A Lake is a large body of water (typically freshwater, though saline lakes exist) that is surrounded by land. Unlike rivers, lakes are relatively still bodies of water that occupy depressions on the Earth’s surface. They are temporary features on a geological timescale, as they eventually fill with sediment or dry up due to environmental changes.
Classification by Origin and Formation
Lakes are formed by a variety of geological, tectonic, and biological processes. Understanding these is essential for UPSC Physical Geography.
1. Tectonic Lakes
Formed by the movements of the Earth’s crust, such as folding, faulting, or subsidence.
- Rift Valley Lakes: Formed when the land between two parallel faults sinks. These are often long, narrow, and very deep.
- Examples: Lake Tanganyika (deepest in Africa), Lake Baikal (deepest in the world), and Lake Nyasa.
- Upwarping Lakes: Formed by the upward movement of the crust which creates a shallow depression.
- Example: Lake Victoria (Africa).
2. Glacial Lakes
Glaciers carve out depressions or block valleys with debris, leading to lake formation upon melting.
- Cirque Lakes (Tarns): Formed in bowl-shaped depressions (cirques) carved by glaciers on mountain slopes.
- Example: Lake Chandra Tal (Himachal Pradesh).
- Ribbon Lakes: Long, narrow lakes occupying glacial troughs.
- Kettle Lakes: Formed when blocks of ice buried in glacial till melt, leaving a depression.
3. Volcanic Lakes
Associated with volcanic activity and depressions.
- Crater/Caldera Lakes: Formed in the craters or calderas of dormant or extinct volcanoes.
- Example: Lonar Lake (Maharashtra) is often cited in this context, though it is specifically a Meteoric Impact Crater.
- Global Example: Crater Lake (Oregon, USA).
4. Fluvial (River-borne) Lakes
Formed by the action of running water.
- Oxbow Lakes: Formed when a river meander is cut off from the main channel due to erosion and deposition.
- Example: Kanwar Lake (Bihar).
- Deltaic Lakes: Formed by the deposition of silt in a river delta, trapping water in depressions.
- Example: Lake Kolleru (Andhra Pradesh).
5. Aeolian (Wind-borne) Lakes
In arid regions, wind erosion (deflation) creates shallow depressions which may fill with water after rain. These are often saline.
- Example: Dhands or Playas in the Thar Desert (e.g., Sambhar Lake, Rajasthan).
Classification by Chemistry (Salinity)
| Type | Characteristics | Examples |
| Freshwater Lakes | Low salt content (< 0.5 ppt); usually have an outlet. | Wular Lake (J&K), Lake Superior (USA). |
| Saline Lakes | High salt content; often found in endorheic (closed) basins with high evaporation. | Dead Sea, Caspian Sea, Great Salt Lake. |
| Brackish Lakes | Salinity between freshwater and seawater; common in lagoons. | Chilika Lake (Odisha), Pulicat Lake. |
Biological Classification: Trophic Levels
Lakes are also classified based on their nutrient content and biological productivity, which is a key concept in Environment and Ecology.
- Oligotrophic Lakes: Low nutrient levels, clear water, and high oxygen content. Very little plant life.
- Mesotrophic Lakes: Moderate levels of nutrients and medium biological productivity.
- Eutrophic Lakes: High nutrient levels (Nitrogen and Phosphorus), supporting dense aquatic plant growth and frequent algal blooms.
- Dystrophic Lakes: Highly acidic, organic-rich (humic) lakes, often found in peat bogs.
Significant Lakes of India for UPSC Prelims
- Wular Lake (J&K): The largest freshwater lake in India; formed by tectonic activity and fed by the Jhelum river.
- Chilika Lake (Odisha): The largest brackish water lagoon in India; a prominent Ramsar site and wintering ground for migratory birds.
- Loktak Lake (Manipur): Known for its floating islands called Phumdis and the Keibul Lamjao National Park (only floating park in the world).
- Vembanad Lake (Kerala): The longest lake in India, famous for the Nehru Trophy Boat Race.
- Pangong Tso (Ladakh): An endorheic, high-altitude saline lake.
Trivia and Technical Facts
- The Caspian Sea: Despite its name, it is geographically the world’s largest inland body of water (lake).
- Lake Baikal: Contains about 20% of the world’s unfrozen surface freshwater.
- Eutrophication: The process where a lake becomes over-enriched with nutrients, leading to oxygen depletion (hypoxia) and the death of aquatic life.
- Thermal Stratification: In deep lakes, water separates into layers based on temperature: the Epilimnion (warm top), Thermocline (middle), and Hypolimnion (cold bottom).