More dams are being removed from rivers as they get older and no longer produce hydropower. Researchers have found after these dams come down, rivers return to their natural state surprisingly fast.
Over the years lots of sediment backs up behind dams. Ecologists have worried the release of that sediment would harm habitat and cause flooding.
But a study from Oregon State University found that didn’t happen. Researchers studied the Rogue and Calapooia rivers in Oregon before and after dams were removed.
Desirée Tullos, the study’s lead author, said insects were flying around the riverbanks one year after the dams were removed.
“Which suggests that, not only did they not have a negative impact from the dam removal, but that the recovery from the actual dam itself –- that long-term disturbance of the dam –- had been removed from the system within a year,” Tullos said.
She said the rivers took a little more time to return to their natural physical state. As periodic storms flushed the system, the sediment that had created some sand and gravel bars quickly washed away, Tullos said.
When the dams were in place, they increased water temperatures, changed the river flows, and prevented fish from swimming upstream.
Tullos said this study could help inform dam managers when they consider tearing down aging dams.
“This infrastructure, whether we’re talking about bridges, roads, dams, levees, it’s aging. We have to do something about it. Dam removal is one option, so is dam replacement. But to see that dam removal is a viable option that doesn’t have long-term impacts on the ecosystem is really exciting,” Tullos said.
Terry Flores is with Northwest RiverPartners, a group that represents farmers, ports, and other businesses and groups that support the Columbia and Snake rivers’ hydroelectric dams.
Flores said it might make sense to take out dams that don’t produce hydropower or provide irrigation or navigation.
“When you’re looking at a project, and you’re looking at whether a project should be removed, what kind of value does it bring, economically or otherwise?” Flores said.
Tullos studied a small and medium river, but she said the effects could be similar on larger rivers where dams have been removed, like the Elwha River on Washington's Olympic Peninsula -- site of the world’s largest dam removal project.
Anne Schaffer, the executive director of the Coastal Watershed Institute, has been studying the recovery at the mouth of the Elwha River. Schaffer said the sediment from the dam’s removal has created good habitat for fish.
“As soon as the habitat is available, fish are using it,” Schaffer said. “We documented surf smelt spawning on brand new beach. It was habitat that literally wasn’t there a year ago.”
Researcher Tullos said sediment impacts increase when larger dams are removed, but in the case of the Elwha Dam, the process has been managed to not harm the environment.
“In terms of thinking about dam removal as one of our options for managing our old infrastructure, sediment is probably not a concern that we need to be too focused on,” Tullos said.