Article reprinted from "Gezhi Discourse Forum"
Changing trends of river sediment transport on the Qinghai-Tibet Plateau,
It is completely different from other major rivers in the world.
Well, we are faced with a new scientific problem:
Why has the Tibetan Plateau undergone such unique changes?
Zhang Fan· Researcher, Institute of Tibetan Plateau Research, Chinese Academy of Sciences
Gezhi Dao No. 77 | February 26, 2022 Beijing
Hello everyone, I am Zhang Fan from the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences. We are pleased to share with you the story of our tracking and study of the sediment of the rivers on the Tibetan Plateau.
When we think of river sediment, we all think of the Yellow River. Since ancient times, the Yellow River has been a river with a lot of sediment on the mainland. The Tang Dynasty poet Liu Yuxi's "Nine-Curved Yellow River Wanlisha" in "Waves and Sands" very vividly describes the curved Yellow River, wrapped in mud and sand from afar.
▲ Jiuqu Yellow River (picture from the Network)
The large amount of sediment carried by the Yellow River will continue to be deposited downstream, forming a so-called above-ground river. When this kind of above-ground river encounters heavy rains and floods, it is very easy to break the embankment and flood. Monitoring data from the 1960s showed that the average amount of sediment entering the sea in the Yellow River at that time was more than 2 billion tons per year, which was far higher than other famous rivers in the world. To control the Yellow River, our country must first control the sediment of the Yellow River.
▲ Changes in the amount of sediment transported by the Yellow River
In the past 70 years, the water and soil conservation of the Yellow River Basin has achieved remarkable results. Since the 1980s, the amount of sediment transport in the Yellow River has begun to decrease continuously, and recent monitoring results show that the amount of sand transport in the Yellow River is only about 1/10 of the peak period, which is very close to the level of the Yangtze River. It can be said that this is a very great achievement.
▲ Changes in global river sediment transport
Not only the Yellow River. British scholars' study of 145 major rivers around the world found that since the 1960s, due to the impact of measures such as soil and water conservation and reservoir storage, nearly half of the river sediment transport has also decreased, and nearly half of the river sediment transport has not changed significantly; only a very small number of rivers have increased due to surface disturbances such as logging and mining.
How have the rivers of the Tibetan Plateau changed?
Completed: 10%///////
When I read this document from a British scholar, I noticed that the study did not cover the rivers of the Tibetan Plateau. At that time, I thought that we should study how the amount of sediment transported by rivers on the Tibetan Plateau changes.
▲ Location of the Tibetan Plateau (Image from the Internet)
Why are you so concerned about the rivers of the Tibetan Plateau? Because the Qinghai-Tibet Plateau is the birthplace of many large rivers. It originates in the southern part of the Qinghai-Tibet Plateau, such as the Indus, Ganges, Brahmaputra and Nu rivers, and eventually flows into the Indian Ocean. The Amu Darya, Tarim, Shule and Hei rivers, which originate in the northern part of the plateau, eventually flow into inland lakes or disappear into the desert. These dozen or so major rivers in Asia originate from the Qinghai-Tibet Plateau, so the Qinghai-Tibet Plateau is also known as the "Asian Water Tower".
▲ The main river originating from the Qinghai-Tibet Plateau
The plateau rivers we are talking about here actually refer to the parts of these great rivers on the plateau. Whether this water is clear or turbid is related to the use of water resources. So we started collecting relevant data. The most critical of these is the long-term monitoring of sediment transport data by hydrological stations. In order to obtain these data, we can say that it took a lot of effort.
▲ Left: Sichuan and Yunnan cover an area of 880,000 square kilometers, with a total of 320 hydrological stations
Right: Qinghai and Tibet cover an area of 1.95 million square kilometers, with a total of 84 hydrological stations
The first is because the hydrological sites on the Tibetan Plateau are very rare. How little? We can compare this with Sichuan and Yunnan, which are close to Qinghai and Tibet: Qinghai and Tibet are more than twice the size of Sichuan and Yunnan, but the number of hydrological sites is only 1/4 that of Sichuan and Yunnan. And most of these few hydrological sites have only runoff data and no sediment data. Why?
