Once formed, the Hawkesbury River proper flows generally northwards, albeit with a significant number of meanders. Initially the river passes the towns of Richmond and Windsor, which are the largest settlements on the river. At Windsor, the river is joined by South Creek, which drains much of the urban runoff in Sydney's western suburbs that does not fall into the Parramatta River catchment. As it flows north, it enters a more rural area, with only small settlements on the river. On this stretch it passes Sackville and Lower Portland, where it is joined by the Colo River. The Colo River and its tributaries drain the northern section of the Blue Mountains.