The Dez River (Persian: دز), the ancient Coprates (Greek: Κοπράτης or Κοπράτας), is a tributary of the Karun River and is 400 km long. It is the site of the Dez Dam.
It is believed that a fortress protected a strategic bridge across the river at Dezful, whence the name, although no trace of this castle remains. The old part of the city adjacent to the bridge is known as the Qaleh (castle). Dez River side resort is located in a part called " Alikale" , that During the summer, people come from all over the Khuzestan province, where they are swimming in the cool water.