Ecostani | Kosi river system’s contribution to Mt Everest’s increasing height
Himalayas have a network of underground water systems that emerge on the surface in form of springs, which provide six times more water than glaciers and snow
A new study published in Nature Geoscience earlier this month said that a river network caused the world’s tallest mountain, Mount Everest, to spring up between 15 and 50 metres over the past 89,000 years. The study provides sharper insights into the evolution of the young mountain range and the impact that the large number of river networks emanating and flowing across the dynamic mountain system have had on it.
At 8,849 metres, Mount Everest, also known as Chomolungma in Tibetan or Sagarmāthā in Nepali, is the tallest mountain on Earth and rises about 250 metres above the next tallest peak in the Himalayas. Everest is considered anomalously high for the mountain range, as the next three tallest peaks — K2, Kangchenjunga, and Lhotse — all only differ by about 120 metres from each other. Ten of the 14 world’s highest peaks are found in the Himalayas and most of them are witnessing an uplift like Everest.
The study by researchers at UCL said that Mount Everest is about 15 to 50 metres (50 to 165 feet) taller than it would otherwise be because of the uplift caused by a nearby eroding river gorge, and continues to grow because of it.
Mount Everest, the study said, was rising by 2mm every year because a river system, some 75 km from the mountain, was eroding and carving away a substantial gorge; the loss of the gorge’s landmass was causing the mounta8in to rise upwards by as much as 2 mm annually.
“A significant portion of this anomaly can be explained by an uplifting force caused by pressure from below Earth’s crust after a nearby river eroded away a sizeable amount of rocks and soils. It is called “isostatic rebound”, where a section of the Earth’s crust that loses mass flexes and “floats” upwards because the intense pressure of the liquid mantle below is greater than the downward force of gravity after the loss of mass,” the study said.
“It’s a gradual process, usually only a few millimetres a year, but over geological timeframes can make a significant difference to the Earth’s surface,” the study said.
The river in question is Arun which starts from Tibet as Phung Chu and is one of the seven rivers that form the Kosi river network in Nepal before flowing into India from Bihar. Today, the Arun River runs to the east of Mount Everest and merges downstream with the larger Kosi river system. Over millennia, the Arun has carved out a substantial gorge along its banks, washing away billions of tonnes of earth and sediment, the study said.
Co-author Dr. Jin-Gen Dai of the China University of Geosciences, said: “An interesting river system exists in the Everest region. The upstream Arun River flows east at a high altitude with a flat valley. It then abruptly turns south as the Kosi River, dropping in elevation and becoming steeper. This unique topography, indicative of an unsteady state, likely relates to Everest’s extreme height.”
PhD student Adam Smith (UCL Earth Sciences) explained that their research showed that as the nearby river system cuts deeper, the loss of material causes the mountain to spring further upwards.
The study also mentioned that the uplift is not limited to Mount Everest, and affects neighbouring peaks including Lhotse and Makalu, the world’s fourth and fifth highest peaks respectively. The isostatic rebound boosts the heights of these peaks by a similar amount as it does Everest, though Makalu, located closest to the Arun River, would experience a slightly higher rate of uplift.
The study said the elevation of Mount Everest and other peaks in the greater Himalayas started about 89,000 years ago when the Arun river joined and merged with the Kosi river network, a process called “drainage piracy”. In doing so, more water was funnelled through the Kosi river, increasing its erosive power and taking more of the landscape’s soils and sediments with it. With more of the land washed away, it triggered an increased rate of uplift, pushing the mountains’ peaks higher and higher.
The Himalayas have an intricate network of underground water systems that emerge on the surface in the form of springs across the region, which provide six times more water than glaciers and snow. The study provides a useful insight into how the Himalayan water system is leading to the evolution of the Himalayan geology, a highly understudied subject.
It is believed that the groundwater variation in the Himalayas is leading to the uplift of mountains and subsidence of the foothills and Indo-Gangetic plains, which is associated with several river systems emanating from the Himalayas.
The study provides useful insight into how Himalayan river erosion (which is very high in several Himalayas rivers such as Ganga and Sutlej) is impacting the mountains and could push policymakers to desist from damming Himalayan rivers as it varies the natural evolution of the mountain system.
Chetan Chauhan, national affairs editor, analyses the most important environment and political story in the country this week
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