One month after the devastating DANA of October 29 in Valencia, Spain, the Turia River continued to carry twice the usual amount of water. The sudden rise in river levels that caused the tragedy was triggered by torrential rains that had occurred upstream just hours before. Small rivers upstream are experiencing significant increases in flow near their sources, not only in Valencia or as a result of this DANA, but worldwide, over the past 35 years.
A study by researchers from the University of Massachusetts Amherst and the University of Cincinnati, recently published in the journal Science, has mapped more than three decades of changes in rivers on a global scale. The result is that 44% of the largest rivers are experiencing a decrease in the amount of water flowing through them each year at their lower reaches, while 17% of the smaller rivers are experiencing significant increases in their upper reaches.
As we know in Spain, "these changes have implications for floods and ecosystem disruption," warns the study, but they also interfere with the development of hydroelectric power or the supply of freshwater to large cities.
Researchers detected an average increase of 42% in flow of small rivers in the upper reaches during major floods in recent years. Colin Gleason, a professor of civil and environmental engineering at UMass Amherst, cites examples such as those that have occurred in Vermont in recent summers, leaving a landscape reminiscent of the DANA in Valencia.
Although the article cannot quantify the exact cause and effect of these changes, researchers believe that climate change and human activity are largely driving them. The latter involves obtaining water for drinking, agriculture, or wastewater discharge.
"Floods are disastrous for humans, but for species living upstream, they can also be advantageous," explains Dongmei Feng, lead author of the study and assistant professor at the University of Cincinnati, providing nutrients and a means of transport for migratory fish. "Residents of the western Amazon, for example, have reported an increase in fish migration because high-flow floods are more frequent."
This increase in flow upstream can also pose an unexpected obstacle for hydroelectric plans, particularly in the high mountains of Asia, in places like Nepal and Bhutan. "It implies that erosion is much more significant than before, and it transports more sediments downstream," says Feng. This becomes a problem for countries seeking to develop clean energy because these sediments can clog power plants.
River flow, also known as discharge, describes the amount of water flowing through a river, measured in cubic meters per second. Currently, flow 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 the river's depth. Because this approach only measures flow at a specific location and time , the data was extremely limited, note the study's authors. That's why Feng and Gleason developed a method using satellite data and computer models that captured this flow in three million river segments worldwide. "That means every river, every day, everywhere, over a period of 35 years," says Gleason. "Some are changing their flow at a rate of between 5% and 10% annually. It's a very, very rapid change. We had no idea what those flows were or which rivers are no longer as they were before, but now we know."
Regarding the significant decreases observed in large rivers in the lower reaches, the result is that there is less freshwater available in the larger parts of many rivers worldwide, which has significant implications for drinking water and irrigation. "We don't know exactly why it's happening, but we do know what it could mean," says Gleason.
The river has less power to move the earth and small rocks from the riverbed. The movement of these sediments forms deltas and is an important process to counteract sea level rise, according to the study.
"People living along these rivers, of course, know there are problems, but if you're an analyst trying to determine the best location for a new hydroelectric plant among 100 candidates, it's difficult to measure 100 different rivers accurately," says Gleason. "Colleagues working in water systems tell you that you would be surprised how many places, especially those with limited resources, make important decisions for the future of climate, water resources, and infrastructure projects almost without data at hand. My hope is that everyone can use this data, understand it, and maybe make a more informed decision."
"If we are interested in a location, we have to think about how it is affected both upstream and downstream. We think of the river system as an integral, organically connected system. The conclusion of this article is that rivers respond to factors such as climate change or human regulation differently," says Gleason.