▲ Flow measurement and sediment sampling at the Yellow River Head Road Hydrological Station
Let's take a look at how this data is measured to see why. The picture above is a scene of runoff and sand content measurement at the Yellow River Toudao Hydrological Station. The group of staff on the left operates the flow meter in the water to measure the flow, and after the flow measurement work is completed on the spot, the data can be directly exported. The crew on the right is collecting river water samples from the boat, which, when transported back to the lab, require filtering, drying, weighing and other operations to calculate the sand content. Therefore, the measurement of sand content is very time-consuming and laborious, and only about 1/3 of the hydrological stations on the Tibetan Plateau have measured sand content.
▲ Changes in the amount of sediment transported by rivers on the Qinghai-Tibet Plateau
We collected data on the runoff and sand content of eight major rivers, more than a dozen stations, and nearly 60 years of the Tibetan Plateau. Through the analysis of these data, we found that the vast majority of rivers on the Qinghai-Tibet Plateau showed a significant increase in sediment transport, and a few rivers did not have a statistically significant change in sediment transport, but most of these rivers also had a weak increase trend. Therefore, the trend of sediment transport in the qinghai-Tibet Plateau is completely different from that of other major rivers in the world. So, we are faced with a new scientific question: Why has the amount of sediment transported by rivers on the Tibetan Plateau undergone such a unique change?
Is rain erosion the only answer to the question?
Completed: 30%/////////
First, we began our research on soil erosion on the Tibetan Plateau.
▲ Soil erosion
Soil erosion refers to the separation, transport and deposition of soil and soil matrix under the action of external forces such as hydraulic, wind and gravity, so soil erosion will provide a source of sediment for rivers.
▲ Soil erosion sampling survey
In order to assess the intensity of soil erosion in the region, we conducted a sample survey of areas with different land use types and different vegetation cover, such as shrublands in southeast Tibet, grasslands in Nagqu, farmland in Shigatse, deserts in Changtang, and so on.
▲ Soil erosion field scouring experiment
In addition, in order to quantitatively analyze the influence of slope, flow and other factors on erosion intensity, we also carried out slope erosion monitoring and experiments in the field. These two photos are the scenes of our experiments in the ice ditch of Qilian Mountain.
▲ Soil erosion indoor artificial rainfall simulation experiment
In addition to our field work, we also conducted indoor artificial rainfall simulation experiments in the artificial rain hall to observe the erosion process under different rainfall conditions.
Through these field and indoor research efforts, we have identified two factors that have an important influence on soil erosion changes on the Tibetan Plateau: precipitation on the one hand, and vegetation on the other. Let's take a look at how these factors change on the Tibetan Plateau.
▲ Average precipitation on the Tibetan Plateau (left)
and vegetation index (right) change
The image above on the left is the result of our analysis using meteorological data. It can be seen that the precipitation on the Tibetan Plateau has generally shown an increasing trend in the past few decades, and this change is also accompanied by an increase in precipitation intensity. Simply put, there is more heavy rain and moderate rain. Regarding vegetation, we analyzed the remote sensing data to get the results of the figure on the right above. It can be seen that the vegetation coverage of the Qinghai-Tibet Plateau has also increased overall. Increased precipitation increases erosion, while increased vegetation produces more protective effects and reduces erosion. The two sides are opposite, and there is no way to explain why the rivers of the Tibetan Plateau have a consistent increase in sand transport. Therefore, we need to supplement the analysis of new data to get a more comprehensive picture of the problem.
Changes in glaciers and permafrost endemic to the plateau
Completed: 40%/////////
We have carried out field monitoring of river turbidity on the Tibetan Plateau.
▲ Distribution of turbidity monitoring points in plateau rivers
The points in the image above are all sites where we made long-term observations in the field. We observed turbidity in 8 high-altitude sections of 6 small watersheds.
▲ Turbidity monitoring system of water body in the runoff section of The Görjungangri Glacier
This is one of them, a water turbidity monitoring system laid on the runoff section of the Kojangangari Glacier. In the upper left corner of the upper left figure is a host machine mounted on a high place, which contains a data collector and a module for wireless transmission. The image on the right above is our turbidity meter probe fixed in the river water. At the top of it is an optical window that can sense changes in the turbidity of the river, just like our eyes.
These instruments, deployed on the Tibetan Plateau, helped us capture the dynamic process of turbidity in the river. These are more detailed data than the sand transport data information published by the hydrological station. We compare these turbidity data with meteorological data, including temperature and precipitation, and we gain some understanding of the changes in river turbidity on the Tibetan Plateau and other regions.
