Rain may fall directly into lake basins, or it may flow into lakes as runoff from surrounding higher ground or through underground springs. Most water leaves lakes through transportation or overflow. In other words, water that enters the lake, on average, stays in the lake for nearly 200 years. Water temperature varies within a lake and between seasons. The surfaces of lakes located at high latitudes or altitudes may freeze in winter.
Ponds are generally small, shallow lakes that have a more uniform temperature.
Salt and bitter lakes are most common in arid areas where water evaporates faster than it can be replaced, leaving high concentrations of mineral salts. The most famous such lakes are Asia s Caspian Sea, the Dead Sea and Utah s Great Salt Lake. Lake Natron is one of several soda lakes that lie in Africa s Great Rift Valley. Their salinity is caused by volcanic gases or fluids. Lakes are important sources of drinking water, electricity and water for irrigation.