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永定河是华北白河水系中最大的一条河流。上游有二条支流——北支流洋河和西支流桑干河,在怀来盆地(現为官厅水库)匯合后成为永定河干流;从官厅水库西北面复有嬀水河来匯。流域上游,年平均雨量約300至400公匣,下游为500至650公厘,雨水較丰是六、七、八月。年蒸發量大約在1500公匣左右。气溫夏季平均在22℃至27℃,冬季最低达零下20℃以下。最枯月平均流量只有2.8秒公方(1930年5月),最高洪峯在1939年
Yongding River is the largest river in the North China White River. There are two tributaries in the upper reaches - the tributaries of the Yangzi River and the Sanggan River in the western part of the North Branch and the Yongding River in the Huailai Basin (now Guanting Reservoir), and the Huangshuihe River in the northwest of the Guanting Reservoir. The upper reaches of the basin, the average annual rainfall of about 300 to 400 boxes, the downstream is 500 to 650 mm, rainfall is abundant in June, July and August. Annual evaporation about 1500 box around. The average temperature in summer is 22 ℃ to 27 ℃, and the lowest in winter is below 20 ℃. The wettest month, the average flow of only 2.8 seconds public (May 1930), the highest peak in 1939