▲ Turbidity data of Dongkmadi Glacier (2021)
The turbidity changes shown on the left side of the image above are similar to those in other regions, with a sharp increase in the turbidity of the river after rainfall. This is what rain erosion is causing, and it is also the content of hydraulic erosion that we studied earlier. On the right is a phenomenon unique to the Tibetan Plateau, where the turbidity of the river changes accordingly as the temperature changes day and night. The temperature rises, the turbidity increases; the temperature decreases, and the turbidity decreases. This is a very unique phenomenon on the Tibetan Plateau. This unique phenomenon makes us realize that there must be some unique and temperature-affected factors on the Qinghai-Tibet Plateau that affect the turbidity of the river.
▲ Distribution of glaciers on the Qinghai-Tibet Plateau
We think of glaciers first. The Qinghai-Tibet Plateau has a high altitude and low temperature, so many alpine glaciers have developed. According to statistics, there are 48,571 glaciers on the Qinghai-Tibet Plateau, with a total area of 51,840 square kilometers.
▲ Glacier dynamic map
Glaciers are different from frozen ice cubes in your imagination. Because of their enormous size and considerable weight, these glaciers move slowly from top to bottom under the influence of gravity. If we look at the glacier moving fast-forward, we will find that it resembles a flowing river.
▲ Schematic diagram of glacier abrasion and astrophy
In the process of movement, under the pressure of the huge ice body, the glacier produces abrasive and edifying effects on the bedrock at the bottom, so that many rock chips are generated at the bottom of the glacier.
▲ Cave at the bottom of the glacier in Kangma County, Shigatse
This debris is transported to the end of the glacier through a drainage system at the bottom of the glacier. The picture above shows a cave at the bottom of a glacier in Kangma County, Shigatse, Tibet. We can see that it is actually possible that the glacial meltwater is not as clear as the thawed ice water, but is very turbid.
▲ Meltwater of rongbu river glacier on Mount Everest (left)
Yangtze River source Jiang Gendiru glacier meltwater (right)
The meltwater on the surface of the glacier and the meltwater at the bottom of the glacier, which collect at the end of the glacier, further transport the debris at the end of the glacier into the river downstream. Due to the different types of bedrock of different glaciers and the different colors of the debris produced, the color of the glacial meltwater we see will also be different, but we can find that the meltwater of these glaciers is very turbid, which clarifies the contribution of glacier erosion to river sediment transport.
Next, we further analyzed several key factors affecting glacier erosion. First, through the analysis of meteorological data, we get the left chart below, that is, the change in temperature. We can see that in the past few decades, the temperature on the Tibetan Plateau has risen significantly.
▲ Qinghai-Tibet Plateau temperature (left) and
Changes in meltwater runoff (right) in the upper reaches of the Ya River
We also used the temperature data to drive the hydrological model, performed numerical simulations, obtained the glacial meltwater runoff in different river basins, and analyzed it. From the image on the right above, we can see that the glacial meltwater runoff is also increasing. This way we know that glacial meltwater runoff carries more sediment into the river. So, we have found another factor that will promote the increase in the amount of sediment transported by the river.
In addition to glaciers, we also think of another factor – permafrost. The distribution of permafrost on the Tibetan Plateau is shown below, with the pink area being seasonal permafrost, the blue area being permafrost, and the yellow area being unfrozen.
▲ The frozen soil of the Qinghai-Tibet Plateau is widely distributed
We can see that except for a very small part of the unfrozen area in the south, the distribution of frozen soil on the Qinghai-Tibet Plateau is very extensive. A few of the photos on the right of the image above are some forms of frozen soil.
▲ Repeated freeze-thawing of the frozen soil surface,
Causes soil or rock to crack
As the temperature changes, the surface layer of the frozen soil will freeze in the winter, melt in the summer, and some even freeze at night and melt during the day. Because the density of water and ice is different, this repeated freezing-melting will lead to repeated expansion and contraction of the soil, which in turn will lead to the rupture of the soil or rock. The picture on the right below is the fracture of the soil layer caused by freeze-thawing that we photographed in the wild at The Great Winter Tree Mountain in Qilian. We can see that this repeated freeze-thaw increases the erosivability of the soil.
▲ Thaw mudslide (Video source:
https://www.sohu.com/a/192066289_99917830）
In some extreme cases, when the thawed loose soil layer has a large water content, it will creep down the hillside under the action of gravity, and then form a thaw mud flow. The video above is of a thaw mudslide in Yushu, Qinghai Province, in September 2017.
