Water erosion shapes the Himalayas

The highest place on Earth is the High Himalayan range. It contains 10 of the 14 eight-thousand-meter peaks, that is, peaks exceeding 8,000 meters above sea level. The exact values depend on the method of measurement adopted, so different sources give different numbers. Without going into details, it is known that the highest of them is Chomolungma, also known as Mount Everest, elevated to about 8850 meters. To the west of it lies Cho Oju, to the east Lhotse and Makalu. The others are located somewhat farther away. The easternmost is Kanchendzonga, and between it and Makalu runs one of the world’s deepest valleys, gouged by the Arun River. And it is this small river that exemplifies the Himalayan altitude record, as reported by a Chinese-British team of scientists on Nature Geoscience [1].

Alpine Orogeny, or the rise of the Himalayas

The genesis of the Himalayas has been known since geologists accepted Wegener’s continental migration hypothesis and gave it the form of the scientific theory of plate tectonics. The continental Indian plate about 100 million years ago broke away from the supercontinent of Gondwana and moved northward and some 50 million years later collided with the Eurasian plate. The crush zone (in the language of geology – folding) is precisely the Himalayan range. At a similar time, analogous processes occurred elsewhere, which is referred to as the Alpine orogeny, and the Himalayas became part of a great chain of mountains stretching from Morocco to Indonesia.

The Alpine Orogeny continues, and the mountains formed during it continue to grow. In most places, uplift is faster than erosion. According to models, the High Himalayas, including the Chomolungma, should currently be growing at about 1 mm per year. GPS observations, however, indicate that this rate is twice as high. Considering the entire history of the range and the rate of uplift due to tectonic forces, Mount Everest should be dozens (15-50) meters lower than it is today.

Isostasy, or how the Arun River affects the height of mountains

What is responsible for this unexpected surplus? Most likely – paradoxically – water erosion, and in several ways. In the highest parts of the mountains themselves, the rivers are relatively small and their erosive power is limited. The exception is the Arun, whose valley, running between Makalu and Kanchendzonga, is a 5 km deep canyon. It is an area of gigantic erosion, the effect of which adds to the formation of the Ganges and Brahmaputra delta. Rather, erosion itself affects denudation, or leveling of the land. In the delta the terrain rises, but in the upper sections it lowers.

The Arun flows far enough away from the highest peaks that its erosion does not directly affect them. On the other hand, the valley creates such a breach in the mass of the rock mass that this region has less weight than the farther surrounding areas, so that the buoyancy forces of the Earth’s mantle encounter less gravitational pressure here and push this part of the Earth’s crust more easily than the neighboring ones. This phenomenon is known as isostasy and has been observed on a large scale in places after the retreated ice sheet, such as the Baltic coast for several thousand years.

Erosion is not only responsible for initiating isostasy. It is also responsible for the very location of the valley. To the north and south of the Himalayas flow two of the Earth’s most powerful rivers – the Ganges and Brahmaputra. They themselves lie quite far from the eight-thousanders, but their tributaries drain the Himalayas, taking huge amounts of eroded rock downstream. Most of the rivers key between the ridges, but eventually flow north on the north side of the mountain range and south on the south.

The Arun initially runs in typical fashion from west to east, parallel to the Brahmaputra. Chomolungma, Lhotse and Makalu lie to the south of this section. However, unlike most Tibetan rivers, instead of eventually heading to the Brahmaputra (known in Tibet as the Yarlung Zangpo), it turns sharply south and breaks through the highest parts of the Himalayas. Its valley forms a mountain gorge.

If not for Arun, the Himalayas would be lower

Further east, a large gorge is formed by the Brahmaputra. Compared to it, the Arun is a small river, and on the Tibetan side, which is climatically a cold desert, it carries little water. Probably its original course was more standard and belonged to the Brahmaputra basin. However, erosion processes (perhaps related to some unusually large flood) caused its channel to break through the mountain ridge east of Makalu and the river flowed south to the Kosi River, a tributary of the Ganges. Such a phenomenon is a river capstan.

It occurs mainly in karst areas, but in other types of mountains it also occurs. In Poland, it is known from old ranges such as the Sudetes and the Swietokrzyskie Mountains. Captage of the Arun River from the Brahmaputra basin through the Ganges system probably occurred 89,000 years ago. years ago. Throughout this period, erosion to the north and east of Chomolungma and its lower companions gouged the canyon, triggering isostasy. The Kosi River system is extensive because, lying on the southern side of the Himalayas, it has a climate extremely different from Tibet. The area records rainfall, which increases erosion and lowers its base.

This, in turn, increases the destructive power of water in the drier, upper reaches of the Arun. It’s likely that the Tibetan section’s captage occurred by way of backward erosion, which retreated the springs of the Nepalese section, which is much more abundant in water. The ravine is a gap in the barrier that the Himalayas put up to the monsoon, so the moist air from the Indian Ocean penetrates deeper here and collects like a trap. This place is known for its long-lasting haze.

So the fact that the Himalayas are the highest mountains in the world (and the neighboring Karakorum is not much behind them) is the result of tectonic movements, but if it were not for the erosion of a relatively small river, the height of their peaks would be tens of meters lower.

[1] Han X., Dai JG., Smith A.G.G. et al. (2024). Recent uplift of Chomolungma enhanced by river drainage piracy. Nat. Geosci. 17, 1031-1037. https://www.nature.com/articles/s41561-024-01535-w