The Hawkesbury River, or Deerubbin, is a major tidal estuary defining the northern boundary of Hornsby Shire. As one of the most significant estuaries on Australia's eastern seaboard, it's a complex and biodiverse environment where freshwater from inland creeks mixes with saltwater from the Tasman Sea. This dynamic creates a winding waterway renowned for its high productivity and supports a brilliant diversity of native animals, linking inland and marine ecosystems. The river is a cornerstone of the local economy and a premier destination for recreational fishing, boating, and other water-based activities.