Dams serve a variety of purposes, from energy generation to flood control.

But new research in the journal Nature Communications concerning China's lower Yellow River suggests dams actually can raise flood risk by altering the makeup and structure of lowland riverbeds.

“The bed is really flat; and then you move upward, closer to the dam site and, immediately adjacent to the dam, the channel bed become really, really rough,” said lead author Hongbo Ma, a postdoctoral researcher at University of California, Irvine.

Conventional wisdom holds that dams release water that cuts deep channels into riverbeds, lowering water level, or "stage," and reducing flood risk.

But models and field data show damming also preferentially removes fine particles. The remaining coarse sediments form underwater dunes that can impede river flow and cause flooding.

The authors call this process "bed coarsening."

"The bed coarsening is not something that's really surprising to us. It's how the bed forms — how the channel bed changes and evolves as a system — that really surprised us," said Ma.

The geology of water systems and landforms abounds with examples of evolutionary forces. They make coastlines migrate, sandbars form and rivers jump their channels. Like the pebble that starts a landslide, tiny particles can exert outsized effects — including causing rivers to back up and burst their levees.

"Dunes themselves are essentially roughness elements; they act like sandpaper, if you will, on the bed of a river. And what that essentially does is provide friction to the flow. And the more dunes you have, the more friction and, the more friction, the slower the flow goes," said co-author Jeff Nittrouer, an associate professor of geology at Texas Tech University's Department of Geosciences.

Nittrouer called the finding "interesting and seemingly counterintuitive."

"You can actually raise river stage despite eroding the channel bed downstream of a dam," Nittrouer said. 

The authors estimate the findings apply to more than 80% percent of silty lowland rivers like the lower Colorado, but knowing for sure requires further individual study.

"We would like to say it's a generic phenomenon, but we also want to remind the community that there's a lot more work that should be done to draw conclusions about a specific river system," said Ma.

But there are hints. Ma cited one U.S. Geological Survey report suggesting the construction of Parker Dam multiplied the size of the river's sediment downstream five to six times, from 150-200 to about 800 microns.

"That's certainly a signal to us that potentially the dune size — the resistance elements of the channel — would increase significantly and cause potential increases in the flood stage and risk to the region," he said.