Sackville Gorge is a geographical feature on the Hawkesbury River, New South Wales in which the river narrows between high cliffs on causing a build up of water in times of flood. Unlike the upstream constrictions near Wallacia and Castlereagh, which are relatively short, the gorge is a very long and narrow waterway stretching about 35 kilometres downstream from Sackville to Wisemans Ferry. The river opens up for a short distance downstream to Spencer before opening out into Broken Bay.
This feature has a major influence on the behaviour of floods in the Hawkesbury-Nepean Valley. Typically, coastal rivers widen out as they flow towards the ocean, however Hawkesbury-Nepean floodwater flows freely into the Richmond/Windsor basin but cannot escape at the same rate through the Sackville Gorge. As a result, water levels around Richmond and Windsor rise well above the level that would be expected. This is often described as the ‘bathtub’ effect.
The effect of the gorge is further exacerbated by the inflow of the Colo River at Lower Portland within the length of the gorge. Depending on rainfall patterns, the Colo River can release a large volume of water into the Hawkesbury at Lower Portland. During periods of low Hawkesbury River flow, this can cause backup flooding along the Hawkesbury River towards Sackville. At higher Hawkesbury River flows, the effect can be to slow down the rate of drainage from the primary floodplain and this holds up flood levels in the Richmond/Windsor area.
View Hawkesbury City Council Flood Extent Maps
Review the Hawkesbury-Nepean Flood Emergency Plan 2020
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