Ancestral Rivers in AsiabyBill Butler Antecedence and superimposition are geologic processes that explain how and why rivers can cut through mountain systems instead of going around them. The examples here (including pictures) are from Asia, but other examples can be found throughout the world.Featured areas
Arun River, Himalayan Mountains, China (Tibet), Nepal, IndiaEuphrates River, SyriaIndus River, PakistanJhelum River, India controlled KashmirSelenge River, RussiaSutlej River & Bhakra Dam, IndiaTigris River, IraqYangtze River, China (2 pictures)Yenisey River, Russia
Arun River, Himalayan Mountains, China (Tibet), Nepal, India The highest mountain range in the world, 4 out of the 5 highest peaks in the world (all over 27,000 feet), and a river runs through it. (Actually there are several rivers that run through it, but we will only give a picture of one of them.) The picture below is a vertical view of the Himalayan Mountains. The bright yellow line marks the border between China (Tibet) to the north and Nepal to the south. A small piece of India can be seen in the lower right corner. Mt. Everest at 29,035 feet is the highest mountain in the world. The summit is under the second “e” in the “Everest” label. Lhotse (label not shown) tops out at 27,940 feet and is just 2 miles to the southeast. Makalu at 27,766 feet is 12 miles east-southeast of Everest and is the 5th highest mountain in the world. Kanchenjunga at 28,169 is the third highest and can be found on the Nepal/India border in the lower right hand corner.
Water drainage from the slopes of Mount Everest is an interesting study in antecedence. Starting from the top of Mt. Everest, the Rongbuk Glacier flows 11 miles to the north-northwest into China. The Rongbuk Monastery is 16 miles NNW of Everest and can be reached by road from China. Melt water from the glacier becomes part of the headwaters for the Rong River. 22 miles north-northwest of Everest, the Rong River turns eastward. You can locate this point in the above picture by going NNW from Everest until you intersect a faint yellow road. The Rong River flows ENE and then ESE until it joins the Arun River some 33 miles northeast of Mt. Everest. (This is near the intersection of two local “roads” - faint yellow lines in the picture) The Arun River flows nearly due south from this point. If you look closely, you can follow its path from the road intersection (above), across the China/Nepal border (the crest of the Himalayas), and through the “a” in “Europa”. 60 million years ago the Himalayan Mountains did not exist. The rock that would become Mt. Everest was 450 to 500 million year old sedimentary layers that were at or below sea level and somewhere well south of the current location of Mt. Everest. These sedimentary rocks contained fossils of trilobites, crinoids, and other animals that once lived in a much earlier ocean. What was then the southern coast of China was located somewhere near the northern border of present day India. The ancestor of the Arun River flowed southward over these lowlands. Meanwhile, the subcontinent of India had rifted away from the east coast of Africa and was being rafted north-northeastward by convection currents in the upper part of the Earth’s mantle. Movement was a relatively rapid several inches per year. The collision between India and China began about 50 million years ago. By 40 million years ago the intervening ocean was gone. The Indian subcontinent was plowing into China. The leading edge of the subcontinent had to go somewhere, and the most convenient direction was to dive under China. This extra buoyant material began to lift what had been the Chinese lowlands. The uplift of the Tibetan Plateau began. The 450 to 500 million year old sedimentary rock that had been in-between India and China, including the Mt. Everest limestones and sandstones, also had to go somewhere. As these rocks were crumpled together, they broke into slabs, and then the slabs were pushed one on top of the other. In short, the only direction the sedimentary layers could go was up - bigtime up. As the old sedimentary layers rose out of the ocean, the ancestral Arun River initially just extended its course further south out over the new land area. However, it wouldn’t be long before the Arun had to start eroding into the rising land mass if it was going to keep its southward course. The Himalayan Mountain Range has been uplifted rapidly over the last 30 million years. What had been lowlands in southern China have been buoyed upward to over 15,000 feet to form the Tibetan Plateau. The Mt. Everest limestones/sandstones have been both buoyed upward and stacked (faulted slabs), and the old trilobites are now found at 29,000 feet above sea level. The Arun River has cut downward into this rising mass (mess?) but not as fast as the land has been uplifted. It’s been: “uplift 2 feet”, “cut down 1 foot”, “uplift 2 feet”, “cut down 1 foot”, etc. However, the Arun’s erosion has been good enough for the river to maintain its course. The Himalayan Mountains are still rising today. The Arun is still eroding down into this rising mountain mass. Today the Arun is cutting a steep gorge across the Himalayas. In all probability the Arun will win this contest, and millions of years from now it will have dug a canyon 10,000 or more feet deep across the Himalayas. Further out in the future, the Himalayas will stop rising and erosion will begin to level them. Two hundred million years from now the Himalayas will be mere stumps, but the Arun River will be an old drainage system that cuts across these stumps. If you could take a “future” picture, it might look like the current “unknown tributary in the northern Ural Mountains”.
