永定河
Huailai County is located in the eastern part of Zhangjiakou prefecture, with latitude ranging from 40° 04′ to 40° 35′ N and longitude 115° 16′ to 115° 58′ E. It contains the upper reaches of the Yongding River and borders Beijing Municipality. It is east-southeast of the urban area of Zhangjiakou and west-northwest of Beijing city proper.