Drainage SystemClass 11 India Physical Environment Notes

Drainage System

The flow of water through well-defined channels is known as drainage, and the network of these channels is called a drainage system. The pattern a drainage system creates is shaped over long geological periods by factors like the type of rocks, the shape of the land (topography), the slope, and the amount and regularity of water flow.

These systems are crucial. During the rainy season, rivers and channels drain excess water. Without them, or if they are poorly defined or blocked, large-scale flooding can occur.

Key concepts to understand about a river system include:

• Catchment Area: The specific area from which a river collects its water.
• Drainage Basin: The entire area drained by a river and all of its tributaries.
• Watershed: The boundary line that separates one drainage basin from another.

Important Drainage Patterns

The arrangement of rivers and their tributaries forms distinct patterns on the landscape.

• Dendritic: This is the most common pattern, resembling the branches of a tree. The rivers of the northern plains are a good example.
• Radial: When rivers originate from a central hill or peak and flow in all directions, like spokes on a wheel. The rivers that start in the Amarkantak range show this pattern.
• Trellis: This pattern develops where primary tributaries flow parallel to each other, and secondary tributaries join them at right angles.
• Centripetal: This is the opposite of the radial pattern. Here, rivers flow from all directions to discharge their water into a central lake or depression.

Classification of the Indian Drainage System

India's drainage system is vast and can be classified in several ways.

On the Basis of Discharge (Orientation to the Sea)

This divides India's rivers into two major groups based on where they empty their water. The landform that separates them, like the Delhi ridge, the Aravalis, and the Sahyadris, is called a water divide.

1. The Arabian Sea Drainage: This system accounts for about 23% of India's drainage area. Major rivers include the Indus, Narmada, Tapi, Mahi, and Periyar.
2. The Bay of Bengal Drainage: This is the larger system, covering nearly 77% of the drainage area. It includes major river systems like the Ganga, Brahmaputra, Mahanadi, and Krishna.

On the Basis of Watershed Size

1. Major river basins: Catchment area of more than 20,000 sq. km. This includes 14 basins such as the Ganga, Brahmaputra, Krishna, and Narmada.
2. Medium river basins: Catchment area between 2,000 and 20,000 sq. km. This group includes 44 river basins like the Kalindi and Periyar.
3. Minor river basins: Catchment area of less than 2,000 sq. km. These are numerous rivers, often in areas with low rainfall.

On the Basis of Mode of Origin

This is the most accepted basis for classification, dividing Indian rivers into two main groups, even though it has some complexities (for example, rivers like the Chambal and Son originate in the Peninsula but are part of the Ganga system).

1. The Himalayan Drainage
2. The Peninsular Drainage

The Himalayan Drainage

The river systems originating in the Himalayas have evolved over a long geological history and include the massive basins of the Ganga, the Indus, and the Brahmaputra.

Key Characteristics:

• Perennial Flow: These rivers have water throughout the year because they are fed by both melting snow from the high Himalayas and rainfall during the monsoon season.
• Youthful Stage Features: In their mountainous course, these rivers are highly erosional. They carve out giant gorges, deep V-shaped valleys, rapids, and waterfalls.
• Plains Stage Features: As they enter the flat northern plains, their energy decreases, and they begin to deposit the sediment they carry. This creates features like flat valleys, ox-bow lakes, floodplains, braided channels, and large deltas near their mouths.
• Meandering and Shifting Courses: While their mountain course is twisted (tortuous), on the plains, they display a strong tendency to meander and frequently shift their courses. The River Kosi, known as the 'sorrow of Bihar', is famous for this, as it deposits huge amounts of sediment that block its own path, forcing it to find a new one.

Evolution of the Himalayan Drainage

Geologists believe that millions of years ago, a single, mighty river called the Shiwalik or Indo-Brahma flowed across the entire length of the Himalayas, from Assam to Punjab, and finally discharged into the Gulf of Sind.

