Adapted from a press release written by Harrison Tasoff for the University of California, Santa Barbara.
Rivers are the arteries of the earth. Water, sediments and nutrients are self-organized in a variety of dynamic channels from the mountains to the sea. Some rivers dig a single pathway, while others are divided into multiple interwoven lines. These channel models affect flood risk, erosion hazards and ecosystem services for more than 3 billion people living in river corridors around the world.
Understanding why some waterways form a single channel, while others are divided into many threads has puzzled the researchers for more than a century. A new study published in the journal Science uses 84 rivers to map the thread dynamics along 36 years of global satellite imagery to determine what determines the behavior of rivers. The results solved the long-term dilemma of river science. They also provide insights into natural hazards and river recovery efforts and can guide infrastructure and revitalization projects before disasters occur like the recent floods in Texas.
“We found that if rivers erode banks faster than sediments against banks, this would cause channels to expand and split over time,” lead author Austin Chadwick said. Chadwick is currently a postdoctoral research scientist at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School. He plans to further investigate the diversity of multiline rivers, with particular interest in how they are formed.
Two rivers
Earth scientists have long divided rivers into single and multichannel categories, often studying both separately. Although both types significantly outweigh the others, most of the largest rivers in the world are multichannel. Notable exception is the single channel in Mississippi, where many river studies have occurred.
Most field-based research focuses on single-line rivers, partly because they are simpler. Due to the challenge of recreating single-threaded channels in laboratory tank experiments, experimental work focused on multi-line rivers.
Chadwick gained inspiration for the new study when he performed one of these tank experiments at the University of Minnesota St. Anthony Falls Laboratory. While examining the multi-channel rivers in the lab, he noticed their continuous expansion and division. “I banged my head against the wall because I kept measuring erosion rather than sediment. That wasn’t what we taught in school,” he said. “It led me to read some old books from the Army and other sources that had more erosion than sediment.” Ultimately, he was curious whether this happened in nature.
This is a classic example of a scientific method: “You generate a hypothesis in a lab environment and then you can test it in essence,” said Evan Greenberg, a former UCSB PhD student.
Long-term data is 2300,000 feet
The team used Landsat data in the Google Earth Engine repository, focusing on 84 rivers in different regions of the world. They eroded and deposited the banks of each river using an image processing algorithm called “particle images.” The authors adapted this algorithm to track the motion of particles in fluid photos to track channel locations in satellite images in their floodplain.
In a single-line read river, erosion and deposition balance. As a result, the width of the passage remains constant, allowing these rivers to tilt to the bends and form winding paths throughout the landscape. By contrast, bank erosion exceeded the sedimentation in the multi-channel river, causing a given channel to expand over time until it was divided into two parts. As a result, multi-channel rivers reorganized their channels before they meandered too far in flooding places.
Each of these dynamics occurs when the river is in a steady state, neither growing nor shrinking. “It’s not like multi-threaded rivers get water on average in water. They are still conveying the same amount of water over time, but do it by constantly bulking the size of a single thread,” explains Vamsi Ganti, associate professor of geography at UCSB.
When the authors say erosion exceeds sedimentation, they refer to the river bank. For multi-channel rivers, additional sediment eroded from the bank is re-deposited on the bottom of the river, eventually forming islands and bars that separate the different passages.
The researchers made some exceptions to the erosion sediment trend, but they found that each of them had a significant change in the basin that forced the river to escape from its natural homeostasis. For example, the San Francisco River in Brazil did not show excessive erosion as the river shrank as lightning strikes its source and boiling water for irrigation.
“What causes the problem of rivers being read single-line or multi-threaded is as old as the field of geomorphology,” Ganti said.
Typically, geographers have learned about river dynamics, including a variety of variables, including downstream slope, water flow, sediment type, and riparian stability. The new model explains river types only based on the balance between sedimentation and erosion. Various geographical factors can affect this balance, explaining why specific environments tend to favor certain types of rivers.
Provides flow space for rivers
In the 20th century, many rivers were trapped in narrow channels, which were disconnected from the historic floodplain. This reclaims more land for the settlement and alleviates some of the inherent harms of life near the river. However, this is catastrophic to riparian ecosystems and may even exacerbate long-term harm. Cutting the river from the floodplain means that the sediment settles on the riverbed, lifting the river relative to the adjacent sediment floodplain. This makes it more likely to jump off the bank in case of flooding or dike failures, a phenomenon the team has investigated in depth.
“Think about Hurricane Katrina,” Chadwick said. “When the embankment broke, it was partly because the floodplain had been cut from Mississippi for so long that it sank relative to the river, flooding the flooding, flooding the flooding.”
Increasing efforts are reconnecting the rivers of the access to the floodplain and providing them with more space to move. Nature-based restoration efforts require figuring out the width of the corridors a given river needs to restore the natural state and how long it takes. With new discoveries on river dynamics, the team devised a formula for this, which included variables, such as how long a river took to abandon the passage. The formula also describes whether the river returns to a single-channel state. They calculate the recovery width and time of various rivers based on their satellite observations.
Chadwick, Ganti and Greenberg found that the time and space that a river needs to re-establish natural behavior varies greatly between single read and multiline rivers. A single-line read river requires ten times more space and time to rebuild its own multi-line river power, which is the energy that streams must erode and move sediments.
The formula developed by the authors enables engineers and scientists to estimate the width required to repair a project, which is a determinant of project feasibility and cost. The analysis can also help decision makers prioritize candidate recovery. Research, recovery and hazard mitigation have historically focused on single-threaded channels, but the shift to multi-line waterway projects may lead to higher returns to reduce costs.
In fact, the team’s findings suggest that river restoration may be less expensive than expected. It is increasingly recognized that many single-line rivers were multi-threaded before human intervention, especially in the western United States, for example, the 1930s Los Angeles River showed it with multiple threads before being guided. Chadwick explained that projects currently considered too large or expensive could actually be affordable if the river is misclassified.
Rivers play an important role in human history. They irrigate the crops we grow on fertile plains and convey our goods to and back and forth. But they also flooded our city and suddenly gave up the dilapidated channels. Learning more about rivers will allow us to coexist with these natural mercury features during unprecedented times of change.
News interviews with Austin Chadwick can be arranged by emailing [email protected]
