everest. Enters Ganga plain in Champaran and joins Ganga at Sonpur near Patna. Two streams: Kaliganfak and Trishulganga.
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| Ghaghra | Origin: Glaciers of Mapchachungo Tributaries: Tila, Seti and Beri Deep gorge at Shishpani Sarda (Kali) joint it and meet Ganga at Chhapra. |
| Kosi | Origin: North of Mt. Everest in Tibet. Tributaties: Son Kosi, Tamur Kosi, Arun Changes course often. Sorrow of Bihar. |
- 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.
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