Drainage systems

Why it matters

Look at any map of India and you will see blue lines spreading like the veins of a leaf. Each line is a river, and every river belongs to a larger network. Drainage means the flow of surface water through well-defined channels. A drainage system is the main river together with all its tributaries (rivers that join it) and distributaries (branches that leave it near the sea).

Why should a Constable aspirant care? Because river questions are short, factual and easy to set. In the General Studies part of the written test you may be asked where a river rises, which river joins which, which river forms an estuary instead of a delta, or what drainage pattern a region shows. Organised river facts let you answer these in 15–20 seconds and save time for arithmetic.

Core concept

Level 1: Basic words you must know

Term	Meaning	Simple picture
Drainage basin (catchment)	The whole area drained by one river and its tributaries	A big funnel collecting rain
Water divide (watershed)	High land that separates two drainage basins	The ridge of a roof sending water two ways
Tributary	A smaller river that joins a bigger river	A lane joining a highway
Distributary	A branch that splits off from a river, usually near its mouth	A highway splitting into exits
Confluence	The point where two rivers meet	A road junction
Delta	Triangular deposit of silt at a river mouth	Fan of mud and sand at the coast
Estuary	A river mouth where fresh water mixes with sea water and tides wash the silt away	Open funnel-shaped mouth, no fan of silt

Two examples of water divides are very useful: the Western Ghats are the main water divide of peninsular India, and Ambala (Haryana) lies on the divide between the Indus and Ganga systems.

Level 2: Two great groups of Indian rivers

Indian rivers are divided into Himalayan rivers and Peninsular rivers. The difference comes from their source of water and the age of the land they flow over.

Feature	Himalayan rivers	Peninsular rivers
Source of water	Melting snow and glaciers + monsoon rain	Mostly monsoon rain
Flow	Perennial (flow all year)	Mostly seasonal; flow drops sharply in summer
Age of land	Young mountains, still rising	Very old, worn-down plateau
Work done	Heavy erosion in mountains, deep gorges; meanders and ox-bow lakes in plains	Shallow valleys, smooth (graded) courses
Length and basin	Long courses, very large basins	Shorter courses, smaller basins
Main systems	Indus, Ganga, Brahmaputra	Godavari, Krishna, Kaveri, Mahanadi, Narmada, Tapi

Why are Himalayan rivers perennial? In summer, when there is no rain, the snow on the Himalaya melts and keeps feeding the rivers. Peninsular rivers have no snow cover behind them, so when the rain stops, the water level falls.

Level 2: The three Himalayan systems

1. The Indus system

• The Indus rises in Tibet, near Lake Mansarovar. It flows west and enters India in Ladakh, where it is joined by tributaries such as the Zaskar, Shyok and Nubra.
• Its total length is about 2,900 km; a large part of its basin lies in Pakistan.
• Five rivers of Punjab — Jhelum, Chenab, Ravi, Beas and Satluj — together join the Indus in Pakistan (their combined stream is called the Panjnad).
• The Satluj also rises in Tibet and enters India near Shipki La. The Jhelum rises at Verinag spring in Kashmir and flows through Wular Lake.
• Under the Indus Waters Treaty (1960), the eastern rivers (Ravi, Beas, Satluj) were allotted to India and the western rivers (Indus, Jhelum, Chenab) mainly to Pakistan. (As of 2025, India has placed the treaty in abeyance.)

2. The Ganga system

• The headstream Bhagirathi is fed by the Gangotri glacier (snout at Gaumukh). It meets the Alaknanda at Devprayag in Uttarakhand; from here the river is called the Ganga.
• The Ganga enters the plains at Haridwar.
• Yamuna — the westernmost and longest tributary — rises at Yamunotri glacier and joins the Ganga at Prayagraj.
• Left-bank (northern) tributaries come down from the Himalaya through Nepal: Ghaghara, Gandak, Kosi. The Kosi often changes its course and floods north Bihar, so it is called the "Sorrow of Bihar".
• Right-bank (southern) tributaries come from the peninsular uplands: Chambal and Betwa join the Yamuna; the Son joins the Ganga directly.
• At Farakka (West Bengal) the river divides. The Bhagirathi–Hooghly flows south past Kolkata to the Bay of Bengal; the main stream enters Bangladesh as the Padma.
• The Ganga's length is over 2,500 km, and it is the longest river flowing in India.

3. The Brahmaputra system

• Rises in Tibet, east of Mansarovar, close to the sources of the Indus and Satluj.
• In Tibet it is called the Tsangpo. Near Namcha Barwa it takes a sharp U-turn and enters Arunachal Pradesh through a gorge, where it is called the Dihang (Siang).
• Joined by the Dibang and Lohit, it becomes the Brahmaputra in Assam. In Bangladesh it is called the Jamuna.
• In Tibet the climate is cold and dry, so it carries little water and silt. In India it passes through very high-rainfall areas, so it carries huge volumes of water and silt, forms a braided channel and river islands such as Majuli (Assam), and causes floods.
• The Ganga and Brahmaputra together build the Sundarban Delta, the world's largest delta, named after the Sundari (mangrove) tree.

