It then passes through the eastern part of Lopburi Province and Saraburi Province, until it joins the Lopburi River northeast of Ayutthaya Island, before it runs into the Chao Phraya River southeast of Ayutthaya near Phet Fortress.

The Pa Sak drains an area of 16,291 square kilometres (6,290 sq mi).

In addition to being used as a waterway, course of the Pa Sak River, no less than 60 years ago, especially in Phra Nakhon Si Ayutthaya Province, it was well known for its Khanom chin, traditional Thai vermicelli.

[2] Around the same time, Wat Mondop across from the Hua Ro quarter was an important pier because people would board boats here for Bangkok.

During the 11th and 12th Thai lunar months, following the Buddhist retreat, Wan Ok Phansa, when the waters are high and the paddies flood, it used to be a time for courting, or boat song festivals along with boat races between the two temples.