According to Wikipedia, the Pa Sak River is a river in central Thailand. The river originates in the Phetchabun Mountains, Dan Sai District, Loei Province, and passes through Phetchabun Province as the backbone of the province. It then passes through the eastern part of Lopburi Province and enters Keang Khoi District of Saraburi Province before it joins together with the Lopburi River northeast of Ayutthaya Island, before it runs into the Chao Phraya River southeast of Ayutthaya near Phet Fortress. It has a length of 513 km and drains a watershed of 16,291 km². The annual discharge is 2.4 km³.