Thursday, May 21, 2015

Lake Corangamite

Not far from the Barwon River - in geographical terms at least - lies Australia's largest, permanent, saline lake. This of course, is Lake Corangamite, the lake after which the Corangamite Catchment Management Authority which is responsible for managing the region's water (including the Barwon) is named. The name is believed to have come from the local indigenous word "koraiyn" meaning salty or bitter.
The south eastern shores of Lake Corangamite
Like Lake Murdeduke discussed in my previous post, Lake Corangamite forms part of the  RAMSAR-listed Western District Lakes region which has been recognised since 1982 as providing bird habitat considered to be of international importance. Also similar to Lake Murdeduke are the lunettes, or dunes which form on the east bank of some lakes in the south of Australia as a result of the action of prevalent westerly winds. Several of these formations can be found on the eastern shores of Lake Corangamite.
The lake is relatively new in geological terms, formed as a result of recent volcanic activity. Several million years ago, prior to the volcanic activity of the late Pliocene and the Pleistocene Epochs which saw the plains of western Victoria strewn with basalt from the hundreds of now extinct volcanoes scattered across the region, water flows were different. During this much earlier period, water from the region would have flowed out of the area, finding its way to the sea.
As volcanic activity in the district increased however, the natural drainage of the district was disrupted. Lava flows blocked rivers and formed lakes. Lake Corangamite is thought to have formed when flows from the Warrion Hill scoria cone prevented drainage to the east - and presumably thence to the Barwon.
Warrion Hill
Scoria cones such as this are common in the area and form when hot lava comes in contact with significant amounts of groundwater, causing explosive eruptions and resulting in the distinctive flat-topped hills which can be seen in many places today. Stony rises, such as those seen to the south of Lake Murdeduke as well as Lake Corangamite are also common as are distinctive maar craters.
Stony rises along the shoreline of the lake
These latter form when molten magma rising to the Earth's surface comes in contact with rocks containing ground water. So great is the pressure from steam, that the cooling magma is blasted into small particles which form rings of ash around the blast site. This results in wide, flat craters which often fill with water to form lakes - Lake Purrumbete to the west is one such. These freshwater lakes have a distinctive round shape, in contrast to the broad, shallow saline lakes such as Lake Corangamite and Lake Murdeduke which form in hollows within or between lava flows.
A maar crater near the shores of Lake Corangamite as seen from
the Red Rock lookout
The relative geological youth of the Corangamite region means that drainage patterns are not well-established; rivers have not yet evolved to carry water from the region. Lake Corangamite is not connected to the Barwon, and is considered endorheic, meaning that there is no natural water flow out of it, but this is not always the case. Current water levels in the lake are the lowest in centuries and salinity has increased well above the brackish levels of only a few decades ago meaning that flooding in the region is not currently an issue. Water flows naturally into Lake Corangamite from three main sources, namely the Woady Yallock River which rises west of Ballarat and flows south through Lake Martin to Lake Corangamite, Salt Creek flowing from the north west and Pirron Yallock Creek flowing north from the Otways.
Looking west
In addition to these watercourses, the Corangamite region is scattered with lakes of different shapes and sizes. In the past, during periods of high rainfall, water would flood the low-lying land in this area so these normally separate lakes became connected as water drained slowly towards Lake Murdeduke to the east. If the rainfall was high enough, the water would over top Lake Murdeduke and descend into the Barwon Basin below and thence to the river. There is evidence to suggest that this was a relatively common occurrence prior to the era of Ewauropean settlement, however low rainfall since the 1990s has meant little flooding in the district.
Looking west across the receding waters of the lake
In the past however, when flooding such as this did occur, surface water could remain in the region for months, even years, rendering otherwise good farming and grazing land useless for agricultural purposes. The most recent rainfall event to cause the overflow of the Lake Corangamite system (including Lake Colac) into the Barwon occurred in 1953. After consecutive years of the worst flooding on record on the lower Barwo n in 1951 and 1952, followed by record rainfall on the south western plains later that year in November, the region was waterlogged. Water levels in Lake Corangamite were said to have risen by 15 feet over the previous two winters and it was feared that Gnarpurt and Corangamite lakes would join, causing more flooding. Lake Murdeduke needed a rise of only 12 feet before the whole system would begin to overflow into the Barwon Basin below. This it was claimed, could result in the same kind of disastrous flooding downstream at Geelong that had been recorded a hundred years earlier in 1852 when Geelong suffered some of its worst ever flooding. Lake Murdeduke it was noted, had been breached on that occasion too.
A 1958 photo of Lake Corangamite, near Pirron Yallock Creek, image held by
the State Library of Victoria
Whilst there was no flooding on the lower reaches of the Barwon during the winter of 1953, November that year saw the highest ever rainfall in the Colac area. This was the final straw for the Corangamite system. There was nowhere for this water to go so it slowly crept across the farms in the district, seeping into low-lying areas, filling swamps and gradually making its way into Lake Murdeduke from where it ran down to the Barwon.
Whilst it did not result in the predicted catastrophe for Geelong, it caused huge problems for the farmers of the region. Action was demanded! Something needed to be done to drain the water from the region before the work of reclamation could begin. Somehow, the "creeping menace" as the Colac Shire president called it, had to be stopped...