Over time, this super-river was broken up (dismembered) into the three main Himalayan river systems we see today:

1. The Indus System in the west.
2. The Ganga System in the center.
3. The Brahmaputra System in the east.

This breakup was caused by two key geological events during the Pleistocene period:

• The upheaval of the western Himalayas, including the Potwar Plateau (Delhi Ridge), which created a water divide between the Indus and Ganga systems.
• The down-thrusting of the Malda gap (between the Rajmahal hills and Meghalaya plateau), which diverted the Ganga and Brahmaputra systems to flow towards the Bay of Bengal.

The River Systems of the Himalayan Drainage

The Indus System

The Indus, also known as Sindhu, is one of the largest river basins in the world. It originates from a glacier near Bokhar Chu in the Kailash Mountain range in Tibet, where it is known as 'Singi Khamban' (Lion's mouth).

• Course: It flows northwest between the Ladakh and Zaskar ranges, cuts a spectacular gorge near Gilgit, and enters Pakistan.
• Tributaries: In India, its Himalayan tributaries include the Shyok, Gilgit, and Zaskar. After emerging from the hills, it is joined by the Kabul river on its right bank. Its most important tributaries are the five rivers of Punjab, collectively known as the 'Panjnad':
  • The Jhelum: Rises from a spring at Verinag in Kashmir. It flows through Srinagar and the Wular lake before joining the Chenab in Pakistan.
  • The Chenab: The largest tributary of the Indus, formed by two streams, the Chandra and the Bhaga. It is also known as Chandrabhaga.
  • The Ravi: Rises west of the Rohtang pass in Himachal Pradesh and flows through the Chamba valley.
  • The Beas: Originates from the Beas Kund near Rohtang Pass. It meets the Satluj near Harike in the Punjab plains.
  • The Satluj: Originates from 'Raksas tal' near Mansarovar in Tibet, where it is known as Langchen Khambab. It flows parallel to the Indus for about 400 km before entering India through the Shipki La pass. It is an antecedent river, meaning it has maintained its course despite the uplift of the Himalayas.
• Discharge: The Indus finally discharges into the Arabian Sea, east of Karachi.

The Ganga System

The Ganga is the most important river of India, both for its vast basin and its cultural significance.

• Origin: It rises in the Gangotri glacier near Gaumukh in Uttarakhand, where it is known as the Bhagirathi.
• Formation: At Devprayag, the Bhagirathi meets the Alaknanda river. From this confluence onwards, the river is known as the Ganga.
  • The Alaknanda itself originates from the Satopanth glacier and is joined by tributaries like the Dhauli and Vishnu Ganga at Vishnu Prayag, the Pindar at Karna Prayag, and the Mandakini at Rudra Prayag.
• Course: The Ganga enters the plains at Haridwar. It flows south, then southeast and east, before splitting into two distributaries in West Bengal: the Bhagirathi and the Padma. The Ganga basin covers about 8.6 lakh sq. km in India.
• Tributaries: The Ganga system has numerous perennial and non-perennial tributaries.
  • Right-bank Tributaries: The Yamuna is the westernmost and longest tributary, originating from the Yamunotri glacier. It joins the Ganga at Prayag (Allahabad). The Son is another major right-bank tributary originating from the Peninsular plateau.
  • Left-bank Tributaries: Important tributaries from the Himalayas include the Ramganga, the Gomati, the Ghaghara, the Gandak, the Kosi, and the Mahananda.
• Other Key Tributaries:
  • Chambal: A tributary of the Yamuna, famous for its badland topography called the Chambal ravines.
  • Damodar: Occupies the eastern Chotanagpur Plateau and flows through a rift valley. Once known as the 'sorrow of Bengal', it is now managed by a multipurpose project.
• Discharge: The Ganga finally discharges into the Bay of Bengal near the Sagar Island.

The Brahmaputra System

The Brahmaputra is one of the largest rivers in the world.

