The Hawkesbury-Nepean River is one of the major river systems of NSW, draining a catchment area of 22,000 square kilometres. Exceptionally heavy rainfall in this area can lead to severe flooding.
This is worsened below Richmond by the fact that water flows into the valley much faster than it can flow out.
In the largest flood of record, which occurred in June 1867 flood waters reached approximately 12 metres higher than the deck of the present-day Windsor Bridge. Even larger floods than the 1867 are possible. Maps of the areas inundated in the 1867 flood are available for download.