Original: From there to its confluence with the Lutkho River just north of the important regional center of Chitral it is called the Mastuj River. It is then called the Chitral River, before flowing south into the upper Kunar Valley in Afghanistan, where it is referred to as the Kunar River.
Simplified: The Lutkho River joins the Mastuj River just north of the important regional center of Chitral in Pakistan and is then called the Chitral River, before flowing south into the upper Kunar Valley in Afghanistan, where it is referred to as the Kunar River.