Ponds and rivers are part of the water cycle — the water that is constantly evap-orating from the Earth, forming clouds and coming back to Earth as rain or snow. Some of this water seeps into under-ground streams and pools, which in turn may feed a spring that is the source of a river. Other rivers are fed by melting glaciers or very large lakes.
After a heavy rain, you may find puddles of water standing in low spots. The same principle explains why water collects in ponds and lakes. Water travels downhill, so a depression in the ground fills with standing water. A pond is a small body of standing water. A lake is a large body of standing water. Most lakes have freshwater, but a few are salty. The Great Salt Lake in Utah is an example of a saltwater lake.
The water in a large lake may be so deep that sunlight cannot penetrate all the way to the bottom. Without sunlight, water plants and algae cannot live on the bottom of the lake. That’s because plants need sunlight for photosynthesis.
Ponds and lakes may get their water from several sources. Some falls directly into them as precipitation. Some enters as runoff and some from streams and rivers. Water leaves ponds and lakes through evaporation and also as outflow.
The depression that allows water to collect to form a lake may come about in a variety of ways. The Great Lakes, for example, are glacial lakes. A glacial lake forms when a glacier scrapes a large hole in the ground. When the glacier melts, the water fills the hole and forms a lake. A kettle lake forms in the sediment left by a glacier when a block of ice melts. Kettle lakes are found where continental glaciers once covered the land. Over time lakes get water from rain, streams, and groundwater coming to the surface.
Other lakes are crater lakes or rift lakes. Crater lakes form when volcanic eruptions create craters that fill with water. Rift lakes form when movements of tectonic plates create low places that fill with water.