At that point, decommissioning (uninstalling
the power-generating capacity) and removing the dam become a
reasonable option.
Multiple reasons exist to justify dam removal as the best alternative in some situations:
- Maintenance costs. The hydroelectric equipment installed when the dam was built may now be in need of expensive upgrades, or the dam itself may be in need of repairs. These types of intervention are usually very expensive, and removal may be the most economical solution.
- Safety. When damage from age and wear affect the dam’s structural integrity, removing it may be the best option for the safety of downstream residents.
- Siltation. A dam, by definition, slows down river water upstream where a reservoir forms. Much of the silt brought down by the river drops to the bottom once it enters the reservoir, building up ever thickersediment deposits. Eventually, the entire reservoir basin fills with sediment, leaving little water holding capacity. At that point the dam’s flood control and hydropower benefits have declined significantly. In addition, river ecosystems have evolved with regular influxes of sediment, and restoring this process usually is ecologically beneficial.
- Fish passage. Many rivers serve as corridors for migratory fish, notably salmon, to travel from the ocean to reach spawning grounds in the upper parts of the watershed. Several of the fish species are endangered and protected by law, and when dams cannot accommodate fish ladders or other dam-crossing structures, removal may be warranted. There is also concern for more than just fish; the entire aquatic ecosystem can benefit from a restored river’s flow, along with natural temperature, nutrients, and water level variations.
Environmental Risks of Dam Removal
It can be difficult to manage the tons of sediment that have accumulated over decades. Under certain conditions, large amounts of sediments could be released quickly, affecting ecosystems downstream, choking river channels and causing flooding. Ecological benefits can then only be expected after a long term period. Breaching a dam can also recirculate pollutants that had been locked away deep in the reservoir. Dam removal is a complex, expensive engineering process that has to be done carefully. Agencies and engineering firms have been developing valuable expertise in dam removal, while scientists are collecting data that should help guide future decommissioning and removal projects.
The Elwha River, Washington, at the site where the dam was removed. Richard Goerg/Photographer’s Choice RF/Getty
Examples of Dam Removal Projects
- The 123 ft. Condit Dam, on Washington State’s White Salmon River, was breached in 2011, emptying its reservoir in less than an hour. Impressive footage of the event can be found here.
- Two dams on the Elwha River, also in Washington State, were removed in 2011 and 2012, giving back to several salmon species access to nearly 70 miles of river and streams. Heavy sediment loads in the river are slowing ecological recovery, but it is estimated that once the new riverbanks are stabilized by vegetation regrowth, the problem will diminish significantly.
- As part of a large cooperative effort by multiple agencies and other stakeholders, the Penobscot River Restoration Project in Maine opened hundreds of river miles to migratory fish, including Atlantic salmon, striped bass, Atlantic sturgeon, alewife, and rainbow smelt. In order to do this, two dams were removed, and fish passage was improved on the remaining structures.
- The Newport No. 11 Dam, on the Clyde River in Vermont, was removed in 1996. Since then, landlocked Atlantic salmon migration was restored, and recreational fishing and whitewater boating on the river have improved.
Sources
American Rivers. Dam Removal Success Stories.
Heinz Center for Science, Economics and the Environment. Dam Removal: Science and Decision Making.
Penobscot River Restoration Trust. The Penobscot River Restoration Project
Link: http://environment.about.com/od/biodiversityconservation/fl/Why-Are-We-Removing-Dams.htm?utm_campaign=list_environment&utm_content=20150316&utm_medium=email&utm_source=exp_nl
