Because the river flows through Klang Valley, which is a heavily populated area of more than four million people, it is considerably polluted, because of deep siltation caused by human waste from informal settlers of the riverbank and even from some business establishments without septic tanks or sewage treatment plants and by soil carried by mudflows from mountains. Heavy development has narrowed certain stretches of the river to the point that it resembles a large storm drain in some places. This contributes to flash floods in Kuala Lumpur, especially after heavy rain.