A lake is a mostly enclosed, inland body of standing water that fills a natural or human-made basin and is surrounded by land. Lakes can be fresh, brackish, or salty; some drain to rivers, others don’t. The world’s deepest lake, Baikal, reaches about 5,387 ft (1,642 m), while the largest inland water body by area is the Caspian Sea.
What makes a lake a lake? (and how it differs from ponds, wetlands, and reservoirs)
In geography and limnology (the study of inland waters), a lake is a relatively large, standing-water body contained within an inland basin. There is no single global size rule that separates “lake” from “pond”—usage varies by country and by scientific tradition. Encyclopaedia Britannica notes that distinctions among lakes, ponds, swamps, and rivers are not firmly standardized, which is why you’ll sometimes see gray areas in naming.
National Geographic’s classroom definition keeps it simple: a lake is “a body of water surrounded by land,” found on every continent and in many climates—a practical description that fits most everyday cases.
Lake vs. pond
Commonly, lakes are deeper and larger; ponds are smaller and often shallow enough for sunlight to reach the bottom across much of the basin, but there’s no universally accepted cut-off. Dictionaries reflect this flexibility, defining a lake as a “considerable” inland body of standing water.
Lake vs. wetland
Wetlands are areas where water controls soils and life—think marshes, swamps, and bogs. Under the Ramsar Convention, “lacustrine” wetlands are those associated with lakes, while other types (palustrine, riverine, etc.) have different traits. This framework is useful when a lake has wide marshy margins.
Lake vs. reservoir
A reservoir is a lake in function but built or managed by people—typically behind a dam for water supply, irrigation, hydropower, or flood control. Many maps label both natural and artificial basins as “lakes,” so the key difference is origin and management rather than appearance. (See Britannica’s broad definition acknowledging natural and artificial basins.)
How lakes form (with clear examples)
Lakes occupy basins made by ice, tectonics, volcanoes, rivers, wind, or dissolution of rock. After formation, rainfall, rivers, and groundwater keep them filled—unless they evaporate faster than they’re replenished.
Glacial lakes
During and after the last Ice Age, glaciers carved bowl-shaped hollows (kettles and overdeepened valleys). When the ice retreated, these basins filled with water, creating chains like North America’s Great Lakes and thousands of smaller tarns.
Tectonic & rift lakes
Tectonic stretching can open giant rift valleys that fill with water. Siberia’s Lake Baikal sits in an active rift and is the world’s deepest freshwater lake at ~1,642 m (5,387 ft); it also holds the largest volume of liquid freshwater.
Volcanic & crater lakes
When volcanic craters or calderas cool and fill—by rain, snowmelt, or springs—they become clear, steep-sided lakes. Their chemistry can be unusual if gases percolate from below.
River-made, coastal, karst & wind-formed lakes
Meandering rivers can abandon loops to make oxbow lakes; coastal sandbars can isolate lagoons; groundwater can dissolve limestone to form sinkhole/karst lakes; and wind deflation can scoop shallow pans that collect seasonal water.
Types & classifications you’ll actually use
By salinity: Freshwater (most lakes); brackish; saline/hypersaline (e.g., the Dead Sea with ~34% salinity—so dense you float easily).
By outflow: Exorheic lakes drain to rivers/oceans; endorheic lakes are closed basins that mainly lose water by evaporation (like the Caspian basin), causing salinity to build up over time.
By nutrients: Oligotrophic (clear, low nutrients), mesotrophic, eutrophic (nutrient-rich and plant-heavy). These states affect clarity, oxygen, and fish habitat (and can shift with pollution).
By permanence: Permanent vs. ephemeral/seasonal lakes. Some fill and drain with rainfall or blocked/unblocked sinks (famous “vanishing” lakes occur in karst terrains).
How a lake “lives” through the year
Lakes change with seasons. In many temperate lakes, warm summers create layers (warm surface, cool deep water) separated by a thermocline. Autumn winds and cooling then “turn over” the water column, mixing oxygen and nutrients; winter ice can cap the system until spring mixing returns. This annual rhythm shapes which fish and plants thrive.
