Hacking River

Aboriginal history

The river valley underwent significant change with sea level rise 18,000 to 7,500 years ago which completely displaced inhabitants of previous coastal and river valley areas and resulted in dramatic changes in distributions of peoples. For more than 8,000 years^{[citation needed]} prior to 1840, the Tharawal (or Dharwal) people^{[citation needed]} occupied the catchment area evidenced by hundreds of Aboriginal artefacts, middens, rock carvings and cave paintings.

The Gweagal people lived mainly by the saltwater bays and estuaries of Port Hacking. They also used the freshwater resources of the upper Hacking River, Heathcote Creek and the Woronora River. The Gweagal people were said^{[by whom?]} to be the guardians of the sacred white clay pits on their territorial land, now known as the Kurnell Peninsula. They used the clay to make body paint, medicine and when mixed with local berries it was also used as a dietary supplement.

Post-British colonial settlement

In the mid-19th century shell grit was in high demand as a source of lime for building in the Sydney district. Consequently, mud and oyster rocks were collected in large numbers from Port Hacking catchment destroying a number^[vague] of aboriginal midden sites in the region.^[10]

The river is crossed by four causeways at and near the village of Otford near its headwaters. At Otford below the railway station is a small dam, constructed for the supply of water for steam locomotives. A causeway is incorporated as part of the structure, which results in a substantial pond upstream. Other significant structures are the Upper Crossing (below Waterfall and above the confluence with Waterfall Creek) and at Audley. The causeway at Audley was built in 1899, within the Royal National Park. Here, mangrove flats were cleared to make way for boat-sheds and accommodation in the late 19th century.^[11] A boatshed and picnic grounds remain at Audley, having been used continuously since.^[6] Visitors can canoe and kayak further upstream along the Hacking River or its tributary Kangaroo Creek. Within the Royal National Park at Audley, visitors have enjoyed picnic and boating facilities for more than a hundred years.

A consequence of the weir was to divide the river into freshwater/saltwater tidal components. Impacts were to impede fish migration and to cause the accretion of sediment behind the weir. In flood, the road across the causeway is impassable, thus isolating the villages of Bundeena and Maianbar. As the Upper Causeway would also flood in these conditions, an all-weather bridge was built at the Upper Causeway.

The tide reaches the weir, and although there is some lag, there is virtually no tidal prism. From the weir, the river continues downstream in the main through the Royal National Park, although towards its fluvial delta at Grays Point, it flows on one side through the suburbs of Kirrawee and Grays Point. It is opposite Grays Point that Muddy Creek joins the River. From this point downstream the river is characterised by large stands of mangroves and an extensive fluvial delta. Historically, the fluvial sediments below the weir have been restrictive to navigation. After the declaration of the national park, the trustees were concerned to facilitate boat access to the new park and consequently training walls were built. Subsequently, as navigation increased, there was repeated pressure for dredging. The fluvial delta has not been dredged for many years. Nonetheless, there is a well-used boating facility at Swallow Rock (Grays Point).^[12]