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一、流域及梯级概况永定河为海河五大支流之一,流域面积占海河全流域三分之一以上,而年水量仅为七分之一。永定河发源于蒙古高原和山西高原交界处,主要支流为桑干河及洋河,全流域年降雨量为300至650公厘,主要集中在夏季。因流域內黄土区和丘陵帶面积較广,水土流失情况严重,所以洪水流量大,夾帶泥砂极多?偬庹咀?926年至1955
I. Introduction to Watershed and Cascade Yongding River is one of the five major tributaries of the Haihe River. Its catchment area accounts for more than one third of the Haihe River Basin, while its annual water volume is only one-seventh. Yongding River originated in the Mongolian Plateau and the junction of Shanxi Plateau, the main tributaries of the Sanggan River and Yang River, the annual rainfall of 300 to 650 mm, mainly concentrated in the summer. Due to the wide area of loess area and hilly belt in the basin, the soil erosion is serious, so the flood flow is large with a lot of sand and muddy soil. From 1926 to 1955