Drainage, basins and watersheds: the vocabulary and the classifications

Detail

1. Core vocabulary: drainage and drainage system

• Drainage is water that flows through well-defined channels, such as streams and rivers.
• A drainage system is the whole network of these channels. Small streams join bigger ones, and the bigger ones join a main river.
• A drainage pattern is the shape this network makes on a map. It depends on six factors:

Factor	What it means	How it shapes the pattern
Geological age	How old the land and rivers are	Old rivers have wide, mature valleys. Young rivers cut deep gorges.
Nature of rocks	Hard or soft rock	Water cuts soft rock faster, so streams follow the weak rocks.
Structure of rocks	Folds, faults and joints	Streams follow faults and cracks. Example: the Narmada and Tapi flow through rift valleys.
Topography	The shape of the land	A dome or peak sends streams outward. A basin pulls streams inward.
Slope	How steep the land is	Steep slope means fast flow and more cutting down. Gentle slope means meanders.
Amount and periodicity of flow	How much water flows, and how regularly	Perennial rivers (they flow all year) build stable channels. Seasonal rivers make weaker channels.

• Why channels matter for floods:
• A channel can be choked (blocked by silt or debris) or ill-defined (shallow, with no clear banks).
• Then it cannot carry the monsoon water.
• The water spills over the banks and causes flooding.

2. Catchment, river basin and watershed

• Catchment area is the area from which a river collects its water. Rain that falls anywhere in this area ends up in that river.
• The terms change with size:
• A large catchment of a big river is called a river basin. Example: the Ganga basin.

• A small catchment of rivulets (small streams) and rills (tiny channels cut by runoff) is called a watershed.

• A drainage basin is the whole area drained by one river and all its tributaries. A tributary is a stream that joins a bigger river.

• Watershed has two meanings, and this is an exam trap:
• As an area, it is the small catchment of rivulets and rills, the usual Indian usage.
• As a line, it is the boundary between two drainage basins, usually a ridge or high ground. This line is also called a water divide. Rain on one side flows into one basin, and rain on the other side flows into the next.

3. Why basins are ideal planning units

• A basin works as one unit:
• Every drop of water in a basin moves downstream through the same channels.
• So a change upstream reaches every place downstream.

• Examples: cutting forests upstream causes more silt and floods downstream, and a dam or a polluting factory upstream changes water flow and quality for everyone below.

• For this reason, basins and watersheds make ideal planning regions at three levels:

• Micro: a village watershed, used for check dams, contour bunds (low earth walls built along the slope) and tree planting.
• Meso: a sub-basin or a district-level river system.

• Macro: a whole river basin, such as the Ganga or Godavari, used for big dams, sharing water between states and flood control.

• Watershed management means managing land, water and plants inside one watershed together, so that soil and water are conserved where the rain falls.

• The Central Water Commission (CWC) follows Integrated Water Resources Management (IWRM) for basin planning. IWRM means managing water in a sustainable and balanced way that weighs social, economic and environmental interests together. One example is the Brahmani-Baitarani basin study under an India-Australia programme [2].

4. Classification by sea outlet: Bay of Bengal vs Arabian Sea

Outlet	Share of drainage area	Main rivers
Bay of Bengal	about 77%	Ganga, Brahmaputra, Mahanadi, Godavari, Krishna, Kaveri
Arabian Sea	about 23%	Indus, Narmada, Tapi, Mahi, Periyar

• The water divide separates the two systems. It runs along:
• the Delhi ridge, the northern end of the Aravallis near Delhi;
• the Aravallis, which run south-west across Rajasthan and Gujarat;

• the Sahyadri (Western Ghats), which run north to south along the west coast.

• Why most water goes east:

• The Western Ghats rise close to the west coast.
• The Deccan Plateau tilts gently from west to east.
• So big Peninsular rivers like the Godavari, Krishna and Kaveri rise in the Western Ghats and travel across the whole plateau to the Bay of Bengal.

• The west-flowing rivers are short. The two exceptions, the Narmada and Tapi, flow west through rift valleys, which are long troughs formed where land sank between two faults.

• Indus scale: the Indus catchment is about 11,65,000 sq km. Of this, about 3,21,248 sq km lies in India, or roughly 28% [3].

• State-wise drainage area (CWC data): Jammu and Kashmir 1,93,762 sq km; Himachal Pradesh 51,356; Punjab 50,304; Rajasthan 15,814; Haryana 9,939; Chandigarh 114 [3].

• The Ravi alone drains 14,442 sq km in India [3].

• Inland drainage is a third, small category. Some streams in the Rajasthan desert reach neither sea and end in salt lakes or sand. CWC treats this as a separate composite basin [3].

5. Classification by size of catchment (NCERT, older CWC data)

Class	Catchment area	Number	Examples
Major	more than 20,000 sq km	14	Ganga, Brahmaputra, Krishna, Tapi, Narmada, Mahi, Pennar, Sabarmati, Barak
Medium	2,000–20,000 sq km	44	Kalindi, Periyar, Meghna
Minor	less than 2,000 sq km	many	mostly in low-rainfall areas

• Traps:
• The Sabarmati, Mahi and Barak are "major" basins, even though they are not famous big rivers.
• The Periyar, Kerala's longest river, is only a "medium" basin.
• Minor basins are common in low-rainfall areas, because little rain means streams are short and their catchments stay small.

