River flooding is becoming more common globally

Researchers from the University of Cincinnati and the University of Massachusetts Amherst mapped 35 years of daily streamflow in 2.9 million sections of rivers, creeks and streams around the world to determine how river levels are fluctuating from climate change and human impacts. Streamflow is the measure of water flowing past a designated spot during a given time.

They found more flooding and increased water flow in upstream sections of rivers and decreased water flow in downstream sections typically associated with human development. The most pronounced changes occurred in small waterways, which saw more dramatic increases in erosion potential, flood frequency and saturation of nutrients in the water.

Likewise, researchers found that extreme flooding is becoming far more common in upstream sections of rivers. These are called “100-year floods” because traditionally there is a 1-in-100 chance of such a disastrous flood occurring in a given year.

Downstream sections of rivers around the world saw insignificant changes in flood frequency while the likelihood of extreme flooding in upstream sections increased a whopping 42% over the 35 years.

The study was published in the journal Science.

“We found that the rivers around the world are changing,” said Assistant Professor Dongmei Feng, the study’s lead author, who teaches in UC's College of Engineering and Applied Science.

In her Terrestrial Hydrology Lab, Feng and her students use the latest technology to study how climate change is affecting waterways around the world.

Upstream sections of rivers typically have increasing water flow because they’re inundated each year with more precipitation and snowmelt at higher elevations, Feng said. Meanwhile, far more people live in lower-elevation sections of riverway, drawing water for agriculture, industry and other human uses.

Feng and UMass Professor Colin Gleason, a study co-author, found that 17% of rivers studied had significant increases in upstream water flow while 44% saw significant decreases in downstream river flow.

“That’s every river, every day, everywhere over a 35-year period,” Gleason said. “Some of these are changing by 5% or 10% per year. That’s rapid, rapid change.”

These changes can have profound effects on navigability, pollution, portability and even hydroelectric power. More sedimentation can cut off water flow to dams and damage turbines.

Gleason said the study demonstrates how science can help policymakers make more informed land-use decisions.

The greater flow of water upstream also can change the important geochemistry of the waterways upon which aquatic life depends, researchers said.

Researchers measured water flow using satellite data and computer modeling.

“Rivers have different responses to climate change and human impacts depending on their hydrologic location,” Feng said. “The opposite changing patterns between upstream and downstream rivers may impact their functions in both ecosystems and the society.”