Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 .
Simplified: 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 .