Nature · Earth Science
Still waters held in the hollows of the land
A lake is a body of water surrounded by land and largely separated from the sea. Filling basins scooped by ice, torn open by tectonics, blasted by volcanoes or dammed by rivers and people, lakes store most of the planet’s accessible liquid freshwater and teem with life.
A lake needs two things: a hollow (basin) and a supply of water that exceeds losses to outflow and evaporation. How that basin forms — by glaciers, faulting, volcanism, river action or human engineering — defines the type of lake and its shape, depth and lifespan.
In warm months deep lakes stratify into a sunlit surface layer, a rapid transition zone and a cold, dark bottom layer.
Basins scoured and dammed by ice
As glaciers grind across the land they gouge deep hollows and leave ridges of debris (moraine) that trap meltwater. Most of the world’s lakes, including the Great Lakes, are glacial in origin.
Water in the rifts of a splitting Earth
Where the crust is pulled apart, long deep troughs form and fill with water. Because these basins persist for millions of years, tectonic lakes like Baikal and Tanganyika host species found nowhere else.
Rainwater cupped in a volcano’s throat
When a volcano’s summit collapses after a great eruption, the resulting caldera can fill with rain and snowmelt. Crater Lake in Oregon, formed ~7,700 years ago, is a flawless example.
A meander left behind by its river
On a floodplain, a river’s looping meander can be pinched off when the channel takes a shortcut. The abandoned curve becomes a calm, crescent-shaped oxbow lake that slowly fills with sediment.
Rivers dammed to serve human needs
Humans build reservoirs by damming rivers to store water, generate hydroelectricity and manage floods. Lake Kariba on the Zambezi is the world’s largest reservoir by volume.