While the exact definition of a dam may vary, it is generally accepted that a ...
Dam means any artificial barrier, together with appurtenant works, which does or may impound or divert water, and which either (1) is or will be twenty (20) feet or more in height from the natural bed of the stream or watercourse at the downstream toe of the barrier, as determined by the Commissioner (in Tennessee, this refers to the Department of Environment and Conservation), or (2) has or will have an impounding capacity at maximum water storage elevation of thirty (30) acre-feet or more. Provided, however, that any such barrier which is or will be less than six (6) feet in height, regardless of storage capacity, or which has or will have a maximum storage capacity not in excess of fifteen (15) acre-feet, regardless of height, shall not be considered a dam, nor shall any barrier, regardless of size, be considered a dam, if, in the judgment of the Commissioner, such barrier creates an impoundment used only as a farm pond. Diversion weirs, roadbeds, water tanks, and wastewater impoundment barriers as defined in this section are not dams.
(Source: The Safe Dams Act of 1973)

Generally speaking, in the United States dams have a design life of 50 years. The hydroelectric dams built in the 1960's and before were granted a 50 year license by the Federal Energy Regulatory Commission. Now, with that design life exceeded, the cost of maintenance and compliance (either to meet current environmental regulations or seismic codes) outweigh the revenue they garner from power production. Thus, they are good candidates for removal.

There are three stages in the removal of a dam:

1. Feasibility assessment - Including sediment characterization, impact to aquatic wildlife & plants, water quality, impact to manmade facilities (including downstream properties);
   
2. Design - Addressing removal, site & streambed restoration and recreational potential; and
3. Dam removal & stream restoration - Addressing construction sequencing, site protection, debris cleanup & removal and vegetation.

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CASE STUDIES
Elwha River Dam Removal (Washington State)