India can be divided into distinct physiographic units. The northern boundary is dominated by the Himalayas, while the peninsula hosts ancient block and fold mountains:

The Himalayan Ranges

The Himalayas are young fold mountains formed by the collision of the Indo-Australian and Eurasian tectonic plates, compressing the Tethys geosyncline. They consist of three parallel ranges:

• Greater Himalayas (Himadri): The loftiest and most continuous range, with an average height of 6,000m. Home to the world's highest peaks, including Mount Everest (8848.86m, Nepal) and Kanchenjunga (8586m, India).
• Lesser Himalayas (Himachal): Located south of the Himadri, with heights ranging between 3,700m and 4,500m. Contains prominent ranges like Pir Panjal (longest range), Dhauladhar, and Mahabharat. Famous hill stations (Shimla, Mussoorie, Nainital) are situated here.
• Outer Himalayas (Shiwaliks): The outermost range, composed of unconsolidated sediments. Average altitude is 900m to 1,100m. The valleys lying between the Himachal and Shiwaliks are known as Duns (e.g., Dehradun, Patli Dun).
• Trans-Himalayas: Lie immediately north of the Greater Himalayas. Includes the Karakoram (hosts K2 / Godwin Austen, 8611m, highest peak in India), Ladakh, and Zaskar ranges.

Visualizing the parallel Himalayan Ranges and the location of high-altitude mountain passes connecting India with Tibet, Central Asia, and regional valleys.

Key Mountain Passes (La)

Mountain passes are crucial routes across the rugged Himalayan ridges, heavily tested in CGL:

Peninsular Mountains

• Aravalli Range: One of the oldest fold mountain systems in the world, now highly eroded as residual mountains. Highest peak is Guru Shikhar (1,722m, Mt. Abu, Rajasthan).
• Vindhyas and Satpuras: Block mountain systems in Central India. The Narmada River flows in a rift valley between the Vindhyas (north) and Satpuras (south). The Tapi River flows south of the Satpuras.
• Western Ghats (Sahyadris): A continuous escarpment forming the western edge of the Deccan Plateau. Highest peak is Anamudi (2,695m, Kerala), which is also the highest peak in South India. Meets the Eastern Ghats at the Nilgiri Hills (highest peak: Doddabetta, 2,637m).
• Eastern Ghats: Discontinuous, highly dissected hills running parallel to the east coast. Highest peak is Jindhagada Peak (1,690m, Andhra Pradesh), followed by Mahendragiri (1,501m, Odisha).

India's rivers are classified into two major groups based on their origin and water regime:

Visualizing the major Himalayan River systems (Indus, Ganga, Brahmaputra) and Peninsular drainage routes (east-flowing into Bay of Bengal, west-flowing into Arabian Sea).

Himalayan River Systems

These are perennial rivers fed by both glacial melt and monsoon rainfall. The three main systems are:

1. The Ganga System: Originates as Bhagirathi from Gangotri Glacier. Merges with Alaknanda at Devprayag to form the Ganga.
   • Right-bank tributaries: Yamuna (originates at Yamunotri, joins Ganga at Prayagraj), Son, Punpun.
   • Left-bank tributaries: Ramganga, Gomti, Ghaghara, Gandak, Kosi (known as the "Sorrow of Bihar"), Mahananda.
2. The Indus System: Originates near Mansarovar Lake (Tibet). Flows northwest through Ladakh and enters Pakistan. Its five major Indian tributaries (Panjnad) are Jhelum, Chenab (largest tributary by volume), Ravi, Beas, and Sutlej.
3. The Brahmaputra System: Originates near Chemayungdung Glacier (Tibet) where it is called Tsangpo. Enters India in Arunachal Pradesh as Dihang. In Bangladesh, it is known as Jamuna and joins the Ganga (Padma) to form the Sundarbans Delta (world's largest delta). Major tributaries include Subansiri, Teesta, Manas, Lohit, and Dhansiri. Hosts Majuli (world's largest river island) in Assam.

Peninsular River Systems

These are seasonal rivers reliant entirely on monsoon rain. They are divided by the water divide:

• East-Flowing (Drain into Bay of Bengal): They form extensive deltas at their mouths.
  • Godavari: The longest peninsular river (1,465 km), known as Dakshin Ganga or Vriddha Ganga. Originates at Trimbakeshwar, Nashik (Maharashtra). Tributaries: Pemganga, Wainganga, Wardha, Manjira, Indravati, Sabari.
  • Krishna: Originates near Mahabaleshwar (Maharashtra). Tributaries: Tungabhadra, Bhima, Koyna, Ghataprabha, Malaprabha, Musi.
  • Cauvery: Originates at Talacauvery, Brahmagiri Hills (Karnataka). Known as the "Ganga of the South" due to its sacred nature. Tributaries: Kabini, Hemavati, Arkavati, Bhavani, Noyyal, Amravati.
  • Mahanadi: Originates in Sihawa, Chhattisgarh. Noted for the Hirakud Dam (world's longest earthen dam).
• West-Flowing (Drain into Arabian Sea): They do not form deltas; instead, they form estuaries due to steep slopes and hard rock pathways.
  • Narmada: Originates at Amarkantak Plateau (Madhya Pradesh). Flows through a marble rock gorge forming the Dhuandhar Falls near Jabalpur.
  • Tapi (Tapti): Originates in Multai, Betul district (Madhya Pradesh). Parallel to Narmada.
  • Luni: An ephemeral/inland river originating in the Aravalli range and disappearing into the Rann of Kutch (Gujarat).

India experiences a Tropical Monsoon type of climate. The climate is highly regulated by the unique layout of the Himalayas in the north (blocking cold Siberian winds) and the Indian Ocean in the south.

The Southwest Monsoon (June - September)

This is the primary rainy season, contributing over 75% of India's annual rainfall. It is triggered by the intense heating of the Tibetan Plateau during summer, creating a low-pressure area that draws moist winds from the high-pressure zone over the southern Indian Ocean.

• Arabian Sea Branch: Hits the Western Ghats first, causing heavy orographic rainfall on the windward side (e.g., Mumbai, Coastal Karnataka). It passes over Rajasthan without causing rain because the Aravalli Range lies parallel to the wind direction.
• Bay of Bengal Branch: Moves up the Bay of Bengal, hitting Northeast India. The funnel-shaped Khasi hills trap this moisture, causing extreme rainfall in Mawsynram (Cherrapunji, Meghalaya), which is the wettest place on Earth. It then deflects westward along the Ganga plains.

Northeast / Retreating Monsoon (October - November)

As temperatures drop in the north, high pressure develops over the land. The winds reverse, blowing from land to sea (northeast to southwest). These winds pick up moisture as they cross the Bay of Bengal and deposit rainfall on the Coromandel Coast (Tamil Nadu) during winter.

Important Climatic Concepts

• Western Disturbances: Shallow cyclonic depressions originating in the Mediterranean Sea that travel eastward via subtropical jet streams. They cause crucial winter rainfall in Punjab, Haryana, and snowfall in Jammu & Kashmir and Himachal Pradesh, which is highly beneficial for Rabi crops (especially wheat).
• El Niño and La Niña:
  • El Niño: Unusually warm ocean currents in the equatorial eastern Pacific. Historically correlates with weak Indian Monsoons and drought conditions.
  • La Niña: Cooling of equatorial eastern Pacific waters. Generally results in a strong, abundant monsoon and floods in India.
• Mango Showers: Pre-monsoon showers occurring in Kerala and Karnataka during late spring, helping in the early ripening of mangoes. Known as Kaal Baisakhi in West Bengal (characterized by destructive thunderstorms).