AMHERST, Mass. — A new study in Science by researchers from the University of Massachusetts Amherst and University of Cincinnati has mapped 35 years of river changes on a global scale for the first time. The work has revealed that 44% of the largest, downstream rivers saw a decrease in how much water flows through them every year, while 17% of the smallest upstream rivers saw increases. These changes have implications for flooding, ecosystem disruption, hydropower development interference and insufficient freshwater supplies.

Previous attempts to quantify changes in rivers over time have only looked at specific outlet reaches or a rear basin part of a river, explains Dongmei Feng, lead author, assistant professor at the University of Cincinnati and former research assistant professor in the Fluvial@UMass lab run by the paper’s co-author Colin Gleason, Armstrong Professional Development Professor of civil and environmental engineering at UMass Amherst.

“But as we know, rivers are not isolated,” she says. “So even if we are interested in one location, we have to think about how it’s impacted both upstream and downstream. We think about the river system as a whole, organically connected system. The takeaway from this paper is: The rivers respond to factors — climate change or human regulation — differently [and] we provide the finer detail of those responses.”

River flow rate, also known as discharge, describes how much water flows through a river, measured in cubic meters per second or gallons per day. Currently, flow rate is measured by manually dragging a tool (called an acoustic doppler current profiler) across the surface of a river and then combining it with another automatic measurement of river depth to calculate flow rate over time. Because this approach and only measures flow rate at a specific location, at a specific time, data on flow rates are extremely limited.

“There are about 10-15,000 infinitesimally small slices around the world where we know river discharge — that’s it — out of millions and millions of miles of rivers,” says Gleason.

So Feng and Gleason developed a new approach using satellite data and computer modeling to capture this flow rate across 3 million stream reaches worldwide. “That’s every river, every day, everywhere, over a 35-year period,” Gleason says. “Some of these are changing by 5 or 10% per year. That’s rapid, rapid change. We had no idea what those flow rates were or how they were changing — which rivers are not like they used to be — now we know.”

The significant decreases found in downstream rivers mean that less fresh water is available on the largest parts of many rivers worldwide. This has significant impacts on drinking water and irrigation.

“Communities that use river water for irrigation and drinking water, if that’s dropping, then is there a sustainable use?” says Gleason. “Can you grow your town? Can you grow your city? Can you increase your number of [acres] in production? Can the river support it? We don’t know exactly why [this is happening], but we do know that’s what it might mean.”

The decrease in flow rate also means that the river has less power to move dirt and small rocks in the river bed. The movement of this sediment downstream builds deltas and is an important process in countering sea rise, so this loss of power is detrimental to deltas, especially in light of modern dam building limiting how much sediment is available to move.

Smaller, upstream rivers (typically closer to mountains) are showing an inverse pattern: 17% are seeing an increase in flow. (Though, Gleason points out, this is not uniform, as 10% are decreasing.) This increase in volume in these small rivers can have big impacts on their surrounding communities. The researchers found a 42% increase in large floods of these small streams. Gleason cites those that have occurred in Vermont in recent summers as an example.

“Floods are disastrous for humans, but for upstream species, they may be advantageous,” adds Feng. Flooding provides important nutrients and a means of travel for migrating fish. “The local people [near the western Amazon River], for example, have reported that the fish migration has increased in that region because the flooding is more frequent, which means the high flow required for fish migration is more frequent.”

This increase in upstream flow rate may also throw an unexpected wrench in hydropower plans, particularly in High Mountain Asia for places like Nepal and Bhutan. “The increased flow of the river channel means erosion power is much more significant than before and it’s transporting more sediment downstream,” says Feng. This becomes an issue for countries looking to develop more clean energy because this sediment can clog up hydropower plants.

While the paper cannot quantify the exact cause and effect, the researchers know that the general drivers of these changes are largely climate change and human activity. “Upstream river regions have increasing precipitation in general,” says Feng. “And the snow melt in the high elevation, which is typically cold, is probably more sensitive to climate change, so the  snow melt has been increasing in these  regions.” Human activity includes sourcing water from rivers for drinking or agriculture or wastewater dumping.

And Gleason adds that this paper is an important step: “If you don’t know what it is, you can’t figure out why it is. People who live along these rivers, of course, know there are problems, but if you’re a policy analyst and you’re trying to determine the best location for a new hydropower plant out of 100 candidates, it’s hard to measure 100 different rivers accurately. [Colleagues in water systems say] you would be shocked at how many places, particularly those that are resource-limited, make major decisions about climate futures, water resources, and infrastructure projects with almost no data on hand. My hope is that everyone can use these data, understand them, and maybe make a more informed decision.”