Level 3: Peninsular rivers — east-flowing and west-flowing

The Western Ghats lie close to the west coast, and the plateau slopes gently towards the east. So most big peninsular rivers flow east into the Bay of Bengal.

River	Source	Flows into	Note
Mahanadi	Highlands of Chhattisgarh	Bay of Bengal (Odisha)	Hirakud dam; length about 860 km
Godavari	Western Ghats, Nashik district (Maharashtra)	Bay of Bengal	Largest peninsular river (about 1,500 km); called "Dakshin Ganga"
Krishna	Spring near Mahabaleshwar (Maharashtra)	Bay of Bengal	Tributaries: Koyna, Bhima, Tungabhadra, Ghatprabha
Kaveri	Brahmagiri range, Western Ghats (Karnataka)	Bay of Bengal (Tamil Nadu)	Tributaries: Kabini, Bhavani, Amaravati; flow is steadier than other peninsular rivers
Narmada	Amarkantak hills (Madhya Pradesh)	Arabian Sea, Gulf of Khambhat	Flows west in a rift valley between the Vindhya and Satpura; Dhuandhar falls near Jabalpur
Tapi	Satpura range, Betul district (Madhya Pradesh)	Arabian Sea, Gulf of Khambhat (near Surat)	Also flows west in a rift valley, parallel to the Narmada but shorter

Why is the Kaveri steadier? Its upper catchment gets rain from the south-west monsoon (summer) and its lower catchment gets rain from the north-east (retreating) monsoon (winter). Two rainy seasons mean less fluctuation.

Why do east-flowing rivers make deltas but the Narmada and Tapi make estuaries? East-flowing rivers cross wide, gentle plains, slow down and drop their silt before reaching the fairly calm Bay of Bengal — so deltas form. The Narmada and Tapi run through narrow rift valleys of hard rock, carry less silt, and enter the sea where tides wash sediment away — so their mouths stay open as estuaries.

Other facts in this group: the Damodar flows through a rift valley in the Chota Nagpur plateau and was called the "Sorrow of Bengal" for its floods. In the dry west, the Luni rises in the Aravalli hills and ends in the Rann of Kachchh; many desert streams simply vanish into the sand — this is inland drainage.

Level 3: Drainage patterns

The shape the network makes depends on the slope and the rocks beneath.

Pattern	Shape	Cause	Indian example
Dendritic	Branches of a tree	Uniform rock and gentle slope	Rivers of the northern plains
Trellis	Main river with tributaries joining at right angles, like a garden trellis	Bands of hard and soft rock lying side by side (folded land)	Rarely asked with an Indian example; sometimes linked to the Singhbhum belt (Chota Nagpur plateau)
Rectangular	Sharp right-angle bends	Rock broken by strong joints	Jointed rocky terrain
Radial	Rivers flow outward in all directions from a high point	A central peak or dome	Amarkantak range (Narmada, Son)
Centripetal	Rivers flow inward from all sides into a lake or basin	A central depression	Loktak Lake, Manipur

Advanced idea: antecedent rivers

The Indus, Satluj and Brahmaputra are older than the Himalaya. As the mountains slowly rose, these rivers kept cutting down through them and made very deep gorges. Rivers that existed before the land rose are called antecedent rivers.

Worked examples

Example 1 — Fact to question.
Which of these rivers forms an estuary rather than a delta? (a) Mahanadi (b) Godavari (c) Tapi (d) Kaveri
Reasoning: Mahanadi, Godavari and Kaveri flow east and form deltas in the Bay of Bengal. The Tapi flows west through a rift valley and enters the Arabian Sea through an estuary. Answer: (c) Tapi.

Example 2 — Arrangement.
Arrange the Punjab rivers from west to east: Ravi, Jhelum, Satluj, Chenab, Beas.
Reasoning: The westernmost is the Jhelum (Kashmir valley), then Chenab, then Ravi, then Beas, and the Satluj is the easternmost. Answer: Jhelum, Chenab, Ravi, Beas, Satluj.

Example 3 — Odd one out.
Ghaghara, Gandak, Kosi, Son
Reasoning: The first three are left-bank tributaries of the Ganga that come from the Himalaya. The Son is a right-bank tributary that rises in the Amarkantak plateau of peninsular India. Answer: Son.

Example 4 — Statement type.
Statement I: The Kaveri has a more regular flow than most peninsular rivers. Statement II: Its basin receives rain from both the south-west and north-east monsoons.
Reasoning: Both statements are true, and two rainy seasons are exactly why the flow is steadier. Answer: Both true, and II explains I.

