Complex sentence: It takes its name from the three main rivers which flow through that area-the Penukal River, Abab River and Lematang River-while the name "Ilir" means "downstream" (it denote the downstream part of the Lematang River).
Split into simpler sentences: It takes its name from the three main river which stream that area, Penukal River, Abab River and Lematang River. And the name "Ilir" means downstream (it denote the downstream part of Lematang River).