Drainage – Class 9 Geography Chapter 3 (NCERT)

What is a Drainage System? Key Concepts Explained

A drainage system refers to the way a river and its tributaries are arranged within a drainage basin — the area drained by a single river system. The boundary line separating one drainage basin from another is called a water divide (e.g., the Ambala watershed separates the Indus and Ganga river systems).

Drainage patterns depend on the slope, geological structure and nature of terrain:

• Dendritic: branch-like, develops on uniform rock (e.g., northern plains rivers).
• Trellis: when a river is joined by tributaries at right angles.
• Rectangular: develops on strongly jointed rocky terrain.
• Radial: rivers flow outward from a central peak (e.g., rivers from Amarkantak).

Indian rivers are classified into two major groups based on their origin, nature and behaviour: Himalayan rivers and Peninsular rivers. This classification is the backbone of the entire chapter.

Himalayan Rivers vs Peninsular Rivers

Worked Explanation: Tracing India's Major River Systems

• The Indus River System: Step 1 – Origin: rises near Mansarovar lake in Tibet. Step 2 – Enters India in Ladakh, joined by Zaskar. Step 3 – Major tributaries: Satluj, Beas, Ravi, Chenab, Jhelum join in Pakistan. Step 4 – Flows through Pakistan into the Arabian Sea. Step 5 – Note: Under the Indus Water Treaty (1960), India can use only 20% of its water.
• The Ganga River System: Step 1 – Origin: Gangotri glacier as the Bhagirathi; joined by Alaknanda at Devaprayag to form the Ganga. Step 2 – Enters plains at Haridwar. Step 3 – Left bank tributaries: Gomti, Ghaghara, Kosi, Gandak (from Himalayas). Step 4 – Right bank tributaries: Yamuna, Son (from Peninsula). Step 5 – Splits into Bhagirathi-Hooghly and Padma in the delta; forms the Sunderban delta with the Brahmaputra before entering the Bay of Bengal.
• The Brahmaputra River System: Step 1 – Origin: Tibet, east of Mansarovar lake, known as Tsangpo (carries more water/silt there due to more rainfall and less silt deposition). Step 2 – Enters India in Arunachal Pradesh as Dihang, joined by Dibang and Lohit. Step 3 – Flows through Assam creating riverine islands (e.g., Majuli). Step 4 – Known for shifting channels and severe floods due to heavy silt deposition. Step 5 – Joins the Ganga in Bangladesh before entering the Bay of Bengal.
• The Godavari (Peninsular Example): Step 1 – Rises in the Nashik hills, Maharashtra. Step 2 – Called the 'Dakshin Ganga' as it is the largest Peninsular river. Step 3 – Tributaries: Penganga, Wardha, Wainganga, Indravati. Step 4 – Drains into the Bay of Bengal after covering parts of Maharashtra, Chhattisgarh, Telangana, Andhra Pradesh.

Lakes and the Economic Role of Rivers

Common Mistakes & Board Exam Tips

Students frequently mix up which tributaries join the Ganga from the left bank versus the right bank — remember: left bank = Himalayan tributaries (Gomti, Ghaghara, Kosi, Gandak), right bank = Peninsular tributaries (Yamuna, Son, Chambal). Another common error is writing 'Brahmaputra is a Peninsular river' — it is Himalayan, originating in Tibet. In map-based questions, always label river origin, direction of flow and mouth clearly. Do not confuse 'drainage basin' with 'watershed' — a watershed is for a small stream, a basin/catchment is for a large river system. Always mention specific state names when asked to trace a river's course for full marks.

Practice Questions with Solutions

• Q: What is a water divide? Give one example.
  A: Step 1: A water divide is an upland or a mountain that separates two drainage basins.
  Step 2: It prevents water of one river system from mixing with another.
  Step 3: Example: The Ambala watershed separates the drainage basins of the Indus and the Ganga river systems.
  Final answer: A water divide is a boundary ridge separating two drainage basins, e.g., the Ambala watershed between the Indus and Ganga systems.

• Q: Distinguish between Himalayan rivers and Peninsular rivers on the basis of their flow and origin.
  A: Step 1: Himalayan rivers originate from snow-covered glaciers in the Himalayas, making them perennial (flow all year).
  Step 2: Peninsular rivers originate from plateaus/hills and depend on monsoon rain, making them seasonal/non-perennial.
  Step 3: Himalayan rivers form deep V-shaped valleys and gorges due to erosion in young fold mountains; Peninsular rivers flow in shallow valleys over old hard rock.
  Final answer: Himalayan rivers are perennial, snow-fed, with deep valleys; Peninsular rivers are seasonal, rain-fed, with shallow valleys on ancient rock.

• Q: Why does the Brahmaputra carry a large amount of silt and change its course frequently in Assam?
  A: Step 1: The Brahmaputra receives heavy rainfall in its upper catchment (Tibet and Arunachal Pradesh) leading to large volumes of water and eroded material.
  Step 2: In Assam, the river enters a wide, gently sloping plain, so its speed decreases sharply.
  Step 3: Reduced speed causes it to deposit huge amounts of silt on its bed, raising the riverbed level.
  Step 4: This forces the river to frequently shift its channel and create riverine islands like Majuli.
  Final answer: Heavy rainfall upstream brings huge silt load, and reduced speed in the Assam plains causes deposition, bed-raising and frequent course changes, forming islands like Majuli.

• Q: Name the two headstreams of the Ganga and explain how the Ganga is formed.
  A: Step 1: The Ganga's main headstream is the Bhagirathi, which originates from the Gangotri glacier.
  Step 2: It is joined by the Alaknanda, which originates in the Garhwal Himalayas.
  Step 3: The two streams meet at Devaprayag in Uttarakhand.
  Step 4: After this confluence, the combined river is known as the Ganga.
  Final answer: The Bhagirathi and Alaknanda are the headstreams; they meet at Devaprayag to form the river Ganga.

• Q: Why is Chilika Lake important, and what type of lake is it?
  A: Step 1: Chilika Lake, located in Odisha, is the largest saltwater lake (lagoon) in India.
  Step 2: It is formed where a lagoon is separated from the Bay of Bengal by a sandbar.
  Step 3: It supports rich biodiversity including migratory birds and Irrawaddy dolphins, and is economically important for fishing communities.
  Final answer: Chilika Lake is India's largest saltwater lagoon, important for biodiversity, migratory birds and the local fishing economy.

Frequently Asked Questions