Example 5 — Pattern recognition.
Several streams flow from surrounding hills into Loktak Lake. Which drainage pattern is this?
Reasoning: Water moving inward from all sides into a central depression is centripetal. Answer: Centripetal.

Real-world connection

River knowledge is practical knowledge for a police officer. Every monsoon, the Kosi in Bihar and the Brahmaputra in Assam flood villages, and police join disaster-response teams for rescue and relief. Big multipurpose dams — Bhakra on the Satluj, Tehri on the Bhagirathi, Hirakud on the Mahanadi, Sardar Sarovar on the Narmada, Mettur on the Kaveri — control floods, irrigate fields and produce electricity. Sharing river water between states can lead to disputes and protests that need calm law-and-order handling.

How it is asked in the exam

Each question carries about 54 seconds, and there is no negative marking, so never leave a river question blank — eliminate and guess if you must.

• Direct fact (easy, 10–15 s): "Where does the Ganga enter the plains?" → Haridwar. "Which river is called the Sorrow of Bihar?" → Kosi.
• Match the following (medium, 30–40 s): river ↔ source (Narmada–Amarkantak, Godavari–Nashik, Kaveri–Brahmagiri) or river ↔ dam (Satluj–Bhakra, Mahanadi–Hirakud).
• Odd one out / classification (medium): pick the west-flowing river, or the non-Himalayan tributary.
• Arrangement (medium–hard): order rivers west to east or north to south (Mahanadi, Godavari, Krishna, Kaveri from north to south).
• Multi-statement / assertion–reason (hard, 45–54 s): e.g. "Himalayan rivers are perennial because they are fed by glaciers" — check each statement, then check the link between them.

A hard question usually combines two facts (source + direction); test each option against one fact at a time.

తెలుగు సారాంశం (Telugu summary)

ఒక ప్రధాన నది మరియు దానిలో కలిసే ఉపనదులు మరియు దాని నుండి విడిపోయే పాయలు కలిసి ఏర్పరిచే నెట్‌వర్క్‌ను డ్రైనేజ్ వ్యవస్థ (నదీ వ్యవస్థ) అంటారు. ఒక నది నీటిని సేకరించే మొత్తం ప్రాంతాన్ని పరీవాహక ప్రాంతం అంటారు.
రెండు పరీవాహక ప్రాంతాలను వేరుచేసే ఎత్తైన భూభాగాన్ని జల విభాజకం అంటారు; ద్వీపకల్ప నదులకు పడమటి కనుమలు ప్రధాన జల విభాజకం.
హిమాలయ నదులు (సింధు, గంగ, బ్రహ్మపుత్ర) మంచు కరగడం మరియు వర్షం వల్ల ఏడాది పొడవునా ప్రవహిస్తాయి; అందుకే వీటిని జీవనదులు అంటారు.
ద్వీపకల్ప నదులు ఎక్కువగా వర్షంపై ఆధారపడతాయి, కాబట్టి వేసవిలో వాటి నీరు బాగా తగ్గిపోతుంది.
భాగీరథి, అలకనంద దేవప్రయాగ వద్ద కలిసి గంగగా మారుతాయి; యమున ప్రయాగ్‌రాజ్ వద్ద గంగలో కలుస్తుంది.
బ్రహ్మపుత్రను టిబెట్‌లో త్సాంగ్‌పో, అరుణాచల్ ప్రదేశ్‌లో దిహాంగ్, బంగ్లాదేశ్‌లో జమున అని పిలుస్తారు. గంగ, బ్రహ్మపుత్ర కలిసి ప్రపంచంలోనే అతిపెద్ద డెల్టా అయిన సుందర్‌బన్స్‌ను ఏర్పరుస్తాయి.
మహానది, గోదావరి, కృష్ణ, కావేరి తూర్పుకు ప్రవహించి బంగాళాఖాతంలో డెల్టాలు ఏర్పరుస్తాయి; గోదావరి అతిపెద్ద ద్వీపకల్ప నది.
నర్మద, తాపీ నదులు పగులు లోయల గుండా పడమరకు ప్రవహించి అరేబియా సముద్రంలో డెల్టాలు కాకుండా ఎస్ట్యూరీలు ఏర్పరుస్తాయి.
కోసి నదిని "బీహార్ దుఃఖదాయిని", దామోదర్ నదిని "బెంగాల్ దుఃఖదాయిని" అని పిలుస్తారు.

Key terms (English — తెలుగు):

• Drainage system — నదీ వ్యవస్థ (డ్రైనేజ్ వ్యవస్థ)
• Drainage basin — పరీవాహక ప్రాంతం
• Water divide — జల విభాజకం
• Tributary — ఉపనది
• Distributary — పాయ
• Confluence — సంగమం
• Perennial river — జీవనది
• Delta — డెల్టా
• Estuary — ఎస్ట్యూరీ (నదీ ముఖద్వారం)
• Rift valley — పగులు లోయ
• Western Ghats — పడమటి కనుమలు