Lakes are formed in depressions in the Earth's surface. Where streams or rivers flow into a lake the water tends to rise in the depression until it overflows into an outlet stream. Evaporation and seepage through the bottom help maintain the level, also. Lakes with no outlet become salty and alkaline such as Soap Lake, Washington and the Great Salt Lake, Utah. Small lakes or ponds may last only in the spring and dry up in summer when the streams that flow into it dry up. Man-made artificial lakes, called reservoirs, contain water impounded (held back) by dams. Examples are Banks Lake and Lake Roosevelt behind Grand Coulee Dam.
Lakes are one of the most temporary geologic features with few that are older than 10,000 years. Streams entering a lake carry silt and sand that is deposited in the lake which tends to fill it. Plants and animal life feed off the nutrients brought in by the streams.
A lakebed is formed as layers of silt, sand and dead organisms settle out of the lake water. As the lake becomes more shallow, plants hold the lakebed in place and soon the lake may become a marsh with a stream flowing through it. The mountain lake in the photo at right has begun this process.
Glacial lakebeds have alternating light and dark layers, varves, formed when lighter silt is deposited in summer and darker organic and clay layers are deposited in winter. This is due to very fine rock dust melting out of the glacier during the warm seasons. Each pair of layers can be counted like tree rings to estimate the age of the glacial lake.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Volcanic rock caused by partial melting of the Earth's crust.
- Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
- A whirlpool or tornado effect that forms in deep, fast moving water. Kolks can pick up solid blocks of basalt in flood areas.
- Pale yellow, glassy material that forms when hot steam and other gases contact water during a lava flow. Sand and clay is usually mixed in as well.
- Pillow Basalt
- Basalt formed underwater or as a basalt flow contacts a river or lake.
- Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
- Layers of light and dark sediment on the floor of a glacial lake. Each light and dark pair indicates a year since light deposits are from rock dust from melting ice in summer and dark layers are from organic matter.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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