从西北流向东南的卢沟河(即今永定河),又称桑干河,源出于山西马邑县北部的雷山,流经北京西郊的卢师山,然后奔泻东南入海。当初因其水相当浑浊,水色深褐,每到夏季又经常泛滥成灾,河道数有迁移,故又有浑河、小黄河和无定河之称。清康熙三十七年(1698)曾疏浚河道,增筑长堤,并改无定河为永定 From the northwest to the southeast, the Lugou River (now the Yongding River), also known as the Sanggan River, originates from the Leishan Mountain in the northern part of the Mayi County of Shanxi and flows through Lu Shishan in the western suburbs of Beijing. It then plunges southeast into the sea. Originally because of its water is very turbid, dark brown color, often flooding every summer, the number of rivers have migrated, so there are Hunhe, the Yellow River and Wudinghe said. Thirty-seven years of the Qing Emperor Kangxi (1698) had dredged the river, building a long bank, and no change in Dingding Yongding