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Lakes and Reservoirs
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Dams are constructed to create reservoirs, and are operated for a variety of reasons, such as electricity production, river management, flood mitigation, fisheries management, water supply, and so on. Projects, the term often used for dams and reservoirs together, are operated under a license, such as those overseen by the Federal Energy Regulatory Commission for projects used for electricity production, those operated by federal agencies, such as the Army Corps of Engineers and Bureau of Reclamation, or those overseen by state agencies, such as the Washington State Dam Safety Office. Also, some lakes are partially or completed regulated for similar needs and uses.
To help understand lakes and reservoirs the U.S. Geological Survey operates gages on the project structures, such as the dam itself or on structures near or attached to dams, to collect data on the water surface elevation. Elevation is the water surface to standard datum, usually NGVD of 1929 or NAVD of 1988, but occasionally to previous national datums or datums used by other agencies. This information can be used to relate a variety of information, such as total lake volume, and can be related to facilities on or around the lake or reservoir, such as boat launches which are often listed on the project Web site.
Reservoirs are operated under criteria defined in the project license, where the reservoir elevation is managed under a generalized "rule curve" which describes the the normal daily elevation, and subsequent storage, over the course of a year, to insure water uses, inflow and outflow are accommodated under the license, such as Howard A Hanson Reservoir. Exceptions from the rule curve are described in the license to cover unusual conditions such as droughts or floods. Operators of reservoirs use models to manage the data and information about the reservoir, the inflow, elevation, outflow, weather, etc., to manage the reservoir within the criteria of the license and follow the rule curve.
Reservoirs are operating for multiple purposes, but some reservoirs have a predominant purpose due to the nature of their size, usually smaller reservoirs. One common type of reservoir is a reregulating reservoir. This a reservoir which is used to moderate the fluctuations in outflow from a larger upstream reservoir used for power generation. The downstream, or reregulating reservoir, is operated to absorb the fluctuations and release the flow in a more controlled manner. Examples of these reservoirs are found in the upper Skagit River, the Baker River, the Nisuqaully River, and the Cowlitz River.
The most recent data for lakes and reservoirs in Washington State are available from many sources, such as the: