The headwaters of the Nepean River rise near Robertson, about 100 kilometres south of Sydney and about 15 kilometres from the coast. The river flows north in an unpopulated water catchment area into Nepean Dam, which supplies water for Sydney. North of the dam, the river forms the western edge of the Sydney Basin, flowing past the towns of Camden and Penrith, south of which flowing through the Nepean Gorge. Near Penrith it is joined by the Warragamba River. North of Penrith, at the junction of the Grose River near Yarramundi, the Nepean becomes the Hawkesbury River.
There are 11 weirs located on the Nepean River that are significantly regulating the natural flows. 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.
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 from much of their former habitat, and reducing floods and freshes 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 a quiet location for local residents to relax. At Emu Plains, the western bank of the river provides a location for outdoor theatre productions on warm summer nights. The eastern bank at Penrith provides barbecue facilities and children's play equipment, as well as a wide pathway running for several kilometres for strolls along the riverbank. The eastern bank is also the home of the Nepean Rowing Club.
When the British colony was established at Sydney in 1788, the Royal Navy men in charge of the settlement went exploring by boat. They discovered the mouth of the Hawkesbury River about 50 kilometers north of Sydney Harbour and followed the river upstream, naming it after Charles Jenkinson, 1st Earl of Liverpool, who at that time was titled Baron Hawkesbury.
Meanwhile, Watkin Tench of the Royal Marines set off to walk inland, west of Sydney. About 60 kilometers inland, at the foot of the Blue Mountains, he discovered a large river which he named Nepean after a different British politician, Evan Nepean. It took the Navy and the Army about three years to realise they had discovered the same river and given it two different names.
The first flood on record - apparently a small occurrence - was in 1795. Others followed in 1799, 1806 and 1809. In 1810, after a series of major floods on the Hawkesbury, Governor Macquarie proclaimed the ‘Macquarie Towns’ of Windsor, Richmond, Wilberforce, Castlereagh and Pitt Town in an attempt to ensure that development was restricted to higher ground, free of flooding. The devastation caused by flooding in February, 1817 prompted Governor Macquarie to issue a notice exhorting settlers, in the strongest possible terms, to build their residences above the established flood levels.
The most devastating flood occurred in June 1867, the Nepean River being estimated to have reached a height of about 13.4 metres in the river, and 27.47 metres on land, ie 27.74 metres AHD (Australian Height Datum). This flood carried away the approaches to the recently rebuilt Victoria Bridge. Emu Plains, Castlereagh and the lower parts of Penrith were all under flood, causing immense loss of property. Many houses were carried into the river by landslides. Many residents were forced to take refuge in public buildings such as the Penrith Hospital and the public schools. A major flood such as that of 1867 caused inundation of over 16,000 dwellings and damage costing in the order of $1.4 billion.
There have been other notable floods since - particularly that of July 1900 and March 1914. Again there was much flooding of streets and loss of houses and property along the river.