What is an estuary?
"An estuary is a place where land and sea meet. It is a transition zone where water flowing off the surface of the land meets the regular ebb and flood of the tides." 1
There are over 1000 estuaries along the Australian coastline 1 varying in shape and size including lakes, lagoons, harbours and river inlets. Estuaries are generally classified according to their geomorphology.
There are seven classes of estuaries in Australia 1:
- Drowned river valleys and embayments
- Wave dominated estuaries
- Wave dominated deltas
- Coastal lagoons and strandplains
- Tide dominated estuaries
- Tide dominated deltas
- Tidal creeks
Estuaries provide a variety of habitats that support plants and animals in a rich, diverse and highly interrelated web of aquatic and terrestrial ecosystems 2. Within Hornsby Shire, the main estuarine inlet is the Hawkesbury Nepean River System, a drowned river valley which boasts the third largest stand of mangroves in NSW, considerable seagrass meadows and remnant areas of saltmarsh.
Estuaries are formed where rivers meet the sea and are constantly being altered by the erosion and deposition of sediment caused by fluctuations in sea level. These sea level variations may be eustatic (variations in the volume of water in the oceans), or isostatic (variations in the level of the land).
Historically large changes in sea level have occurred through time. In particular, about 18,000 years ago the sea level was estimated to be 100m below its present level. Much of the water now in the world’s oceans was locked up in polar ice caps during the last ice age. As the ice age came to an end the water rose rapidly 2. This transitional phase ended in approximately 3,000 BC, and the levels have remained static to the present.
1 Turner L, Tracey D, Tilden J & Dennison W (2004). Where river meets sea Exploring Australia’s estuaries. Cooperative Research Centre for Coastal Zone Estuary and Waterway Management, Brisbane Australia.
2 NSW Government 1992, Estuary Management Manual
