The Khwae Yai River, also known as the Si Sawat, is a river in western Thailand. It has its source in the Tenasserim Hills and flows for about 380 kilometres (240 mi) through Sangkhla Buri, Si Sawat, and Mueang Districts of Kanchanaburi Province, where it merges with the Khwae Noi to form the Mae Klong River at Pak Phraek.