▲ Soil temperature and humidity monitoring probe is laid out
Changes like this will not only destroy the vegetation on the surface, but also produce a large amount of sediment, forming a new source of sand, so it will also lead to an increase in river sediment. Because the frozen soil will produce such a phenomenon and effect with the change of surface temperature, we have deployed soil temperature and humidity probes in multiple locations to monitor the frozen soil. The picture above is the scene where we set up the soil temperature and humidity probe. After these probes are laid out, we will fill in the excavated soil layer by layer in the original order.
▲ The warming increases the number of days of thawing of the frozen soil surface,
Increases the possibility of surface erosion
Combining these soil temperature and humidity data with remote sensing data on surface temperature, we further analyzed the number of days of annual thawing of the frozen soil surface layer. As can be seen from the above figure, as the Qinghai-Tibet Plateau heats up, the number of days of thawing of the frozen soil surface is also increasing. This, then, increases the likelihood of surface erosion, so we have identified a factor that will increase surface erosion and thus increase river sediment.
Quantitative analysis of the sources of sediment in rivers
Completed: 60%//////
After we have identified these factors, we must also conduct a comprehensive evaluation and quantitative assessment of them. With the support of the second Qinghai-Tibet Scientific Expedition, we have carried out a key study of the Yajiang River, that is, the Brahmaputra River Basin.
▲ Yajiang expedition route and team photos
Above is what the Yajiang River basin looks like and the route we are investigating. With a total length of more than 2,000 kilometers on the mainland, the Yajiang River is the longest river on the Qinghai-Tibet Plateau. In the past three years, we have conducted many inspections of the Yajiang River, including the source area of the Yajiang River, the main tributaries and the main stream. The photos below show us monitoring turbidity, flow and sediment in the Yajiang River, as well as survey monitoring of glacier areas and glacial runoff and sampling surveys of vegetation and soil erosion. Through these scientific investigations, we collected data on the meteorology, hydrology, vegetation, soil and other aspects of yajiang.
By analyzing these multiple data, we obtained the contribution rate of different factors in the upper reaches of the Yajiang River to the increase in sand transport under warm and humidification conditions. From the histogram above, we can see that vegetation cover does have a positive effect on soil and water conservation, but other factors have a stronger force in reverse, so it leads to an increase in sand transport. The largest contributor in the figure is the increase in erosion force caused by the increase in precipitation intensity, followed by the increase in sand-carrying capacity caused by increased precipitation, and the increase in soil thaw days and the increase in glacial meltwater.
In fact, the glacier area in the upper reaches of the Yajiang River accounts for a very small area, only 1.17%, but the glacier with such a small area can contribute 14% to the increase in the amount of sediment transport caused by the increase in glacial meltwater. Therefore, the role of glaciers is very large. In other watersheds where glaciers account for a larger proportion of the area, the role or contribution rate of glaciers and permafrost will be greater. At this point, we have basically answered the scientific questions raised earlier.
Why do we study plateau river sediment?
Completed: 70%/////////
So, what is the use of answering these questions for local production practices?
▲ The sediment of the river creates fertile land on the plateau
We can first see what the sediment of the plateau river itself is of any use. On long-term scales, the sediment deposition of rivers forms the terraces of rivers and some alluvial fans, which create fertile land on the plateau.
▲ The channel of the Nianchu River is dredged
However, the rapid increase in river sediment transportation on the Tibetan Plateau in recent years has also led to a series of negative problems. First, there is the intensification of river siltation. The Nianchu River is one of the four major tributaries of the Ya River, and in July 2019, when we inspected the Nianchu River, we saw the scene of the dredging of the river in the picture above. Before you see this photo, it may be difficult to imagine that the rivers on the Qinghai-Tibet Plateau can be silted up to such a serious extent. In fact, locals tell us that in recent years, the Chu River has to carry out such dredging every year to dredge the river.
In addition to the siltation of the river channel, the increase in the amount of sand transport will also lead to the siltation of the reservoir. The picture below on the left is the Xiluodu Reservoir of the Jinsha River in the upper reaches of the Yangtze River, and its storage capacity is decreasing with the silt accumulation. This change will result in a shorter service life of the reservoir than designed.
▲ Reservoir siltation (left)
Turbine wear (right)
In addition, if sediment enters the turbine, it will also cause wear on the components of the turbine. The image above on the right shows that the rotor of the turbine has been significantly thinned by the wear and tear of sediment. Therefore, river sediment will also affect the long-term operation of water conservancy and hydropower projects.