6. Current official scheme: 20 basins (CWC and the River Basin Atlas)

• River Basin Atlas of India:
• It was prepared by the CWC and ISRO under the India-WRIS project. India-WRIS (Water Resources Information System) is an online database of India's water resources.
• It was launched on 1 November 2012 at CWC, New Delhi [4].

• It maps watersheds within basins and sub-basins, along with dams, barrages, observation sites, terrain, rainfall and land use [4].

• CWC divides India into 20 river basins: 12 major and 8 composite basins [2][3]. (NCERT: 14 major, 44 medium and many minor basins, based on catchment size alone.)

• A composite basin groups many small neighbouring rivers that drain one stretch of coast or region into a single planning unit.

12 major basins (CWC)	8 composite basins (CWC)
Indus	Subarnarekha (with small rivers between the Subarnarekha and Baitarani)
Ganga-Brahmaputra-Meghna	East-flowing rivers between the Mahanadi and Pennar
Godavari	East-flowing rivers between the Pennar and Kanyakumari
Krishna	Area of inland drainage in the Rajasthan desert
Cauvery	West-flowing rivers of Kutch and Saurashtra, including the Luni
Mahanadi	West-flowing rivers from the Tapi to Tadri
Pennar	West-flowing rivers from Tadri to Kanyakumari
Brahmani-Baitarani	Minor rivers draining into Myanmar and Bangladesh
Sabarmati, Mahi, Narmada, Tapi	—

Source for both columns: [3].

• Why the two schemes differ: NCERT counts basins by size alone. CWC groups rivers into planning units, so it joins small coastal rivers into composite basins and treats the Ganga-Brahmaputra-Meghna as one system.

7. Classification by origin: Himalayan vs Peninsular

• This is the "most accepted" scheme, and NCERT Class 11 follows it.

Feature	Himalayan rivers	Peninsular rivers
Source	Glaciers and snow of the Himalaya	Rain over the plateau and the Western Ghats
Flow	Perennial (all year)	Mostly seasonal (rain-fed)
Age	Young	Much older
Valleys	Deep gorges and I-shaped valleys	Broad, shallow, mature valleys
Examples	Indus, Ganga, Brahmaputra	Godavari, Krishna, Kaveri, Narmada

• The flaw in this scheme:
• The Chambal, Betwa and Son rise on the Peninsular plateau and are much older rivers.
• They flow north and join the Yamuna or the Ganga, so they are counted in the Ganga system, which is "Himalayan".

• So a river's origin and its basin do not always match.

• Link to the Indo-Brahma hypothesis: this hypothesis says a single huge river, the Indo-Brahma or Shivalik river, once flowed along the whole length of the Himalaya. It later broke up into today's Indus, Ganga and Brahmaputra systems. This explains why the Himalayan rivers are grouped as one family.

Prelims Hooks

• 77% to the Bay of Bengal, 23% to the Arabian Sea. The divide runs along the Delhi ridge, the Aravallis and the Sahyadri.
• "Watershed" has two meanings: a small catchment of rivulets and rills, and the boundary line between two basins.
• Size classes: major basins are over 20,000 sq km (14), medium basins 2,000–20,000 sq km (44), and minor basins under 2,000 sq km.
• Kalindi, Periyar and Meghna are medium basins. Sabarmati, Mahi, Pennar and Barak are major basins.
• CWC: 20 basins = 12 major + 8 composite. The River Basin Atlas of India was made by CWC and ISRO under India-WRIS and launched on 1 November 2012.
• The Arabian Sea group includes the Periyar, even though it is a Kerala river. The Mahanadi and Kaveri drain to the Bay of Bengal.
• Trap: the Chambal, Betwa and Son are Peninsular in origin but belong to the Ganga system.
• The inland drainage of the Rajasthan desert and the minor rivers draining into Myanmar and Bangladesh are separate CWC composite basins.
• Indus: total catchment about 11.65 lakh sq km, of which about 3.21 lakh sq km is in India. Jammu and Kashmir has the largest share.

Mains Points

• Basin as the planning unit (GS-III):
• What happens upstream reaches everyone downstream, through silt, pollution, dams and floods.

• So planning by basin, through IWRM and watershed management from micro to macro level, works better than planning by state boundaries. It also gives a base for settling disputes over shared rivers.

• Floods from choked channels (GS-III, disasters):

• Silt from deforested slopes, encroachment on floodplains and blocked drains choke river channels.
• The choked channels cannot carry monsoon peaks, so rivers overflow.

• Remedies: desilting, protecting floodplains, and treating the catchment by planting trees and building check dams.

• East-west imbalance (GS-I):

• Because the land tilts east, 77% of the drainage goes to the Bay of Bengal.
• The west-coast rivers are short and swift, which is good for hydropower but gives little storage.

• Low-rainfall basins in Rajasthan and Saurashtra have only minor and inland drainage, which is one reason they face water stress.

• Classification debate (GS-I): grouping rivers by origin hides the fact that rivers like the Chambal and Son are Peninsular in origin but belong to the Ganga system. Planning groups such as CWC's 20 basins fit water management better than textbook groups based on origin or size.