Chapter – 3

Drainage System

In this post we have given the detailed notes of Class 11 Geography Book 2 Chapter 3 (Drainage System) in English. These notes are useful for the students who are going to appear in Class 11 board exams.

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Chapter 3: Drainage System

Introduction

• Drainage: The term “drainage” describes the movement of water through well-defined channels, such as rivers, streams, and other water bodies.  
• Drainage System: A drainage system refers to the network of these channels, working together to collect and transport water across a landscape.
• Factors Influencing Drainage Patterns: The specific pattern a drainage system takes is shaped by various factors, including the geological history of the area, the type and structure of the underlying rocks, the topography, the slope of the land, the volume of water flowing, and how often it flows.  

Drainage Basins and Watersheds

• Catchment Area: Every river drains water from a specific area of land, known as its catchment area.  
• Drainage Basin: A drainage basin encompasses the entire area drained by a river and all its tributaries (smaller streams that flow into it).  
• Watershed: The boundary line that separates one drainage basin from another is called a watershed.  
• River Basins vs. Watersheds: While both are drainage areas, river basins are associated with larger rivers and cover extensive regions, whereas watersheds are typically smaller and related to streams or tributaries.  
• Interconnectedness: Drainage basins and watersheds are not isolated entities; they are interconnected systems where changes in one part can affect other parts and the entire unit. This makes them important for planning and managing water resources.  

Types of Drainage Patterns

• Dendritic: This pattern resembles the branching of a tree. It is common in areas with flat or gently sloping terrain and uniform rock structure. The rivers of the Northern Plains in India are a good example of this pattern.  
• Radial: In a radial pattern, rivers flow outward in all directions from a central high point, such as a hill or mountain. The rivers originating from the Amarkantak range in India exemplify this pattern.  
• Trellis: A trellis drainage pattern develops where primary tributaries flow parallel to each other, and secondary tributaries join them at right angles. This pattern is often found in areas with folded topography.  
• Centripetal: In a centripetal pattern, rivers converge inward from all directions towards a central depression or basin. This is often seen in areas with internal drainage where rivers do not drain into the sea.  

Classifying Indian Drainage Systems

• Based on Discharge (Orientation to the Sea):

  • Arabian Sea Drainage: This system includes rivers that drain into the Arabian Sea. It covers about 23% of India’s landmass and comprises major rivers like the Indus, Narmada, Tapi, Mahi, and Periyar.  
  • Bay of Bengal Drainage: This system encompasses rivers that drain into the Bay of Bengal. It covers a larger portion of India (about 77%) and includes major rivers such as the Ganga, Brahmaputra, Mahanadi, Krishna, and others.  

• Based on Watershed Size:

  • Major River Basins: These have a catchment area greater than 20,000 square kilometres. India has 14 major river basins, including the Ganga, Brahmaputra, Krishna, Tapi, Narmada, Mahi, Pennar, Sabarmati, and Barak, among others.  
  • Medium River Basins: These have a catchment area between 2,000 and 20,000 square kilometres. India has 44 medium river basins, including the Kalindi, Periyar, and Meghna.  
  • Minor River Basins: These have a catchment area of less than 2,000 square kilometres. These are typically smaller rivers found in areas with low rainfall.  

• Based on Origin and Characteristics:

  • Himalayan Drainage: This system includes rivers that originate in the Himalayas. These rivers are generally perennial, meaning they flow throughout the year, as they are fed by both melting snow and rainfall. The major rivers in this system are the Indus, Ganga, and Brahmaputra.  
  • Peninsular Drainage: This system encompasses rivers that originate in the Peninsular region of India. These rivers are generally non-perennial, meaning their flow fluctuates significantly with the seasons, as they are primarily fed by rainfall. Most of the major Peninsular rivers flow from west to east, with the exception of the Narmada and Tapi, which flow westward.  

