The vast lake-dotted and marshy landscapes found in North America were formed by glacier action 10,000 to 20,000 years ago. Glaciers formed lake basins by gouging holes in loose soil or bedrock, by depositing material across streams beds, or by leaving buried chunks of ice whose melting shaped lake basins. More recently, humans and other animals have created lakes and reservoirs by damming rivers and streams.
Lakes constantly undergo evolutionary change, reflecting the changes that occur in their watersheds.
Most are destined to fill in with remains of lake organisms and with silt and soil washed in by floods and streams. These gradual changes in the physical and chemical components of a lake affect the development and succession of plant and animal communities. This natural process takes thousands of years. Human activities, however, can dramatically change lakes, for better or worse, in just a few years. How the water comes and goes In Washington, about three-fourths of the precipitation that falls reenters the atmosphere by transpiration from plants and evaporation from the earth's surface.