• Origin: It originates in the Chemayungdung glacier of the Kailash range, near Mansarovar lake in Tibet.
• Course and Names:
  • In Tibet, it flows eastward for about 1,200 km through a dry, flat region and is known as the Tsangpo, which means 'the purifier'.
  • It carves a deep gorge near Namcha Barwa in the Himalayas and enters India in Arunachal Pradesh, where it is called the Siang or Dihang.
  • After being joined by its main left-bank tributaries, the Dibang and the Lohit, it is known as the Brahmaputra in the Assam valley.
  • In Bangladesh, it is joined by the Tista river and is known as the Jamuna.
• Tributaries: Its major left-bank tributaries are the Burhi Dihing and Dhansari. Important right-bank tributaries include the Subansiri, Kameng, Manas, and Sankosh.
• Characteristics: The Brahmaputra is well-known for floods, channel shifting, and bank erosion. This is because its tributaries are large and carry a huge quantity of sediment due to heavy rainfall in their catchment areas.
• Discharge: It finally merges with the Padma (Ganga) in Bangladesh and flows into the Bay of Bengal.

The Peninsular Drainage System

The Peninsular drainage system is older than the Himalayan system. This is evident from the broad, largely-graded shallow valleys and the overall maturity of the rivers.

Key Characteristics:

• Water Divide: The Western Ghats, running close to the western coast, act as the primary water divide, separating the major east-flowing rivers from the small west-flowing rivulets.
• Flow Direction: Most major Peninsular rivers, except for the Narmada and Tapi, flow from west to east and drain into the Bay of Bengal. This is due to the slight tilt of the Peninsular block.
• Nature of Flow: These rivers are generally non-perennial, meaning their flow depends on rainfall.
• Course: They have a fixed course and are characterized by an absence of meanders.
• Exceptions: The Narmada and Tapi are exceptions, as they flow westward through a rift valley.

The Evolution of Peninsular Drainage System

Three major geological events have shaped the Peninsular drainage system:

1. Subsidence of the western flank: During the early tertiary period, the western part of the Peninsula sank below the sea, disturbing the original river patterns.
2. Upheaval of the Himalayas: This event caused the northern flank of the Peninsular block to subside, leading to trough faulting. The Narmada and Tapi now flow through these faults.
3. Tilting of the Peninsular block: A slight tilt from northwest to the southeastern direction gave the entire drainage system its orientation towards the Bay of Bengal.

River Systems of the Peninsular Drainage

East-Flowing Rivers (draining into the Bay of Bengal)

• The Mahanadi: Rises near Sihawa in Chhattisgarh and flows through Odisha to the Bay of Bengal.
• The Godavari: The largest Peninsular river system, also called the Dakshin Ganga. It rises in the Nasik district of Maharashtra. Its principal tributaries are the Penganga, the Indravati, the Pranhita, and the Manjra.
• The Krishna: The second-largest east-flowing Peninsular river, rising near Mahabaleshwar. The Koyna, the Tungbhadra, and the Bhima are its major tributaries.
• The Kaveri: Rises in the Brahmagiri hills of Karnataka. It is a unique Peninsular river because it carries water throughout the year with less fluctuation. This is because its upper catchment receives rainfall from the southwest monsoon (summer) and its lower part receives rain from the northeast monsoon (winter).

West-Flowing Rivers (draining into the Arabian Sea)

• The Narmada: Originates on the western flank of the Amarkantak plateau. It flows in a rift valley between the Satpura and Vindhyan ranges, forming the famous Dhuandhar waterfall near Jabalpur. It meets the Arabian Sea by forming a broad 27 km long estuary (where the river's current meets the sea's tide).
• The Tapi: The other important westward-flowing river, originating from Multai in Madhya Pradesh.
• The Luni: The largest river system of Rajasthan. It originates near Pushkar and flows towards the Rann of Kuchchh. The entire river system is ephemeral, meaning it only has water during the rainy season.

Extent of Usability of River Water

India's rivers carry huge volumes of water, but its distribution is uneven in both time and space. This leads to situations where one part of the country suffers from floods while another faces drought. This highlights a problem of water management, not just availability.

One proposed solution is the inter-linking of rivers to transfer surplus water from one basin to a deficit one. However, this faces challenges like uneven terrain, which makes it difficult to lift water from plains to plateau areas.

River water use is further complicated by several problems:

• River water pollution from cities and industries.
• Load of silt in the river water.
• Uneven seasonal flow of water.
• River water disputes between states.