The remains of the Briggsville Dam following its removal. (Credit: Martha Naley / U.S. Fish and Wildlife Service)
When Egypt’s Aswan Low Dam was constructed across the Nile in 1902, it inspired nations around the world to undertake similar feats of engineering, spurring a movement that would produce many large dams into the latter half of the century.
Yet, over the last 40 years, more than 1,000 dams have been removed in the U.S, several of which have been more than 30 meters high. Dam removals have become even more commonplace in the past two decades. The motivation ranges from concerns over safety to lack of usefulness, and a growing body of research indicates that dams disrupt riverine ecosystems and morphology, and, when possible, it’s best to let a river flow uninterrupted.
A new study by the U.S. Geological Survey assessed more than 100 dam removals to determine how quickly rivers are able to recover from the negative impacts of damming. A paper detailing the study’s findings is published in the journal Science.
“In talking with my colleagues a few years ago, we recognized that the last 10-20 years has seen a big increase in the number of dam removals nationwide,” said Jeff Duda, co-author on the study and principal investigator at the USGS John Wesley Powell Center for Analysis and Synthesis. “We wanted to evaluate the science of dam removal, specifically whether there (have) emerged any major themes in tracking the outcomes of dam removal.”
Compiling a database from their selected body of literature, the researchers found that most river channels stabilized within months or years following a dam removal — not decades, as had been previously suspected. Furthermore, faster dam removals seem to result in hastier recoveries.
However, no two dam and river combinations are exactly alike, and according to Duda, “context matters.”
“The history of the watershed, the size of the dam, the amount of sediment stored in reservoirs and the location of the dam in the river network are all important factors to consider with any dam removal project,” Duda said.
Each of those aspects, Duda said, may inform how the dam is removed, whether over a matter of weeks or through phases that may last months or years.
A river’s physical characteristics are the first to return to normalcy after dam removal, along with certain biological characteristics, such as the return of migratory fish to the upper reaches of the channel. The recolonization of plant communities tends to take much longer, Duda said.
While the paper targeted about 10 percent of dam removals in the U.S., Duda noted that many of the studies used were conducted over short periods and often employed unique metrics. He and the other researchers are working on further studies that could improve understanding about river resiliency and recovery following dam deconstruction.
“If we could have common metrics studied at more dam removal sites over longer durations, future efforts to synthesize the science of dam removal would be more fruitful,” Duda said.
Top image: The remains of the Briggsville Dam following its removal. (Credit: Martha Naley / U.S. Fish and Wildlife Service)