In its early course, the river flows in a narrow north sloping valley starting from the Chokkampatti hills and extending north about , with a number of sharp lateral ridges running down to it from east and west. After flowing from its origin and on reaching Mount Plateau at its eastern end, it is joined by the west-flowing Mullayar—an important tributary—at Mullakudy. During this journey the Periyar traverses through the Periyar Tiger Reserve, flowing across or forming the boundaries between various sections of the reserve. Going downstream, it defines a part of the border between Periyar section of the reserve in the east and Sundaramala section in the west and then the borders between Moolavaigai and Thannikudy sections in the east and Ummikuppan, Mlappara and Aruvioda sections in the west before reaching the Periyar Lake.