Himalayan Drainage System

• Geological History: The Himalayan drainage system has evolved over millions of years, shaped by the ongoing uplift of the Himalayas and various geological processes.  
• Perennial Flow: The rivers in this system are perennial because they receive water from both melting snow and glaciers in the Himalayas and monsoon rainfall.  
• Distinctive Features:
  • Mountainous Course: In the mountainous regions, these rivers flow through deep gorges and V-shaped valleys, often forming rapids and waterfalls.  
  • Plains Course: As they enter the plains, they deposit sediments, creating features like flat valleys, ox-bow lakes, flood plains, braided channels, and deltas near their mouths.  
  • Meandering Tendency: In the plains, these rivers tend to meander, shifting their courses frequently. This can lead to flooding and erosion. The Kosi River, known as the “Sorrow of Bihar,” is a prime example of a river that frequently changes its course due to sediment deposition.  

Evolution of the Himalayan Drainage

• Ancient Indo-Brahma River: Geologists believe that a massive river, called the Shiwalik or Indo-Brahma, once flowed along the entire length of the Himalayas, from Assam to Punjab and beyond, millions of years ago.  
• Dismemberment: Over time, geological events, such as the uplift of the Himalayas and the formation of the Potwar Plateau (Delhi Ridge), caused this ancient river to break up into three major drainage systems: the Indus, the Ganga, and the Brahmaputra.  

Major River Systems of the Himalayan Drainage

• The Indus System:

  • Origin and Course: The Indus River, also known as the Sindhu, originates in the Tibetan Plateau near Lake Mansarovar and flows through India (Jammu and Kashmir) and Pakistan before emptying into the Arabian Sea.  
  • Tributaries: Its major tributaries include the Jhelum, Chenab, Ravi, Beas, and Sutlej, which together form the Panjnad. These tributaries originate in the Himalayas and contribute significantly to the Indus’ flow.  

• The Ganga System:

  • Origin and Course: The Ganga River is the longest and most important river in India, both in terms of its basin size and cultural significance. It originates in the Gangotri glacier in the Himalayas and flows through India before emptying into the Bay of Bengal.  
  • Tributaries: The Ganga has numerous tributaries, including the Yamuna, Chambal, Gandak, Ghaghara, Kosi, and Son. These tributaries drain a vast area of northern India and contribute to the Ganga’s large water volume.  

• The Brahmaputra System:

  • Origin and Course: The Brahmaputra River originates in the Himalayas in Tibet, where it is known as the Tsangpo. It flows through Tibet, India (Arunachal Pradesh and Assam), and Bangladesh before joining the Ganges to empty into the Bay of Bengal.  
  • Tributaries: The Brahmaputra has several major tributaries, including the Dibang, Lohit, Subansiri, and Tista. These tributaries contribute to the Brahmaputra’s large water volume and its tendency to flood.  

Peninsular Drainage System

• Geological History: The Peninsular drainage system is older than the Himalayan drainage system. Its rivers have had more time to erode the landscape, resulting in broad, gently sloping valleys and mature river systems.  
• Western Ghats as Water Divide: The Western Ghats Mountain range acts as a crucial water divide in the Peninsula. Rivers originating to the west of the Western Ghats are generally short and flow into the Arabian Sea, while those originating to the east are longer and flow into the Bay of Bengal.  
• Non-Perennial Flow: Most Peninsular rivers are non-perennial, meaning their flow varies significantly with the seasons. This is because they are primarily fed by rainfall, which is often seasonal and unreliable.  
• Exceptions: The Narmada and Tapi rivers are exceptions to the general characteristics of Peninsular rivers. They flow through rift valleys, have a more consistent flow, and exhibit some features more typical of Himalayan rivers.  

