For anyone who has walked or toured inland from our coast, the dramatic scenery of our river gorges is something they will have appreciated. However we have canyons that no one is ever likely to see first-hand unless they can ride in a deep water submersible - something like the one that recently found a plastic bag at the bottom of the world's deepest trench in the ocean.
Along the Eurobodalla coast the continental shelf, which slopes from low tide level to a depth of about 140 metres, is narrower than just about anywhere else around Australia (only about 15 to 20km from the coast). Then the sea floor drops away more rapidly as the continental slope falls to the deep abyssal plain.
There are two underwater canyon systems off our coast. Both of them start below the depth where the gentler slope of the shelf gives way to the steeper continental slope. They are not thought to be related to our present river systems and probably began forming about 2 million years ago, gradually extending when the various ice ages lowered sea levels. Australia and the indigenous inhabitants were in the peak of the last ice age 20,000 years ago and the sea level was 120 metres lower than now. Tasmania and the mainland were joined by land and in the South Coast land extended almost to the edge of the present continental shelf. Sea level rose as the earth warmed after the peak of the ice age, reaching its current level about 7000 years ago.
Sonar mapping shows the canyon off Batemans Bay is 8 km wide. Its walls are relatively smooth with the deepest part beneath the southern wall. The canyon system extends down the continental slope to 4500m depth.
Other canyons, one off Moruya and two off Tuross, appear to be part of a connected system. The deepest one, off Tuross, is 3 km wide and is thought to be the main one. In its lower reaches it is V shaped. The other two are thought to be tributaries with all merging further down the continental slope.
In May this year the first detailed survey of the underwater canyon off Perth discovered many new species. It was carried out with the help of remote operated vehicles (ROVs) similar to the underwater drones that Dave Rowland has been using to photograph some of the amazing habitats in Batemans Bay and east of the Tollgates. I wonder what creatures, new to science, may be living in our submarine canyons?
With their upgraded ROV now capable of diving over 300m and drop cameras even deeper, Dave and the UnderseaROV team are planning some exciting expeditions off our coast over the next few months.
The Nature Coast Marine Group has an extensive program of activities where members can have fun learning about our marine environment. New members are always welcome. To find out more about the group visit the website www.ncmg.org.au or search for Nature Coast Marine Group on Facebook and follow us there.