The Yellow River is second longest river in China. At present, theoretical annual flow of Yellow River is 56 billion cubic meters, while actual flow of Yellow River at sea mouth is only 36 billion cubic meters. You should know that annual net flow of eight main tributaries of Yangtze River is more than 50 billion cubic meters, which shows how small volume of water in Yellow River is. From 1972 to 2000, Yellow River dried up 19 times. Today, due to construction of water facilities, there are no more outages. But there is so little water in Yellow River and it is worth exploring.
Historically, Yellow River was a river with a lot of water. During Xia and Shang dynasties, climate of China was very warm and humid, and entire Loess Plateau was covered with forests. At that time, annual net flow of Yellow River into Hukou exceeded 100 billion cubic meters. In middle reaches of Yellow River, Feng, Wuding, Wei, Jing, Luo and other rivers flow into it, and total flow is very significant.
Forest distribution on Loess Plateau during Western Zhou Dynasty
Due to sufficient water volume of Yellow River, Yellow River became main transport artery in north in ancient times, and even channel formed by Yellow River diversion was also a waterway. Back in days of Western Zhou Dynasty, Western Zhou Dynasty advanced along Yellow River to North China Plain and created fortresses along banks of branches of Yellow River, at crossroads of Yellow River. To increase control over Central Plains, Duke of Zhou built Chengzhou and Luoyi in Luoyang.
After that, Qin dynasty united six countries, and its main march route also ran along Yellow River. During Qin and Han dynasties, a large amount of grain was transported to Guanzhong via Yellow River, and during peak period, 6 million stones were transported per year. The intersection of Yellow River and other water systems would become a metropolis in Central Plains. For example, Luoyang during Tang Dynasty "gathered all boats and ships in world, often over 10,000, filling river routes, merchants, travelers, goods, horses and horses." Kaifeng, at Yellow River fork, has been a transportation hub in Central Plains since Warring States period, and water transportation made it capital of Song Dynasty.
However, in history, volume of water in Yellow River has tended to decrease. During Western Jin Dynasty, annual net flow of Yellow River in Hukou fell below 50 billion cubic meters, while during Qing Dynasty it was only about 30 billion cubic meters. There are two main reasons for decline in volume of Yellow River. First, climate in northwest is getting drier and rainfall is decreasing. Secondly, human activity has caused serious damage to pastures and forests. The loess plateau was a dense forest 2000 years ago, but during Qing Dynasty it turned into a bare "desert". The reduction in forests increases evaporation from land and reduces land's water holding capacity, so flow of Yellow River decreases.
The reduction in volume of water in Yellow River will have a bad effect on economy of north. First, a decrease in water volume will cause sediments in Yellow River to easily silt up, which in turn will block channel and increase risk of flooding Yellow River. Since Han Dynasty, Yellow River has increased frequency of diversions due to sedimentation. Secondly, reduction in water content will lead to a decrease in navigability of river, which will hinder development of transport in north and will not stimulate development of economy. Thirdly, reduction in volume of water in Yellow River has led to a reduction in irrigation water supplied by Yellow River to surrounding agricultural areas, which is not conducive to development of agriculture.
During Ming and Qing dynasties, Yellow River lost its capacity. At that time, in order to prevent accumulation of sediment from Yellow River into Grand Canal, Grand Canal and Yellow River were separated.
However, water consumption on both sides of Yellow River in modern times is small, and Yellow River has not yet stopped flowing. From 1919 to 1979, average flow of Yellow River in Lanzhou was about 32.6 billion cubic meters, and flow of Huayuankou was 56.3 billion cubic meters. From 1956 to 1979, annual flow of Yellow River in lower reaches exceeded 60 billion cubic meters.
With continuous development of society and economy, population continues to increase, water consumption on both sides of Yellow River continues to increase, and people draw more and more water from Yellow River. Therefore, since 1970s, water resources of mainstream of Yellow River have been more abundant in upper reaches and less in lower reaches. In 1960s, Yellow River's discharge into sea was 57.5 billion cubic meters.
Ecological destruction of upper reaches also exacerbates problem of uneven seasonal distribution of water in Yellow River. 60% of precipitation in Yellow River basin falls mainly in June-September, which is also high season in entire north. Therefore, at this time, waters of Yellow River flow relatively calmly into sea. During winter, north as a whole is short of water, and all provinces draw water from Yellow River, causing Yellow River to be cut off.
Why didn't Yellow River stop flowing after 2000? This is mainly due to construction of large reservoirs in middle reaches. Around 2000, Xiaolangdi Water Conservation Project was completed. During summer, Xiaolangdi stores water to prevent flooding and reduce risk of flooding downstream. In winter and spring, Xiaolangdi Reservoir will still deliver water downstream. Thus, water resources of Yellow River have been adjusted to a certain extent according to season.
However, reservoirs alone cannot solve problem. If we want to fundamentally solve Yellow River water problem, we must start from an environmental point of view. After decades of afforestation, forest cover of Loess Plateau has increased significantly. Seventy years ago, Shaanxi's forest cover was only 13%, but now it has increased to 43%. In Shanxi Province, it increased from 2.4% to 23% in same period. This is a remarkable achievement, but it is far from enough.
Currently, river management and navigation in lower Yellow River should be based on ecological management of middle and upper Yellow River, otherwise it is out of question. In addition, issue of diverting water from southwest to upper reaches of Yellow River to replenish water resources of Yellow River can be considered.
In short, development of Yellow River is a thousand-year project. The scope of work to control Yellow River is enormous, but it must be practiced from generation to generation.