Chapter 9: Lakes
Lakes are inland depressions that hold standing water. Geologically speaking, they are temporary features; they eventually disappear because they are either filled by sediments (silt and organic matter) or drained by the down-cutting of their outlet rivers.
1. Classification of Lakes
Lakes are classified based on the physical processes that created the depressions they occupy:
A. Lakes Formed by Earth Movements
- Tectonic Lakes: Formed by the warping, sagging, or bending of the Earth’s crust. The Caspian Sea and Lake Baikal (the deepest lake in the world) are prime examples.
- Rift Valley Lakes: Formed when the land between two parallel faults sinks. These are usually long, narrow, and very deep (e.g., Lake Tanganyika and Lake Nyasa in East Africa).
B. Lakes Formed by Glaciation
- Cirque Lakes (Tarns): Small, circular lakes found in mountain hollows carved by glaciers (e.g., Red Tarn in England).
- Rock-Hollow Lakes: Formed where a glacier has scooped out a hollow in the floor of a valley.
- Ribbon Lakes: Long, narrow lakes in a glacial trough, often dammed by terminal moraines (e.g., Lake Windermere).
C. Lakes Formed by Volcanic Activity
- Crater or Caldera Lakes: Formed in the vent of a dormant or extinct volcano. Rainwater collects in the crater (e.g., Crater Lake in Oregon, USA).
- Lava-dammed Lakes: Formed when a lava flow blocks a river valley.
D. Lakes Formed by Erosion & Deposition
- Ox-bow Lakes: Curved lakes formed when a river meander is cut off from the main channel.
- Karst Lakes: Formed by the collapse of limestone caverns (e.g., Solution Lakes).
- Lagoons: Shallow lakes formed near the coast, separated from the sea by a sandbar (e.g., Chilika Lake in India).
2. Freshwater vs. Saline Lakes
- Freshwater Lakes: Common in humid regions. They have an outlet (a river flowing out), which carries away salts and prevents them from accumulating.
- Saline (Salt) Lakes: Found in arid regions with high evaporation. They have no outlet to the sea. The salts brought in by rivers concentrate over time. (e.g., Dead Sea, Great Salt Lake).
3. The Importance of Lakes
- River Regulation: Lakes act as reservoirs, absorbing excess water during floods and releasing it during dry seasons to maintain steady river flow.
- Hydro-Electric Power (HEP): Large lakes (natural or man-made) provide a steady head of water for generating electricity.
- Climate Moderation: Large lakes cool the air in summer and warm it in winter, creating a “maritime” effect for nearby land.
- Economic Value: Lakes provide fish, a means of transport (e.g., The Great Lakes of North America), and opportunities for tourism and recreation.
Revision Questions & Answers
Q1: Why are lakes called “temporary” features in a geological sense?
- A: Because they are subject to obliteration. They are eventually filled by the deposition of silt from rivers or emptied by the erosion of their outlets, which eventually drains the basin.
Q2: What is the difference between a Doline and a Karst Lake?
- A: A Doline is a small surface depression in limestone. When several dolines merge or the floor of a large depression becomes clogged with clay, it prevents water from draining away, forming a Karst Lake.
Q3: How does a Rift Valley lake differ from a Tectonic lake?
- A: While both involve earth movements, Tectonic lakes result from broad warping or bending of the crust. Rift Valley lakes are specifically formed by faulting, where a block of land sinks between two parallel cracks.
Q4: Why is the Dead Sea so salty?
- A: It is located in an arid region with extremely high evaporation. Because it has no outlet to carry salts away, the minerals brought in by the River Jordan have accumulated for thousands of years, making it one of the saltiest bodies of water.
Q5: What is a “Barrier Lake”?
- A: A lake formed when a natural barrier—such as a landslide, lava flow, or glacial moraine—blocks a valley and traps a river behind it.
