Multi-threaded channels of the Brahmaputra River, Bangladesh. Credit: Earth Science and Remote Sensing Unit, NASA Johnson Space Center
Geographers have finally figured out why some rivers form single channels, while others divide into many interwoven threads. Their findings, published in the journal Science, could transform flood planning and river restoration efforts.
“The question of what causes a river to be single-threaded or multi-threaded is pretty much as old as the field of geomorphology,” says Associate Professor Vamsi Ganti, senior author of the study at the University of California Santa Barbara.
“We found that rivers will develop multiple channels if they erode their banks faster than they deposit sediment on their opposing banks,” adds lead author Dr Austin Chadwick. “This causes a channel to widen and divide over time.”
Ganti, Chadwick and co-author Dr Evan Greenberg tracked the erosion and deposition that occurred on the banks of 84 rivers around the world. They analysed 36 years (1985–2021) of global satellite imagery with an image-processing algorithm.
The algorithm, which was originally designed to track particle motion in laboratory photos of fluid, was adapted to track channel position in floodplains.
They found that erosion and deposition are balanced in single-threaded rivers. As a result, the channel’s width remains constant which allows it to form meandering paths with wide bends.
In multi-channel rivers, the sediment eroded from the banks is instead redeposited on the river bottom. This imbalance causes the river to widen, and forms islands and bars that separate out different channels.
These dynamics occur when rivers are in their “steady state” – neither growing nor shrinking.
“It is not like multi-threaded rivers are gaining water on average. They are still conveying the same amount of water through time, but they are doing that by constantly shuffling the size of the individual threads,” says Ganti.
Rivers can follow one of 2 trajectories depending on the balance between bank erosion and deposition. Credit: Chadwick et al. (CC BY-NC-ND)
Various geographic factors affect the balance between riverbank deposition and erosion, which explains why single- and multi-threaded rivers often occur in distinct environments. For example, multi-threaded channels favour higher water discharge, coarser sediment, and steeper slopes.
Channel patterns shape flood risks, erosion hazards, and ecosystem services for the more than 3 billion people who inhabit river corridors worldwide.
“There is growing recognition that many rivers have historically transitioned from multithread to single-thread patterns after human interference (e.g., damming, diking, sediment mining, clearing and snagging, agricultural development),” write the authors.
“Sustainable, nature-based river management aims to restore rivers to their natural channel patterns, but the success of these multibillion-dollar efforts hinges on understanding channel-pattern origins – especially where predisturbance channel patterns are unknown.”
The team devised a formula for figuring out how wide of a corridor a given river requires to return to its natural state, how long it will take to do so, and whether it returns to a single- or multi-channel state.
They found that restoration widths and times vary widely between single and multi-threaded rivers. A single-threaded river requires about 10 times more space and time to reestablish itself compared to a multi-threaded river of the same stream power (the amount of energy the stream has to erode and move sediment).
These insights will help guide infrastructure and revitalisation projects.
“River scientists, engineers, and managers have historically favoured single-thread channels in restoration, hazard mitigation, landscaping, and research,” the authors write.
“This stems from the hazards of widespread lateral erosion along multithread channels, which we identify as a mechanistic cause of their formation. Despite these risks, excess erosion can facilitate the restoration of riverine ecosystems.
“Nature-based restoration solutions, such as embankment removal, are more effective for multithread channels, which require less time and space to reestablish their channel pattern. Our findings suggest a promising future for nature-based restoration in ecologically rich, historically multithreaded river corridors.”