A shot in the arm for the plight of the beavers currently living, rather inappropriately, on the River Otter, Devon.
Not only have they been cleared of disease-spreading accusations, but a new report from Oregon, USA, claims that beaver dambuilding has no lasting detriment on the surrounding environment.
And a shot in the arm’s got to beat one to the head.
A study of the removal of two dams in Oregon suggests that rivers can return surprisingly fast to a condition close to their natural state, both physically and biologically, and that the biological recovery might outpace the physical recovery.
The analysis, published by researchers from Oregon State University in the journal PLOS One, examined portions of two rivers – the Calapooia River and Rogue River. It illustrated how rapidly rivers can recover, both from the long-term impact of the dam and from the short-term impact of releasing stored sediment when the dam is removed.
Most dams have decades of accumulated sediment behind them, and a primary concern has been whether the sudden release of all that sediment could cause significant damage to river ecology or infrastructure.
However, this study concluded that the continued presence of a dam on the river constituted more of a sustained and significant alteration of river status than did the sediment pulse caused by dam removal.
“The processes of ecological and physical recovery of river systems following dam removal are important, because thousands of dams are being removed all over the world,” said Desirée Tullos, an associate professor in the OSU Department of Biological and Ecological Engineering.
“Dams are a significant element in our nation’s aging infrastructure,” she said. “In many cases, the dams haven’t been adequately maintained and they are literally falling apart. Depending on the benefits provided by the dam, it’s often cheaper to remove them than to repair them.”
According to the American Society of Civil Engineers, the United States has 84,000 dams with an average age of 52 years. Almost 2,000 are now considered both deficient and “high hazard,” and it would take $21 billion to repair them. Rehabilitating all dams would cost $57 billion. Thus, the removal of older dams that generate only modest benefits is happening at an increasing rate.
In this study, the scientists examined the two rivers both before and after removal of the Brownsville Dam on the Calapooia River and the Savage Rapids Dam on the Rogue River. Within about one year after dam removal, the river ecology at both sites, as assessed by aquatic insect populations, was similar to the conditions upstream where there had been no dam impact.
Recovery of the physical structure of the river took a little longer. Following dam removal, some river pools downstream weren’t as deep as they used to be, some bars became thicker and larger, and the grain size of river beds changed. But those geomorphic changes diminished quickly as periodic floods flushed the river system, scientists said.
Within about two years, surveys indicated that the river was returning to the pre-removal structure, indicating that the impacts of the sediment released with dam removal were temporary and didn’t appear to do any long-term damage.
Source: Oregon State University