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- The early republican period
The Junggar Basin
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North of the Tarim Basin is another large depression, the Junggar (Dzungarian) Basin. It is enclosed by the Tien Shan on the south, while to the northeast it is cut off from Mongolia by the Altai Mountains. The surface of the basin is flat, with a gentle slope to the southwest. The larger portion of the land lies at elevations between about 1,000 and 1,500 feet (300 and 450 meters), and in the lowest part the elevation drops to just below 650 feet (200 meters). In general the main part of the basin is covered by a broad desert with barchans (crescent-shaped sand dunes that move); only in certain parts are dunes retained by vegetation.
The Tien Shan
The Chinese (eastern) part of the Tien Shan consists of a complex system of ranges and depressions divided into two major groups of ranges: the northern and the southern. The groups are separated by a strip of intermontane depressions that itself is broken up by the interior ranges. Ancient metamorphic rock constitutes the larger portion of the ranges in the interior zone; Paleozoic (i.e., about 250 to 540 million years old) sedimentary and igneous sedimentary beds form its northern and southern chains, while Mesozoic (about 65 to 250 million years old) sandstones and conglomerates fill the intermontane depressions in the interior zone and constitute the foothill ridges. The height of the main Chinese chains of the Tien Shan is between 13,000 and 15,000 feet (4,000 and 4,600 meters), with individual peaks exceeding 16,000 feet (4,900 meters); the interior chains reach 14,500 feet (4,400 meters). In the western part, where precipitation is adequate, large glaciers are formed, reaching a length of more than 20 miles (32 km). Large rivers with heavy flows, such as the Ili (Yili) River and its tributaries, begin their courses there, and the predominantly alpine meadow steppe is one of the best grazing lands of China.
Drainage
China has more than 50,000 rivers with individual drainage areas exceeding 40 square miles (100 square km). Of the total annual runoff, about 95 percent drains directly into the sea (more than 80 percent into the Pacific Ocean, 12 percent into the Indian Ocean, and less than 1 percent into the Arctic Ocean) and 5 percent disappears inland.
The three principal rivers of China, all of which flow generally from west to east, draining into the China Sea, are the Huang He, the Yangtze, and the Xi. The Huang He, which rises in the Kunlun Mountains, is the northernmost of the three; it drains into the Bo Hai (Gulf of Chihli), north of the Shandong Peninsula. The Yangtze, the longest river in the country, rises in the Tibetan Highlands and flows across central China, draining into the East China Sea north of Shanghai. The Xi River, the southernmost of the three, rises in the Yunnan-Guizhou Plateau and empties into the South China Sea via the Pearl River Delta at Guangzhou (Canton).
The distribution of surface water in China is extremely uneven. Only a small part of the country has sufficient quantities year-round. Much of the country has abundant runoff but only during the rainy summer, when enormous surpluses of water are received. From the southeast to the northwest, the surface water decreases as the relief becomes more mountainous. A vast area of the northwest lacks water throughout the year. North China (north of the Qin Mountains–Huai River line), with its flat relief and long history of agriculture, contains almost two-thirds of China’s cultivated land; paradoxically, because of scanty and erratic precipitation, the average annual runoff in the North accounts for only about one-sixth of the total for the country as a whole.
The mountains of the southeast and the mountainous Hainan Island have the most abundant surface water. Over the year they receive more than 60 inches (1,500 mm) of precipitation (in some places even more than 80 inches [2,000 mm]), of which almost two-thirds constitutes the runoff, so that a dense drainage network has developed. The amount of runoff is highest in the southeast, exceeding 40 inches (1,000 mm). It gradually diminishes toward the west and north, so that in the true deserts of the northwest it is usually less than 0.4 of an inch (10 mm). The arid climate of the northwest is reflected in the landscape of the dry steppes, which is characterized by richer grasses in the east, while in the west the landscape gradually changes to bare deserts.
In the lower reaches of the Yangtze, the Pearl River Delta, and the Chengdu Plain a dense network of waterways has been developed. In the North China Plain and the Northeast Plain most of the rivers have a linear flow, and tributaries are few and unconnected. In the inland drainage area there are very few rivers because of scanty precipitation. Extensive areas such as the Tarim Basin and northeastern Gansu province are often completely devoid of runoff. In those regions the rivers depend on melted snow and ice; in consequence, they are mostly small and are found only in mountains and mountain foothills. As they drain increasingly farther away from the mountains, most of them eventually disappear in the desert, while some form inland lakes. Because the northern part of the Plateau of Tibet is a cold desert, the rate of evaporation is slow, so that a denser network of rivers has developed; most of these, however, run into glaciated depressions, forming numerous lakes.