Single-Thread and Multi-Thread River System

Why in News?

Recently, the geographers at the University of California Santa Barbara (UCSB) have reported in a paper published in Science that they have solved the mystery of river splits.

What was the research methodology?

• Use of satellite – To understand why some rivers flow in a single channel while others split into many threads, the researchers turned to satellites.
• They studied 36 years of global Landsat images, covering the period from 1985 to 2021.
• Rivers studied – From a worldwide survey of nearly 400 river sections, they chose 84 that were wide enough and moved at a speed suitable for their analysis.
• These included both single-thread and multithread rivers across different climates, slopes, and water flows.
• Particle image velocimetry – It is a computer technique called, which tracked small changes in images from year to year.
• This helped scientists measure how much a riverbank eroded and how much material accreted on the opposite side.
• Pictures to maps – They converted the satellite pictures into maps showing where land was dry and where it was covered by water.
• Comparison – By comparing thousands of cross-sections of the rivers over time, they generated millions of small vectors that recorded the directions and speeds of erosion and accretion.
• Combining the data – They combined all this data more than 4 lakh measurements of erosion versus accretion to test whether the two processes balanced out.
• This allowed them to discover the patterns that caused single or multithread rivers.
• Result of the study – They discovered the physical mechanism that causes there to be two types of rivers.

What are the two types of rivers?

• Two types – The rivers are classified into
  • Single-thread
  • Multi-thread
• Single-thread rivers – They are characterised by equilibrium between bank erosion and bar accretion.
• Material lost from one bank is balanced by the material deposited on the other, maintaining a stable width.

• Multi-threaded rivers – They consistently exhibit higher rates of erosion relative to the deposition on the opposite banks.
• This leads to the channel widening and eventually splitting. This imbalance, per the work, is the driving force behind multithreaded rivers.
• Main factor – It is, erosion is what drives the phenomenon of flow splitting in rivers.

Erosion is the geological process where Earth's surface materials, like soil and rock, are worn away and transported by natural forces such as water, wind, and ice

What are the relevance of the study?

• Features – The two main types of rivers, single-thread and multi-thread, also feature different flood and erosion risks, ecosystem services, and water resources.
• Disaster management – These hazards and features are becoming more relevant as people and governments cope with more frequent and more intense water weather events.
• Research – The physical mechanism that dictates single versus multi-threading has been becoming a more important subject of research.
  • Previous research mostly examined where different types of rivers could be found.
  • Now researchers also focused on how these rivers changed over time.
• There is growing recognition that many rivers have historically transitioned from multi-channel to single-channel after human interference.

Human interference in river formation includes damming, diking, sediment mining, clearing and snagging, and agricultural development.

• Correcting the misinterpretation – For many decades, scientists have believed that single-channeled, meandering rivers needed vegetated banks to form and that plants and meandering rivers coevolved.
• Researchers reported that that idea is based on a misinterpretation of the sedimentary record.
• Discovery of role of vegetation – Vegetated River bends move in a different direction than unvegetated river bends, relative to the down-slope direction that the entire river flows.
• This renders the sedimentary deposits that unvegetated meandering rivers produce fundamentally different from the deposits of vegetated meandering rivers, even though they have the same form.
• Given a straight valley, vegetated river bends will move outwards toward the sides of the valley whereas unvegetated river bends will move down the valley, without moving sideways.
• Vegetation causes this difference in river movement mainly because it causes levees to form, which indirectly limits the sinuosity.
• In turn, sinuosity controls how and where bends of the river migrate.

Sinuosity is the ability to curve or bend easily and flexibly.

What are Insights for India?

• Rivers studied – The researchers considered ganga and Brahmaputra river stretches 
• Ganga – The reaserch considered three stretches of the Ganga, near Patna, Farakka, and Paksey (Bangladesh).
• Brahmaputra – They examined stretches near Bahadurabad (Bangladesh), Pandu (India), Pasighat (India), and one further upstream in the Himalaya.
• Findings on Brahmaputra – The Brahmaputra is a classical braided river.
• The team also found that the Brahmaputra’s threads eroded their banks fast.
• The shape of their channels is fundamentally unstable.
• The subchannels are prone to widen and split over years and decades, because the flow laterally erodes riverbanks faster than it deposits along them.
• The find went against the conventional wisdom that erosion and deposition are in equilibrium.
• It is very surprising and intriguing that multi-thread rivers laterally erode faster than they deposit.
• The study has unravelled a new sort of way that rivers can maintain their form, which is fuelled not by equilibrium but instead cycles of instability as sub-channels repeatedly widen and split over time.
• This fundamental instability is an important consideration for river management.
• Reducing flood risk – Along multi-thread rivers like the Ganga and the Brahmaputra, the rating curves used to measure river flows must be updated more frequently in order as the channels change their shape.
• The problem in India is that in many parts, braided river sections have been artificially confined to single channels using built embankments.
• Multi-channel rivers require significantly less space and time to return to their natural state, leading to lower restoration costs.
• Stressed on Nature-based solutions – This includes measures that can significantly lower the risk of flooding in adjacent areas.
  • Removing artificial embankments
  • Restoring the river’s connection with its natural floodplains
  • Creating vegetated buffer zones along riverbanks
  • Reactivating abandoned channels
  • Building wetlands in braided sections

Reference

The Hindu| Mechanism of River Flow and Split