Nepean River (Darug: Yandhai), is a major perennial river, located in the south-west and west of Sydney, New South Wales, Australia. The Nepean River and its associated mouth, the Hawkesbury River, almost encircles the metropolitan region of Sydney. The headwaters of the Nepean River rise near Robertson, about south of Sydney and about from the Tasman Sea. The river flows north in an unpopulated water catchment area into Nepean Reservoir, which supplies potable water for Sydney. North of the dam, the river forms the western edge of Sydney, flowing past the town of Camden and the city of Penrith, south of which flowing through the Nepean Gorge. Near Wallacia it is joined by the dammed Warragamba River; and north of Penrith, near Yarramundi, at its confluence with the Grose River, the Nepean becomes the Hawkesbury River. Changes to the natural flow of the river The river supplies water to Sydney's five million people as well as supplying agricultural production. This, combined with increased pressures from land use change for urban development, means the river has been suffering significant stress. There are eleven weirs located on the Nepean River that significantly regulate its natural flow. The river has been segmented into a series of weir lakes rather than a freely flowing river and is also impacted by dams in the Upper Nepean catchment. The Wallacia Weir was initially built as a wooden weir for the John Blaxland flour mill at Grove Farm. The first Australian fishsteps were built when the current concrete weir was built at the beginning of the Nepean Gorge, an anticendant entrenched meander caused by the slow uplift during the Blue Mountains orogeny carved down through the fifty-million-year-old Hawkesbury sandstone. In the 1950s the building of the Warragamba Dam across the steep gorge of the Warragamba River, the Nepean’s major tributary, intercepted the flow of the great bulk of its waters and diverted them to meet the needs of the growing Sydney metropolitan area, reducing the river to a shadow of its former self. These dams and weirs have had a potent effect, blocking migratory native fish like Australian bass (also locally commonly known as perch) from much of their former habitat, and reducing floods and freshets needed for spawning. Nevertheless, the Hawkesbury/Nepean remains an important and popular wild bass fishery. The luscious banks of the Nepean River provide a natural haven for local flora and fauna and