A lake is a body of standing water with an upper surface that is exposed to the atmosphere and does not have an appreciable gradient. Ponds, marshes, and swamps with standing water can all be included under the definition of a lake. Lakes receive water from streams, overland flow, and ground water, and so they form part of drainage systems. Many lakes lose water at an outlet, where water drains over a dam—natural or constructed— to become an outflowing stream. Lakes also lose water by evaporation.

Lake basins, like stream channels, are true landforms, created by a number of geologic processes and ranging widely in size. For example, the tectonic process of crustal faulting creates large, deep lakes. Lava flows often form a dam in a river valley, causing water to back up as a lake. Landslides can also suddenly creates a lake.

Where there aren’t enough natural lakes, we create them by placing dams across the stream channels. Many regions that once had almost no lakes now have many. Some are small ponds built to serve ranches and farms, while others cover hundreds of square kilometers. In some areas, the number of artificial lakes is large enough to have significant effects on the region’s hydrologic cycle.

On a geologic time scale, lakes are short-lived features. A Lake disappears by one of two processes, or a combination of both. First, lakes that have stream outlets will be gradually drained as the outlets are eroded to lower levels. Even when the outlet lies above strong bedrock, erosion will still occur slowly over time.

Second, inorganic sediment carried by streams enters the lake and builds up, along with organic matter produced by plants and animals within the lake. Eventually, the lake fills, forming a boggy wetland with little or no free water surface.

Many former freshwater ponds have become partially or entirely filled by organic matter from the growth and decay of water-loving plants.

A Lake can also disappear when the climate changes. If precipitation is reduced, or temperatures and net radiation increase, evaporation can exceed input and the lake will dry up. The water level of lakes and ponds in moist climates closely coincides with the surrounding water table.