Recently DIRdude Rich teamed up with GUE ITC Candidate Matt White from Sydney to dive Australia's deepest lake, Lake St Clair in Tasmania.
Lake St Clair is located at the southern end of the world famous Cradle Mountain-Lake St Clair National Park and is part of the Tasmanian World Heritage Area which covers a significant proportion of the South West and Central Highlands of the state.
The Lake has an area of 11 square miles (28 square km), measures 9 miles by 1 mile (14.5 by 1.6 km), and lies at an elevation of 2,417 feet (737 m) on Tasmania’s Central Plateau. Filling a valley carved out by ice during several glaciations over the last two million years and then dammed with a moraine, the maximum depth approaches 700 feet (215 m) along the west-side, making St. Clair Australia’s deepest lake. It is also the headwaters of the Derwent River. It is ringed by some remarkable mountain peaks which are frequently covered in snow for much of the year. Lake St Clair is also known by its Aboriginal name of Leeawuleena which means "sleeping water", an apt description, although this often tranquil lake has many moods which seem to change constantly.
It is also the end point (or the start) for bushwalkers tackling the Overland Track which runs between Lake St Clair and Cradle Mountain, the track much beloved by bushwalkers from all over Australia and the world.
The area around Lake St Clair offers a wealth of walks, ranging from leisurely strolls to overnight bushwalks, with beautiful forests and alpine moorlands to explore. But relatively little is known about the aquatic environment in the lake itself and it has only ever been infrequently dived and never (to our knowledge) documented. Likewise the lake has never been bathymetrically surveyed in any detail.
Logistics
We flew our gear into Tasmania and hired O2 Clean S100 cylinders and weights from the guys at
Southern Tasmanian Divers in Hobart.
We were diving EAN32 which meant at 730m elevation we had a MOD of ~27m (ie. less ~3m fto compensate for the altitude), more than deep enough for the reconnaissance dives we were planning for this trip. Later trips were we'll require Trimix are going to be more complex and will require partial pressure mixing of gas on site.
The DIRdudes chartered the lake's ferry the 'Ida Clair' from
Lake St Clair Lodge where we stayed. This ferry, whilst not specifically designed for diving or water entry and exits, was more than adequate for our needs, and certainly made accessing some of the more remote parts of the lake easier.
Accommodation
We stayed at the Mimosa Cottage, a heritage listed cabin on the banks of the Lake, which offered simple self-catering accommodation, plenty of room for setting up the dive gear and fantastic views of the lake.
The Diving
Heres an vimeo album of the dives we did on the lake.
Dive 1: Submerged Islands
Lake St Clair is a deep U-shaped channel for much of its length with a pronounced shelf along its eastern flank. Towards the southern end the lake shallows rapidly as the valley is dammed by glacial moraines which have been folded up into distinct and unique ridges. Historically there were a number of isolated vegetated islands along this southern coastline which likely represent the crests of further morainal ridges descending into the lake. These photos below (courtesy of the Tasmanian Archive and Heritage Office) are from circa 1900 and clearly show this small islands along what is now know as Frankland Beach.
In the 1930's the decision was taken to build a weir at the outflow of the lake and established a pump-house to provide Hobart with emergency water. The construction of the weir artificially raised the lake by approximately 3m and inundated these small islands and permanently changing the character of the southern part of the lake and the headwaters of the Derwent River.
The objective of our first dive was therefore to attempt to relocate and document these islands and get a feel for the character of the lake in terms of visibility, water temperature, aquatic flora and fauna etc.
Dive 2: Pump-House
In 1933 the HEC drew up plans to develop a large Hydro Electric Scheme in the upper reaches of the Derwent River. The scheme was built between 1938 and 1940 and involved the erection of a controlled dam on the Derwent River and a pumphouse and substation at Pumphouse Point.
The pumping station allowed the lake to be drained below its original level. In order to stop the river from draining back into Lake St Clair when the lake level was low, a weir was constructed at the northern end of St Clair Lagoon. This resulted in the permanent inundation of the northern end of the Derwent River. The pumphouse was built out on the Lake allowing the pumps to draw water from 6.5 metres below the surface. This water was then discharged along a 245 metre concrete flume connecting the pump house to the shore. It then flowed via a short canal into St Clair Lagoon.
More coming soon...
Dive 3: Cynthia Bay Jetty
Our third dive was off the Cynthia Bay Jetty which acts as the home mooring for the 'Ida Clair'.
More coming soon...
Rich & Matt at Echo Point
