A lake (from Latin lacus) is a terrain feature (or physical feature), a body of liquid on the surface of a world that is localized to the bottom of basin (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river. The only world other than Earth known to harbor lakes is Titan, Saturn's largest moon, which has lakes of ethane, most likely mixed with methane. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds.

Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.

The Wetland Connection

Wetlands help keep lakes clean by filtering sediments and excess nutrients. Acting like natural sponges, wetlands slow down water. This function reduces flooding, stabilizes stream flows and lake levels, and provides recharge for groundwater.

Shoreline wetlands are habitat for a diverse community of plants and animals such as northern pike, which spawn among aquatic vegetation. Nutrient-rich sediments and soils in wetlands support insects, frogs, and other small animals eaten by fish and wildlife. Wetland vegetation provides food and cover for waterfowl, muskrats, and other wildlife.

In addition to shoreland wetlands, the amount and quality of wetlands within a lake's watershed also affect lakes.