Rivers are resilient and quickly respond to removal of large dams with rapid dispersal of sediment and increased abundance of fish and other aquatic life, according to a study by the US Geological Survey.
In the US, more than 1,000 dams have been removed since 1915, with 548 removals occurring between 2006 and 2014. A centenary of dam removal – and there’s a paper in Science, 1000 dams down and counting , that kind of celebrates it and explains why dam removal has “a long future ahead.”
Hear that Barnaby Joyce? Dam removal – the US is taking them out while the Abbott government is having Tonka Truck fantasies about building at least 27 new dams . Big dams. Nothing says environmental overlord quite like a big hulking dam with a massive spillway and more concrete than the Hubert H Humphrey building in Washington DC.
“We know water is wealth and stored water is a bank,” Joyce said in 2014, after convening a water infrastructure summit Canberra.
But some US government agencies, including the US Forest Service, seem to think river restoration is a better investment. The three authors of the Science paper – James O’Connor , Jeff Duda and Gordon Grant – are government scientists, so Joyce and his pro-dam infrastructure ilk can’t dismiss the trio’s findings as the ramblings of feral treehuggers.
“Forty years ago, the demolition of large dams was mostly fiction, notably plotted in Edward Abbey’s novel The Monkey Wrench Gang,” says the Science paper.
“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.”
The study says these two dams were among the largest removal and river restoration projects, releasing over 10 million cubic metres of stored sediment.
“Most river channels stabilize within months or years, not decades, particularly when dams are removed rapidly; phased or incremental removals typically have longer response times,” the study says.
Up to 50 per cent or more of sediment previously stored behind the dam wall is eroded “within weeks or months” of the wall being breached and rapidly moves downstream.\
“Migratory fish have also responded quickly to restored river connectivity,” the study says.
“Within days of the blast removing the last of Glines Canyon Dam, Elwha River Chinook salmon swam upstream past its rocky abutments.”
But there can also be unforeseen consequences, and “watershed contaminants, organic accumulations, nutrients, once-inundated structures, and landforms from past land uses may be uncovered and sometimes mobilized by dam removal.”
The study suggests the brisk pace of dam removal will continue in the US, but decisions to remove large dams will need to be assessed in terms of risks, costs, potential for river restoration and the impacts of climate change on water demand.
It’s a short paper, and costs $20 to download, but it’s worth it just to read a reassuring argument for ecological sanity amid all the Barnaby bluster about dams as symbols of nation-building.