China has more than 50,000 rivers with a catchment area of some 100 square kilometers. Of these, 1,600 have a catchment area of more than 1,000 square kilometers and 79 have a catchment area of more than 10,000 square kilometers. The rivers in China have a total length of 226,800 kilometers and a total flow of more than 2,600,000 million cubic meters. With a reserve of 680 million kilowatts, China leads the world in hydroelectric power potential and has great prospects for the development of navigation, power industry, aquaculture, and industrial and agricultural production. Almost all large rivers in China belong to the exterior river system, which directly or indirectly emptying into the seas. Because China's terrain is high in the west and low in the east, most of its rivers flow westeast into the Pacific Ocean, including the Heilongjiang, Liaohe, Haihe, Yellow (Huanghe), Huaihe, Changjiang (Yangtze River), Minjiang, Pearl River and Lancang rivers. The area drained by these rivers belongs to the Pacific catchment area, and covers 5.4459 million square kilometers, or 56.8 percent of the country's total area and 88.9 percent of the exterior drainage area. The Nujiang River and Yarlung Zangbo River in the south of the Qinghai-Tibet Plateau, which flow south out of China into the Indian Ocean, have a catchment area of 624,500 square kilometers in China, or 6.5 percent of its total area. The Eirtix in the northwest corner of Xinjiang, which flows north out of china into the Arctic Ocean, drains 50,000 square kilometers, or 0.5 percent of the country's total area. The exterior river catchment area, the land surface that supplies these rivers with water, is 6.1213 million square kilometers, or 63.8 percent of the country's total land mass, and is basically identical with the monsoon influenced regions. With abundant supply of water resulting from the plentiful precipitation there, the total flow of the exterior rivers makes up more than 95 percent of the total of the country's rivers. With the Qinling-Huaihe line as the divide, the exterior rivers south and north of it have their own distinctive characteristics. The rivers on the south of the line, such as Changjiang (Yangtze River), Qiantang, Minjiang and Pearl River, are in the humid zone, where the rainy season is long, annual precipitation is plentiful, plant growth is luxuriant, water conservation is fine and the winter temperature is above zero degree centigrade. Therefore, these rivers have a large flow, a number of tributaries, long high-water season, small seasonal variation in water-level, small amounts of silt, and nonfreezing period. The rivers on the north of the line, such as Liaohe, Haihe and Yellow are in the semi-humid and semi-arid zones, where the annual precipitation is small and concentrated in July and August, the vegetation is sparse, soil erosion is serious and the winter temperature is below zero degree centigrade. Therefore, these rivers have a small flow, a few tributaries short high-water season, big seasonal variation in water-level, a low-water period in winter, large amounts of silt, and a freezing period of varying lengths. All these characteristics are unfavorable to water conservation, power generation, irrigation and navigation. The Heilongjiang River system and the Yalu River in the northeast have their special features and freeze over for long periods in winter. On the other hand, because they flow across the humid, densely forested mountain areas, they carry heavy volume of water with little silt. The Nujiang River and Lancang River belong to another type. Rising in the Qinghai-Tibet Plateau, they flow between towering mountains. Their drainage basins are small and their tributaries are few. However, due to the feature of ice-free, the rivers carry little silt and have great drops in height, they also have enormous flows that offer abundant sources of power. Interior rivers have no outlet to the sea but flow into inland lakes or disappear in deserts. Most of China's interior rivers are found in the arid northwest, north of the Kunlun range or Northern Tibet Plateau to its south, which are fed by the glaciers and melting snow from the Tianshan, Kunlun and Qilian ranges. Although chiefly seasonal waterways that dry up most of the year, these rivers are of great importance to agriculture and animal husbandry in the northwest. The interior rivers of China drain an area of 3.4748 million square kilometers, and are roughly identical with the non-monsoonal regions. Their flow is only less than 5 percent of the total of the whole country. With a length of 2,137 kilometers, the Tarim River in southern Xinjiang is the longest inland river in China.