In addition to these direct effects, river sediment has some indirect effects. Every year in winter and spring, the Gonggar Airport in Lhasa, Tibet, is forced to shut down intermittently for dozens of days due to sandstorms. The source of these sandstorms is mainly sediment deposited on the riverbed in the wide valley section of the middle reaches of the Ya River. Because these sediments are very loose and exposed after the water level drops during dry periods, it is very easy to form sandstorms if it encounters windy weather.
▲ October 20, 2018
Wind sand in the valley of the Nyingchi section of the Brahmaputra
In October 2018, when we were inspecting Yajiang, we saw a wind and sand in the river valley. At that time, the water level was not very low, the wind was not very large, and very obvious sand and dust could be seen.
▲ October 17, 2018
Brahmaputra river blocking disaster
In addition, the river sediment of the Qinghai-Tibet Plateau may also be affected by some disasters unique to the plateau. In October 2018, there was a serious blockade of the Yajiang River near the Big Bend, and a large amount of accumulation blocked the main stream of the Yajiang River, causing the water level to rise rapidly. The backwater not only flooded some of the farmland on the shore, but also washed away a cross-river bridge upstream, which is the bridge on the right side of the picture above, and we can see how it is broken.
▲ Yajiang River Blocking Emergency Research Team (left)
Yajiang dam plugging body (right)
After the disaster occurred, the Autonomous Region Water Conservancy Department contacted our Qinghai-Tibet Institute for the first time, and the institute immediately organized an emergency scientific expedition team to block the Yajiang River. Under the leadership of Academician Yao Tandong, we rushed to the scene of the disaster and made a close observation of the ice collapse of the Yajiang River and the surrounding situation.
We can see that in the right figure above, there are many white clastic ice cubes on the plugged river barrage, and combined with the seismic station data laid out by colleagues who did the earthquake, and the dynamic changes of the glacier obtained by the colleagues who did remote sensing, it was judged that the river blockage was caused by natural ice avalanches, and it is very likely to occur again. Based on this information, the autonomous region carried out an emergency relocation of local villagers. After the expedition, we also completed the scientific assessment report of the ice avalanche and blocking the river.
▲ Yajiang ice collapse blocking river news report (left)
Scientific assessment report (right)
Less than two weeks after the incident, on October 29, another serious ice avalanche occurred at the same site. When we saw the news reports at that time, we were very glad that the local villagers had relocated in time and had not been affected by the second disaster. We also deeply felt the significance of our research through this incident.
Heaven for the eyes, hell for the body
Completed: 90%/////////
Some people say that the Qinghai-Tibet Plateau is a paradise for the eyes and a hell for the body, and my feelings are indeed like this. I returned to China in 2009 to work at the Qinghai-Tibet Institute, and went to the Tibetan Plateau for the first time in 2010. Because I had no experience, in the Lhasa office building of the Qinghai-Tibet Institute, which is the small building in the photo below, I moved as fast as I did in Beijing. Then, suddenly, I had Venus in my eyes and lost consciousness.
▲ The Lhasa Department of the Qinghai-Tibet Institute
Fortunately, my colleague was right next to me, and I was held up so I didn't fall. However, I began to experience a series of altitude sickness symptoms such as headaches, nausea, and vomiting. Because I was vomiting all the time, the institute sent me to the military hospital, underwent simple treatment, and the next day let me return to Beijing immediately by plane and the earliest flight. This was my first trip to the Tibetan Plateau.
Although my first trip to Tibet can be said to be a dismal end, but eat a long and wise, I will carry a small hemximeter with me every time I go to Tibet to measure the blood oxygen content and keep abreast of my body's condition. I also prepare medical oxygen cylinders in the car and inhale oxygen in time when needed.
▲ Painful and happy scientific research path
You can look at the altitudes I've been to over the years. From 3,750 meters in Lhasa to 5,500 meters at the end of the Koqionggangri Glacier that we observed, I have repeatedly pushed through the limits of my body, and at the same time I have gained a new understanding of the river sediment on the Qinghai-Tibet Plateau.
▲ Guarding the beautiful and fragile Qinghai-Tibet Plateau
The Tibetan Plateau is very beautiful and at the same time very fragile. In the context of global warming, the climatic environment of the Tibetan Plateau is undergoing rapid changes. It is hoped that the study of river sediment on the Qinghai-Tibet Plateau will enable us to better cope with the impact of climate change on the source area of rivers.
That's it for my sharing, thank you!
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