1000 dams down and counting

Forty years ago, the demolition of large dams was mostly fiction, notably plotted in Edward Abbey’s novel The Monkey Wrench Gang. Its 1975 publication roughly coincided with the end of large-dam construction in the United States. Since then, dams have been taken down in increasing numbers as they have filled with sediment, become unsafe or inefficient, or otherwise outlived their usefulness. Last year’s removals of the 64-m-high Glines Canyon Dam and the 32-m-high Elwha Dam in northwestern Washington State were among the largest yet, releasing over 10 million cubic meters of stored sediment. Published studies conducted in conjunction with about 100 U.S. dam removals and at least 26 removals outside the United States are now providing detailed insights into how rivers respond). A major finding is that rivers are resilient, with many responding quickly to dam removal. Most river channels stabilize within months or years, not decades (4), particularly when dams are removed rapidly; phased or incremental removals typically have longer response times. The rapid physical response is driven by the strong upstream/downstream coupling intrinsic to river systems. Reservoir erosion commonly begins at knickpoints, or short steep reaches of channel, that migrate upstream while cutting through reservoir sediment. Substantial fractions of stored reservoir sediment—50% or more—can be eroded within weeks or months of breaching. Sediment eroded from reservoirs rapidly moves downstream. Some sediment is deposited downstream, but is often redistributed within months. Many rivers soon trend toward their pre-dam states.