When viewing the Loess Plateau from the air, as far as the eye can see, everything is yellow. The land undulates like waves, with deep and overlapping gullies whose steep slopes can reach tens to hundreds of meters in depth. Walking at the bottom of the gullies, you look up and see a strip of blue sky. The Loess Plateau has been "cut apart" into fragments, with densely interlaced gullies. In most areas, there are 3-5 kilometers of gullies per square kilometer, with gully depths reaching 50-100 meters. Gully areas typically account for 30%-50% of the basin area, and in some areas, this figure exceeds 60%. The Loess Plateau is mainly composed of three landform types: plateaus, ramparts, and mounds.
The plateau, also known as the Loess Plateau, is a broad, flat highland with steep slopes cut by gullies on all sides. It is an excellent farming area. Dongzhi Plateau in Lundong and Luochuan Plateau in northern Shaanxi are famous examples. The long, narrow hills composed of loess are called ramparts. Plateaus undergo long-term, intense erosion and are cut into ramparts. Most ramparts have narrow, gentle tops with many saddle-shaped depressions. Their sides are fragmented by gullies, and in some cases, the gullies even connect with each other. The rounded, steeply sloping hills of loess are called mounds.
The Loess Plateau has attracted worldwide attention. Firstly, the massive accumulation of loess here is the largest in the world in terms of both area and thickness. Secondly, the severe soil erosion on the Loess Plateau has made it the area with the most extensive water and soil loss and the highest intensity of erosion in China and the world. The area of water and soil loss in the upper and middle reaches of the Yellow River is about 430,000 square kilometers, with an annual loss of 1.6 billion tons of soil.
How was the Loess Plateau formed? When did this intense soil erosion begin? And what caused it?
Most scientists believe that the loess on the Loess Plateau originated from arid regions in central Asia and northwestern China. The rocks in those areas expand and contract due to temperature changes, undergoing long-term weathering that breaks them down into small stones, sand, silt, and clay. During the winter and spring seasons, strong northwest winds blow these fine silt and clay particles eastward and southward until they reach the current Loess Plateau. Due to the high Qin Mountains blocking the south, the wind weakens and deposits the particles on the ground. Over millions of years, this continuous deposition process formed the thick loess on the Loess Plateau, while coarser rocks and sand remained in their original locations, forming deserts and gullies. The content of coarse silt in the loess decreases gradually from northwest to southeast, while the content of fine clay increases in the same direction. Based on the content of coarse silt and clay, the loess can be divided into sandy loess, loess, and clay loess. The closer to the source of sand and dust, the loess is coarser, while the farther away from the source, the loess is denser. This is one of the important bases of the wind-deposition theory of the Loess Plateau.
Although loess particles contain a small amount of coarse material, they are generally fine and easily dispersed. However, when the loess collapses, it retains its prismatic shape. The high content of calcium carbonate in the loess acts as a binder for soil particles, but calcium carbonate is easily dissolved in water, making the loess easily dispersed and washed away by rain. Due to the sparse vegetation cover on the Loess Plateau and frequent summer rainstorms, the loess is often carried away by water, and even the finest gullies can be quickly deepened and expanded by rainstorms, with their heads constantly rising until they become gullies. Such gullies continuously cut and dismember the Loess Plateau, leaving it fragmented and crisscrossed with gullies, with large amounts of sediment carried to the middle and lower reaches of the Yellow River, making the Yellow River a river rich in sediment. The name "Yellow River" originated from this.
In addition to natural factors, human activities, especially irrational land use, have also accelerated the process of soil erosion on the Loess Plateau. Currently, the Loess Plateau is facing severe challenges of soil erosion and desertification, and urgent measures are needed to protect and restore this precious natural resource.The soil erosion on the Loess Plateau is human-induced accelerated erosion on the basis of natural erosion. From 3,000 to 2,000 years ago, the annual erosion volume was about 970 million tons. After periods of intense soil erosion such as the Ming and Qing dynasties and the 1950s to 1980s, the current annual erosion volume is about 1.6 billion tons.
The regional distribution of soil erosion intensity on the Loess Plateau is uneven, showing a gradual increase from south to north. In the north, the soil erosion intensity reaches more than 30,000 tons per square kilometer, while in some mountainous areas in the south, the soil erosion intensity is less than 500 tons per square kilometer. What causes this difference? Studies have found that it is firstly due to natural geographical zonal factors such as precipitation, vegetation, and soil, and secondly to the combined influence of non-zonal factors such as landforms and human activities.
Intense soil erosion destroys local land resources and has many adverse effects on industrial and agricultural production and road traffic construction. At the same time, a large amount of sediment from water and soil loss enters the lower reaches of the Yellow River, silt up the riverbed, raise the riverbed, and trigger flood disasters. Since 1950, great achievements have been made in the prevention and control of soil erosion on the Loess Plateau, mainly by increasing vegetation and reducing sediment in the Yellow River. However, there is still a long way to go to fundamentally change the landscape of the Loess Plateau, and more efforts are needed to strengthen management and make unremitting efforts.