Regional climate also matters. The Laurentian Great Lakes, together covering ~94,250 mi² (244,106 km²), show strong ice-and-temperature swings that drive evaporation, storms, and lake-effect snow. NOAA and EPA datasets summarize their depth, volume, and ice trends; for example, 2024 set a record-low average ice cover (~4.3%) across all five lakes since 1973, while 2025 rebounded closer to normal maxima.
| Record / Metric (as of 2025) | Lake (Location) | Value | Note / Source |
|---|---|---|---|
| Largest inland water body by area | Caspian Sea (Eurasia) | ~143,000–149,000 sq mi (371,000–386,400 km²) | Range reflects different delineations; Britannica vs. Wikipedia. |
| Largest freshwater by area | Lake Superior (North America) | 31,700 sq mi (82,100 km²) | EPA Great Lakes physical features. |
| Deepest lake | Lake Baikal (Russia) | Max depth ≈ 5,387 ft (1,642 m) | Depth confirmed by Britannica/Wikipedia. |
| Lowest lake surface (elevation) | Dead Sea (Israel/Jordan/West Bank) | ≈ −1,443 ft (−440 m) below sea level; salinity ~34% | Hypersaline, extremely buoyant water. |
| Total Great Lakes water area | Great Lakes (U.S./Canada) | ~94,250 sq mi (244,106 km²) | EPA dataset; NOAA bathymetry portal. |
Lakes and people: why they matter (and the pressures they face)
Lakes store drinking water, support fisheries and irrigation, enable shipping and tourism, and anchor cultures and faiths. The Great Lakes alone hold ~21% of Earth’s surface freshwater and shape weather, economies, and identities across two nations. U.S. government summaries detail their depths, volumes, and vast shoreline.
Modern pressures include nutrient pollution (algal blooms), invasive species, shoreline development, and climate shifts (warming water, altered ice and evaporation). NOAA’s GLERL reports show record-low average ice cover in 2024 and long-term declines since the 1970s, while some inland seas like the Caspian are also experiencing notable level drops that endanger ecosystems and ports.
For conservation policy and definitions, the Ramsar Convention provides the global wetland framework, with a category (lacustrine) specific to lakes.
FAQ
Is there a strict size that separates a lake from a pond?
No. Scientists and mapmakers don’t use one universal cut-off; context and local naming matter. As a rule of thumb, lakes tend to be larger/deeper and have more wind-driven mixing than ponds.
Are all lakes freshwater?
Most are, but some are brackish or saline. Famous examples include the hypersaline Dead Sea (~34% salt) and other salt lakes in arid basins that lose water mainly via evaporation.
What’s the largest lake in the world?
By area, the Caspian Sea is the largest enclosed inland water body (~371,000–386,400 km² depending on delineation). It’s brackish, not freshwater.
Which lake is deepest?
Lake Baikal in Siberia reaches ~1,642 m (5,387 ft). It also holds the most liquid freshwater by volume.
Do lakes freeze less than they used to?
In the Great Lakes, 2024 set a record-low average ice cover (~4.3%) since 1973, and scientists find a long-term downward trend in ice cover and season length.
Where can I read a simple, reliable definition for students?
Try National Geographic’s education page on lakes, which explains the basics clearly.
What Did We Learn Today?
- A lake is an inland, standing-water body in a natural or artificial basin; there’s no strict global size rule separating it from a pond.
- Basins form by ice, tectonics, volcanoes, rivers/coasts, karst, or wind; Baikal shows how tectonics produce remarkable depths (~1,642 m / 5,387 ft).
- Classifications include salinity (fresh/brackish/saline), outflow (exorheic/endorheic), nutrients (oligo- to eutrophic), and permanence (ephemeral vs. permanent).
- Seasonal mixing and ice strongly affect lake ecology; 2024 set record-low Great Lakes ice cover since 1973.
- By area, the Caspian is largest; by freshwater area, Lake Superior leads; by depth and volume, Baikal dominates; the Dead Sea is lowest and hypersaline.
