For most of human history dams were seen as good things. They provided power, at first by mill wheel, later by turbine. They have regulated water levels in rivers, allowing for navigation and in many cases mitigating floods. They have enabled irrigation for faming and drinking water supplies for cities and towns. Over the millennia humans have built hundreds of thousands of dams. But in recent decades scientists, civic leaders, and policy makers have been finding some very good reasons for removing dams. Over the past 40 years, more than 1000 dams have been removed throughout the United States. A study recently published in the journal Science reveals that once a dam is removed, rivers are highly resilient and will respond relatively quickly to the removal.
"The apparent success of dam removal as a means of river restoration is reflected in the increasing number of dams coming down," says Jim O'Connor, a geologist with the U.S. Geological Survey and the lead author of the new study, "more than 1,000 in the last 40 years."
"Rivers quickly erode sediment accumulated in former reservoirs and redistribute it downstream," O'Connor explains, "commonly returning the river to conditions similar to those prior to impoundment."
O'Connor's study is part of a national effort to document the effects of removing dams and the resulting river ecosystem restoration, which is being studied by scientists from a number of universities and government agencies, including the USGS and U.S. Forest Service. Data is universally showing that most river channels tend stabilize in months or years, not decades, especially when dams are removed quicly.
Why are dams being removed? Some have been removed due to safety concerns. Dams are man-made structures. Without maintenance, which, in many cases, can be quite costly, they can degrade and become hazards. Some dams have been removed due sediment buildup. Others have been taken down due to inefficiency or having simply having outlived their usefulness. But an increasingly compelling reason for dam removal is our growing understanding of huge environmental costs of dams.
Dams slow down, or in some cases, completely cease the movement of water, which causes cascades of problems. In any area where fertilizer can drain into a stream, which includes most places where there are humans, decreased flow rates and water volume in rivers and streams causes increased stagnation and eutrophication, an overgrowth of aquatic plant life including algae. This causes hypoxia, very low levels of dissolved oxygen making it difficult, or in some cases impossible for aquatic animals like fish and invertebrates to survive. This, in turn, robs birds and other animals that feed on aquatic life of an important food source. Dams also vastly limit, or in some cases completely curtail fish migration, regardless of the use of fish ladders, fish stocking, and other human attempts to mitigate dams' negative impacts.
However, "In many cases," says Jeff Duda, an ecologist with the USGS and co-author of O'Connor's study, "fish and other biological aspects of river ecosystems also respond quickly to dam removal. ... When given the chance, salmon and other migratory fish will move upstream and utilize newly opened habitat."
The more dams have been removed, the more study there has been of the results of their removal, both in the US and around the world. "As existing dams age and outlive usefulness," says Gordon Grant, a US Forest Service hydrologist, " dam removal is becoming more common, particularly where it can benefit riverine ecosystems. ... But," Grant cautions, "it can be a complicated decision with significant economic and ecologic consequences. Better understanding of outcomes," he explains, "enables better decisions about which dams might be good candidates for removal and what the river might look like as a result."
The thousand dams that have been removed is a drop in the bucket. There is an estimated 75,000 dams in America, and an estimated 800,000 world-wide.
(Sources: phys.org, Wikipedia)