How an earthquake changed the course of the Ganges River

The Ganges River, one of the world’s most significant rivers, has maintained the steady course of its central delta bed for centuries. It is a river of great importance both geographically, culturally and economically. However, recent research by scientists in Bangladesh indicates that the river’s current shape is the result of an avulsion caused by an earthquake that occurred 2,500 years ago. years ago.

What is the process of river avulsion?

Avulsion is a natural geomorphological process that involves the formation of a new river channel. It is a fundamental phenomenon for delta formation and stabilization of the coastal landscape through sediment redistribution. This phenomenon makes it possible to balance the subsidence of the river basin and the local accumulation of materials in the channels.

Avulsion contributes to stabilizing the coastal landscape, creating new areas of sediment that can support diverse ecosystems and provide flood protection. This process can also respond to changes in sea level, as rising sea levels increase hydrostatic pressure downstream, which can lead to transformations of the river channel, especially in estuarine areas.

The mechanisms responsible for river avulsion are complex and are still the subject of research. This phenomenon most often occurs when the channels of a river become higher in elevation than the flood plain, resulting in a shift in their course. This gradual abandonment of the old channel and capture of the new stream is mainly driven by natural river processes. It can also result from external forces such as tectonic deformation or earthquakes. Although until recently there have been no reported cases of earthquake-induced avulsion in river deltas, which are usually seismically inactive, recent findings indicate that it may have occurred in the past.

Ganges – the first such discovery

The first avulsion of a river channel in the delta was discovered by accident by Vanderbilt University geologist Steven Goodbred and a team of colleagues. While surveying one of the old channels of the Ganges, about 100 kilometers south of Dhaka, the capital of Bangladesh, they came across an unusual pit dug for a pond. Inside it, they discovered a three-meter-high structure called a seismite, which was formed by the agitation of layers of river sediments by an earthquake. The structure was in the form of sand volcanoes, the presence of which scientists had long suspected.

A team of scientists took samples of seismites and dated them using the optically stimulated luminescence(OSL) method. This method makes it possible to determine when the material was last on the earth’s surface using light. The analysis showed that the vertical columns of sand protruding from the silt were buried about 2,500. years ago as a result of an earthquake. Similar results were also obtained after analyzing the last sediment in the old Ganges channel, located 85 kilometers further downstream.

Published in the journal Nature Communications the study found that a magnitude 7 or 8 paleoearthquake that occurred 2,500 years ago, reorganized the waterways of the Ganges delta by suddenly and completely severing the main current. Thus, it confirmed theoretical considerations about the possibility of earthquakes influencing the avulsions of delta rivers.

What consequences would an earthquake of the magnitude of 2,500 years ago have today? years ago?

The Ganges, a 2,700-km-long river. km, takes its origin in the Himalayas, from the Gangotri Glacier in the state of Uttarakhand, northern India. It flows through the states of Uttar Pradesh, Bihar, Jharkhand and West Bengal before reaching Bangladesh, where it joins the Brahmaputra River. By the time their shared waters flow into the Bay of Bengal, they form the largest delta in the world, covering an area of 80,000. km².

The Ganges plains of India and Bangladesh are now densely populated, home to some 156 million people. In addition to being an important transportation route, the river is also a key source of drinking water and is used to irrigate farmland. In addition, the Ganges is of great religious significance to Hinduism. Considered a sacred river, it is believed to have cleansing powers. Numerous religious ceremonies and festivals are regularly held along its banks. The Ganges is also a destination for pilgrimages, attracting hundreds of thousands of devotees.

Given the above information, the discovery by scientists in Bangladesh, is particularly disturbing. The region around the world’s largest delta is still seismically active and prone to earthquakes. Thus, there is a risk of a repeat of the 2,500-year-old event. years. Changing the course of the Ganges under the influence of an earthquake would lead to flooding, which could be catastrophic with the current development of the delta.

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