Dam removal

Negative effects of dams include environmental degradation, such as reduced primary productivity, loss of biodiversity, and declines in native species; some negative effects worsen as dams age, like structural weakness, reduced safety, sediment accumulation, and high maintenance expense.

[5] A number of major dam removal projects have been motivated by environmental goals, particularly restoration of river habitat, native fish, and unique geomorphological features.

[7] Many of the dams in the eastern United States were built for water diversion, agriculture, factory watermills, and other purposes that are no longer seen as useful.

In addition, many of these dams block anadromous fish runs, such as Atlantic salmon and American shad, and prevent important sediments from reaching estuaries.

Many dams in the western United States were built for agricultural water diversion in the arid country, with hydroelectric power generation being a very significant side benefit.

Three native fish species have become extinct in the Grand Canyon and others are endangered since the dam was completed, including humpback chub and razorback sucker.

A common problem for dams is how sediment carried naturally by the river is deposited in the reservoir and eventually fills it up with silt.

Oftentimes the sediment stored in a reservoir is good for the riparian corridor below the dam, can rebuild fish habitat, provide nutrients, and add onto a beach or estuary.

For example, when the Fort Edward Dam on the Hudson River was removed in the 1970s, PCBs in the sediment were released, affecting human and wildlife health downstream.

The sediment trapped behind the dam flows downstream in a fixed rate that allows the ecosystem to adjust to the changes.

This can devastate the riparian ecosystem along the river as well as dangerously scour bridge pilings, buried pipes, levees, and other infrastructure.

However, if the reservoir held back by the dam is relatively small and quickly drains into a larger river or lake, this approach can be carried out with minimal impact on either the ecosystem or human infrastructure.

It entails emptying the entire reservoir, allowing the sediment to dry, and then transporting it to a safe location for disposal.

[10][11][12] To restore free-flow, the EU Biodiversity Strategy seeks to eliminate unnecessary dams and barriers across 25 000 miles of river by 2030.