Lanzhou


Loess south of Lanzhou - J.H. Wittke

 

The steep hillsides south of Lanzhou are composed of loess, which where not terraced and planted, is a vivid yellow-brown in the morning sunlight. Loess is a wind-blown sediment. The term derives from the German word for loose "Loesch" used to describe sediments of the Rhine Valley. In 1877, the geologist Franz von Richthofen concluded that the yellow soils of central China were very similar to the Rhine Loesch. Loess covers about 440,000 square kilometers in China. The greatest thicknesses (50-300 meters) occur in the about 300,000 square kilometers of the "loess plateau." The greatest thickness (334 meters) is a few kilometers north of Lanzhou. Three stratigraphic units can be distinguished based on color: Lower Pleistocene deposits are reddish yellow, those of the Middle Pleistocene are brownish yellow, and the Upper Pleistocene material is grayish yellow.

All loess is eolian, transported by the wind. Dust storms in northern China are still common and impressive, reminiscent of the American Dust Bowl of the 1930s. However, the origin of the Chinese loess is different from other parts of the world. The loess of North America and Europe is derived from glacial material. As the Pleistocene glaciers retreated, the sparsely vegetated plains left behind were a ready source of wind-blown dust. Most of the Chinese loess originated in the deserts and gobis of Central Asia northwest of the loess plateau, as indicated by decreasing average grain sizes toward the southeast. Finer grains of dust can be carried farther than coarse ones, so grain size decreases away from the source area. Recent geochemical studies show that the loess is not exclusively derived from the arid deserts north of the Tibetan Plateau, but that some comes directly from the glacial sediments of the Plateau itself. Chinese workers call these two types "hot loess" and "cold loess," respectively.


Caves in Loess - Anne Barton Wittke

 

Loess covers about 10 percent of the Earth's land surface and is concentrated in the temperate zones. Porous and homogeneous, lacking the internal structure that most sediments display, loess is easy to cultivate, and yields the fertile farmlands of the world's wheat belts. Loess provides some of the best agricultural soils in China; however, a lack of abundant rainfall or irrigation water limits the productivity of this region. The loess is quite soft, yet cohesive, and has been carved into steep terraces for farming. Caves, which are used as dwellings, temples, and for storage are burrowed into the hillsides. Tool marks can be seen on the right side of the outcrop examined by P. Kresan of the University of Arizona (below).


Loess Outcrop - J.H. Wittke

In the city of Lanzhou we cross the Huang He (Yellow River), which has been described as the most turbid river in the world. The Huang He drains a substantial portion of the loess plateau and is the second largest river in China. However, it has average flow volumes only 5 percent that of the Yangtze River. Called "China's Sorrow," river flows are inconstant and sometimes become deadly floods. The Huang He has very few major tributaries and most of the minor streams that feed it occupy small drainage basins, providing little input. The Huang He is mainly fed from the area to its south, its flow volumes heavily dependent on rainfall on the Tibetan Plateau near its head waters. A long period of drought in western Qinghai and the Tibetan Plateau has reduced the water flow through the city by almost a quarter compared with average levels, but it still looks impressive.

Recently, China has unveiled plans to divert water from the Yangtze River into three northern rivers, the Yellow, Huai and Hai (as reported by CNN). The proposed project will link the flood-prone Yangtze River basin to the arid north of China. China has been on a building spree: other projects include the Three Gorges hydroelectric dam, a railroad from Golmud to Lhasa across the Tibetan Plateau, and a proposed 4,000-kilometer natural-gas pipeline across western China. The diversion project was first conceived by Chairman Mao Zedong 50 years ago, and was revived after several years of drought exacerbated China's water crisis. Over-pumping and unrestrained industrial development have dried up rivers, wells and lakes and depleted groundwater. The Yellow and Huai Rivers are chronically polluted, because pollutants are not diluted by the reduced water flows.

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