Evolution of the Peninsular Drainage

• Key Geological Events: The current drainage patterns in Peninsular India are a result of several major geological events:
  • Submergence of the Western Flank: The western part of the Indian Peninsula subsided and was submerged under the sea millions of years ago. This event disrupted the original drainage patterns and led to the formation of the Western Ghats.  
  • Uplift of the Himalayas: The uplift of the Himalayas caused tilting and faulting in the Peninsular region. This created rift valleys, which are now occupied by rivers like the Narmada and Tapi.  
  • Tilting of the Peninsula: The Peninsula tilted towards the southeast, giving the rivers a general eastward flow.  

Major River Systems of the Peninsular Drainage

• The Mahanadi:

  • Origin and Course: The Mahanadi River originates in the highlands of Chhattisgarh and flows through Odisha to empty into the Bay of Bengal.  
  • Characteristics: It is a major river with a length of 851 km and a catchment area of 1.42 lakh sq. km. It is navigable in its lower course and is prone to flooding.  

• The Godavari:

  • Origin and Course: The Godavari River, often referred to as the “Dakshin Ganga,” is the largest river in Peninsular India. It originates in the Western Ghats in Maharashtra and flows through several states before emptying into the Bay of Bengal.  
  • Characteristics: It is a long river with a length of 1,465 km and a vast catchment area of 3.13 lakh sq. km. It has several major tributaries and is also prone to flooding in its lower reaches.  

• The Krishna:

  • Origin and Course: The Krishna River is the second largest river in Peninsular India. It originates in the Western Ghats near Mahabaleshwar and flows through Maharashtra, Karnataka, and Andhra Pradesh/Telangana before emptying into the Bay of Bengal.  
  • Characteristics: It is a major river with a length of 1,401 km and has several major tributaries, including the Koyna, Tungabhadra, and Bhima.  

• The Kaveri:

  • Origin and Course: The Kaveri River originates in the Western Ghats in Karnataka and flows through Tamil Nadu before emptying into the Bay of Bengal.  
  • Characteristics: It is a relatively shorter river with a length of 800 km and a catchment area of 81,155 sq. km. It is unique among Peninsular rivers for having a relatively stable flow throughout the year due to its upper and lower catchments receiving rainfall in different seasons.  

• The Narmada:

  • Origin and Course: The Narmada River originates in the Amarkantak plateau in Madhya Pradesh and flows westward through a rift valley before emptying into the Arabian Sea.  
  • Characteristics: It is a major river with a length of 1,312 km and a catchment area of 98,796 sq. km. It is known for its scenic beauty, including the Dhuandhar waterfall and marble rocks near Jabalpur.  

• The Tapi:

  • Origin and Course: The Tapi River originates in the Satpura Range in Madhya Pradesh and flows westward through Maharashtra and Gujarat before emptying into the Arabian Sea.  
  • Characteristics: It is a significant river with a length of 724 km and a catchment area of 65,145 sq. km.  

Usability of River Water

• Abundance and Variability: India’s rivers are a major source of water, but their flow is highly variable, both in terms of timing and location.  
• Perennial vs. Non-Perennial Rivers: Perennial rivers, mainly found in the Himalayan system, flow throughout the year, while non-perennial rivers, common in the Peninsular system, have very little water during the dry seasons.  
• Water Wastage: A significant portion of river water is wasted during floods, especially during the monsoon season, as it flows into the sea without being utilized effectively.  
• Floods and Droughts: India faces the challenge of both floods and droughts occurring in different parts of the country simultaneously due to the uneven distribution of rainfall and river flow.  

Challenges in Utilizing River Water

• Pollution: River water pollution is a major concern, with industrial effluents, sewage, and other waste products contaminating many rivers.  
• Siltation: The accumulation of silt in riverbeds reduces their water carrying capacity and affects their flow.  
• Seasonal Variations: The highly seasonal nature of rainfall and river flow in many parts of India makes it challenging to ensure a consistent water supply throughout the year.  
• Inter-State Disputes: Water disputes between states over the sharing of river water resources are a significant challenge.  
• Channel Encroachment: The encroachment of human settlements and activities into river channels and floodplains reduces their capacity to handle floods and disrupts the natural flow.  

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