永定河是海河水系中最大的一条支流,流经山西、内蒙、河北、北京、天津等五个省、市、自治区。永定河流域是海河流域中土壤流失最严重地区之一,除北部少数地区风蚀较重外,以水蚀为主。河道输沙量占海河总输沙量的24％。搞好永定河流域的水土保持,对根治海河十分重要。 Yongding River is the largest tributary of the Haihe River system and flows through five provinces, cities and autonomous regions in Shanxi, Inner Mongolia, Hebei, Beijing and Tianjin. The Yongding River basin is one of the most serious soil loss areas in the Haihe River Basin. Water erosion is the main part of the Yongding River Basin, except for the wind erosion in a few northern parts of the country. Sediment discharge accounts for 24% of the total sediment discharge in Haihe River. To do a good job of water and soil conservation in the Yongding River Basin is of great importance to radicalizing the Haihe River.