Complex sentence: Downstream as far as Mastuj it is known as the "Yarkhun River" from there to its confluence with the Lutkho River just north of the important regional centre of Chitral it is called the Mastuj River.
Split into simpler sentences: Downstream as far as Mastuj it is known as the Yarkhun River. From there to its confluence with the Lutkho River just north of the important regional centre of Chitral it is called the Mastuj River.