The Hawkesbury-Nepean Valley is fed by five major tributaries and the effects of flooding can be fast and far-reaching to the surrounding flat plains.
The Hawkesbury River, on which Thompson Square is situated, is one of the most significant fluvial systems on the eastern coast of Australia.
The Hawkesbury-Nepean Valley covers approximately 425 square kilometres of flood plain. It is an area that is prone to rapid inundation, causing both erosion of river banks and the deposit of silt.
Archaeological investigations previously completed at the site of the Hawkesbury Regional Museum indicate that initial deposition of alluvial sand deposits, found in the soil profile, began approximately 150,000 years ago.
During the late eighteenth century, 1798-1799, New South Wales was in drought. Locally, it was broken by a devasting flood in March 1799. Rising by roughly 15 metres, the river banks could not contain the vast rapidly flowing waters. The Government Store, which was located on the riverbank, was washed away, along with houses, livestock, and provisions. Warnings prior to the flood from the Boorooberongal had gone unheeded. The early settler’s clearing of the land for agricultural purposes also contributed to the effects of flooding along the Hawkesbury River.
The Hawkesbury River again broke its banks in 1800 and inundated the settlement’s crops. This trend continued in 1801, 1806 and 1809. The pattern of rising flood waters was understood by the Boorooberongal. They were well versed in the changing natural environment of the region, interacting with the seasonal cycles by relocating when flood threatened. By comparison, the colony mostly experienced discomfort, deprivation of basic amenities and great loss at each flooding occurrence.
The largest recorded flood occurred in June 1867. The waters rose to over 19 metres above normal levels – its height is represented by the pole above you. This flood was widespread and catastrophic, with many people losing their homes, their livelihoods, and their lives.
Since 1867, the area has experienced many more floods. From 1799 to 1978, there were forty-four major floods (above 10m) recorded for the Windsor area.
The masonry design located on the bridge abutment represents the layers of soils uncovered during the archaeological salvage – from the sand body to current day. The blue lines, based on historical data, represent the voluminous flood history of the area over thousands of years.
Windsor’s Recorded Flood History
Below is a representation of recorded levels of flooding for Windsor from 1799 to 2020.
1857 10.4m (July), 11.9m (August)
1860 8.8m (February), 11.8m (April), 11.1m (July), 11.4 (November)
1864 15.1m (June), 11.4m (July)
1870 9.0m (March), 14.1m (April), 11.2m (May), 8.5m (November)
1871 11.7m & 8.5m (May)
1873 13.1m (February), 9.0m (June)
1877 9.6m (May), 8.6m (July)
1896/7 – Windsor Bridge Raised
1925 8.6m (May), 11.5m (Jun)
1929 8.0m (February), 8.6m (October)
1950 9.6m (June), 8.4m (July), 9.8m (October)
1952 9.5m (June), 11.8m (July), 9.6m (August)
1956 13.8m (February), 8.7m (June)
1960 – Warragamba Dam Completed
1963 8.7m (April), 9.0m (June), 9.6m (August)
1974 8.7m (April), 10.4m (May), 9.6m (August)
1976 9.4m (January), 8.0m (March)
1978 14.5m (March), 9.7m (June)
1988 12.8m (May), 10.9m (July)
1990 8.7m (April), 13.5m (August)