Massive earthquake changed the course of the Ganges River

A major earthquake 2,500 years ago caused a dramatic course change in one of the world’s largest rivers, the Ganges, in what is now Bangladesh, a densely populated area still susceptible to significant seismic activity. 

The experts found that this previously undocumented earthquake rerouted the main channel of the Ganges River.

Massive river-course change

While river-course changes, known as avulsions, have been documented before, this one stands out due to its scale. 

“I don’t think we have ever seen such a big one anywhere,” said co-author Michael Steckler, a geophysicist at the Lamont-Doherty Earth Observatory. According to Steckler, such an event could have flooded the surrounding area, posing a significant threat to any inhabitants.

“It was not previously confirmed that earthquakes could drive avulsion in deltas, especially for an immense river like the Ganges,” said lead author Liz Chamberlain, a geophysicist at  Wageningen University.

The Ganges River system

The Ganges River, originating in the Himalayas, flows approximately 1,600 miles and merges with other significant rivers such as the Brahmaputra and the Meghna, forming a complex system of waterways that empty into the Bay of Bengal. This system is the world’s second-largest river system by discharge, after the Amazon.

Rivers in major deltas often change course over time due to sediment buildup, causing the river bed to rise and eventually breach, redirecting the flow. 

Typically, these changes happen gradually through successive floods over years or decades. However, an earthquake-induced avulsion, as Steckler explains, can occur almost instantaneously.

The ancient main channel of the Ganges

The researchers identified what they believe to be the ancient main channel of the river, located about 100 kilometers south of Dhaka, Bangladesh’s capital. This area, now used primarily for rice cultivation, is prone to frequent flooding. 

During fieldwork in 2018, Chamberlain’s team discovered vertical dikes of light-colored sand cutting through horizontal layers of mud in a freshly excavated pond. These features, known as seismites, indicate past earthquake activity. The seismites were 30 to 40 centimeters wide, extending through three to four meters of mud.

Further analysis showed that the seismites formed around 2,500 years ago, coinciding with the abandonment and infilling of the river channel. Similar findings were observed 85 kilometers downstream. 

Avulsion caused by an earthquake

The researchers concluded that this was a significant avulsion caused by an earthquake, likely of magnitude seven or eight.

The potential sources of the quake are twofold: a subduction zone to the south and east, or giant splay faults at the base of the Himalayas. Both zones are accumulating stress and could produce similar earthquakes in the future, as evidenced by a 2016 study led by Steckler. 

The last comparable quake occurred in 1762, generating a deadly tsunami that reached Dhaka, with another possible event around 1140 CE.

Modern risks of river avulsion

The 2016 study estimated that a modern recurrence of such a quake could impact 140 million people. 

“Large earthquakes impact large areas and can have long-lasting economic, social and political effects,” said Syed Humayun Akhter, vice-chancellor of Bangladesh Open University and co-author of both studies.

The Ganges is not alone in facing these risks. Other rivers in tectonically active deltas include China’s Yellow River, Myanmar’s Irrawaddy, several rivers off the U.S. West Coast, and the Jordan River, which spans the borders of Syria, Jordan, the Palestinian West Bank, and Israel.

More about the Ganges River 

The Ganges River is one of the most significant and sacred rivers in the world. It originates from the Gangotri Glacier in the Indian Himalayas. 

The river is revered in Hinduism, with millions of devotees considering its waters to be purifying and holy. 

Cradle of civilization 

Historically, the Ganges has been a cradle of civilization, supporting agriculture and sustaining major cities along its banks, such as Varanasi, Haridwar, and Kolkata. 

The fertile plains of the Ganges basin have been vital for India’s food production, particularly in growing rice and wheat. 

Environmental threats 

However, the river faces significant environmental challenges, including pollution from industrial waste, sewage, and religious offerings, which threaten its ecosystem and the health of those who depend on its waters.

Efforts to clean and conserve the Ganges have been ongoing, with government initiatives like the National Mission for Clean Ganga aiming to reduce pollution and restore its ecological balance. 

Despite these challenges, the Ganges remains a symbol of life, spirituality, and resilience for millions of people.

The study is published in the journal Nature Communications.

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